On 1 September 2015, the pilot of a SA227 (Metroliner) aircraft, registered VH-HPE (HPE), was conducting a scheduled freight run from Brisbane, to Emerald, via Thangool Airport, Queensland.
Approaching Thangool, the pilot conducted a Distance Measuring Equipment (DME) arrival with a left circuit onto runway 28, touching down at 0545 Eastern Standard Time, just before first light.
Shortly after touchdown, and with all landing gear wheels in contact with the ground, the pilot saw the glimpse of an animal flash from left to right in front of the aircraft. At the time, the aircraft was travelling at about 80 kt. The right propeller then struck the animal, later identified as a small kangaroo. The pilot reported that following the large bang associated with the propeller striking the animal, there was a lot of vibration throughout the aircraft, but no abnormal engine indications. The pilot continued the landing roll, and used ground idle rather than reverse thrust to slow the aircraft.
The pilot taxied the aircraft to the parking bay, shut down the engines, then carried out an external inspection and found that one of the propeller blades attached to the right engine was twisted (Figure 1). The pilot was not injured.
Following the incident, the operator replaced the right engine and propeller and arranged for the original engine to be further assessed by engineering staff.
Figure 1: Damage to propeller blade on VH-HPE
Source: Pilot
Pilot experience and comments
The pilot held an Airline Transport Pilot Licence (Aeroplane) (ATPLA) and had about 4,900 flying hours, with about 1,310 hours on Metroliner aircraft. The pilot stated that there were no relevant NOTAMS for Thangool Airport, nor any alerting radio calls regarding wildlife. During the approach, the pilot had activated the pilot activated lighting (PAL), which provides runway and airport lighting, however no wildlife was evident on the runway.
Thangool Aerodrome Safety Officer comments
The aerodrome safety officer (ASO) reported that, due to continual pro-active mitigation strategies, such as animal culling, there have been no reported kangaroo strikes at the airport in the last 28 years.
The ASO also commented that they had conducted a runway inspection about 20 minutes prior to the Metroliner landing. No animals were evident during that inspection.
Banana Shire Council comments
Sections of the Thangool Airport had wildlife protective fencing however, it is not fully fenced. At the time of publication, the Banana Shire Council, who own and operate the airport, had a funding application lodged with the Australian Government’s Regional Aviation Access Programme to enable the remainder of the airport fencing to be completed (Figure 2).
Figure 2: Thangool airport with current and proposed fencing marked
Source: Banana Shire Council
Additional wildlife strike
On 2 September 2015, another kangaroo strike was reported to the ATSB (ATSB occurrence number 201503915). In this occurrence, a Raytheon B200 aircraft, VH-FDB, was on a medical retrieval mission from Townsville to Barcaldine Airport, Queensland. As the aircraft touched down in the early hours of the morning, at about 0141 EST the pilot caught a last moment glimpse of the animal before the aircraft struck a small kangaroo. The pilot reported that engine indications were normal, with no noticeable vibration as they completed the landing roll and then shut down the left engine while on the runway. They then taxied clear of the runway using the remaining engine. The strike caused damage to the three propeller blades attached to the left engine and disabled the aircraft (Figure 3)
Operator comment
The operator commented that although the Enroute Supplement Australia (ERSA) entry for Barcaldine Airport warns that both an animal and bird hazard exists, the local base had never sighted any animals, and therefore were somewhat inclined to discount it as a likely occurrence.
The operator/crew had not requested a “roo inspection” prior to the landing. They have also advised the ATSB that apart from the propeller damage, the left engine power module may also need replacing.
Figure 3: VH-FDB showing twisted propeller blades
Source: Operator
Barcaldine Regional Council
A council representative advised the ATSB, that a 6 ft chain mesh fence, with locked gates surrounded Barcaldine Airport. The airport staff conducted regular wildlife inspections at the airport, and also upon request with prior notice. They advised that due to drought conditions, there has been a noticeable increase in kangaroo numbers in the prior months. This increase was due to the animals seeking feed.
Safety action
Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce their safety risk. The ATSB has been advised of the following proactive safety action in response to this occurrence.
Operator of VH-FDB
As a result of this occurrence, the aircraft operator of VH-FDB has advised the ATSB that they are taking the following safety actions:
Strip inspection
The operator is amending their internal procedures, to automatically request a strip inspection for animals at any airport in the ERSA where an animal hazard is listed. This new procedure will also extend to requesting a strip inspection at any aeroplane landing area (ALA) when deemed appropriate.
Safety message
Occurrences involving aircraft striking wildlife, particularly birds, are the most common occurrences reported to the ATSB. They are a significant economic risk for aerodrome and airline operators as well as a potential safety risk. The ATSB regularly publishes a statistical report on the number and frequency of wildlife strikes. The aim of the report is to give information back to pilots, aerodrome and airline operators, regulators, and other aviation industry participants to assist them with managing the risks associated with bird and animal strikes. This is available on the ATSB website.
Both animal strikes and bird strikes remain a mandatory reporting item under the Transport Safety Act 2003. Reporting obligations are available on the ATSB website.
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
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On 28 August 2015, a Pacific National fully loaded coal train MB520 departed from Maules Creek, NSW destined for Port Waratah, NSW. Shortly after passing Ardglen and descending the grade towards Pages River, a wheel failure occurred on the leading right hand wheel on the 19th wagon. This led to the wheel moving off the wheel seat towards the centre of the axle. The bogie rotated causing the other wheels on the bogie to derail. The train continued travelling until a damaged inter car brake cable activated the train’s brakes. There were no injuries, but the derailment caused damage to over 590 metres of track including 963 sleepers.
What the ATSB found
The ATSB investigation found that the derailment was caused by a wheel failure. A metallurgical analysis conducted after the derailment found that the wheel had multiple cracks in the wheel rim; one crack extended from the rim to the boss of the wheel. This likely originated from a transverse thermal crack on the wheel tread surface. Thermal cracks of this type are generally associated with high thermal input under service brake conditions.
An increasing level of wayside alerts starting a month before the derailment indicated the likelihood of the fault being present then. Despite the increasing impact level readings, no immediate action was taken to inspect the wheel before it eventually failed.
It was found that the wheel inspection processes were not effective in detecting surface damage or cracks. The failed wheel was approaching the end of its service life with a rim thickness of 25 mm. This low rim thickness increases propensity to thermal distortion and is likely a significant factor into the thermal crack formation and propagation that occurred.
At the time of writing this report there have been seven reports of fractures detected in wheels, within a 3-year period, on Pacific National coal wagons in NSW. Three occurred before the derailment with the first incident on 31 May 2013. Three more fractured wheels were identified within a two-month period after the derailment. Following the first three wheel failures internal reports recommended changes to reduce the risk of wheel defects. Only limited action was taken by the operator prior to the derailment on 28 August 2015.
What's been done as a result
Since the derailment, the rollingstock operator has advised that they have established a process to remove wheels considered to be at greater risk of fracture. To this end, they have implemented a program to remove wheels with a rim thickness under 25 mm. Pacific National also intends to improve their internal standards to include revised criteria and actions for thin rims and wheel impacts.
Safety message
Rollingstock operators with heavy haul wagons using wheels near the end of their service life should be aware of the increased risk of wheel failure due to cracking. They should ensure that wheel inspection and maintenance programs include systems and techniques for detecting and assessing wheel defects with the potential to lead to cracking. These systems and techniques should be validated to ensure they are effective to detect such defects.
Safety issues and actions
Depending on the level of risk of the safety issue, the extent of corrective action taken by the relevant organisation, or the desirability of directing a broad safety message to the rail industry, the ATSB may issue safety recommendations or safety advisory notices as part of the final report.
Wheel inspection processes
The wheel inspection processes and systems were not effective in detecting surface damage or cracks on the R4 wheel on wagon NHIH97081 prior to the wheel failure.
Despite a number of incidents, Pacific National did not take adequate action, before the derailment, to reduce the risk of wheel defects, especially in light of previously identified contributors such as low rim thickness.
The following wayside monitoring systems were not designed to detect wheel failure. Each system has its own focus and method of operation.
Wheel Impact Load Detector (WILD)
Used to measure the effect of wheel impacts on the rail surface under dynamic conditions. This can provide early detection of wheel defects such as skids (wheel flats), and wheel out-of-round. Originally designed to protect track from high energy impacts from wheels. It consists of arrays of accelerometers and train presence switches clamped to the rails. It provides alerts to the network owner and also to rollingstock operators.
Hot box detectors
Hot-box detectors are a reactive method of condition monitoring. They usually detect the infrared signature of bearing components and alarm if the temperature exceeds a predetermined setting.
Consequently, hot-box detection is usually used as a ‘last line of defence’ to protect railway infrastructure assets critical to production processes such as coal and ore carrying railways.
Rail Bearing Acoustic Monitoring (RailBAM)
Bearing Acoustic Monitoring (BAM) is a predictive condition monitoring system that ‘listens’ to the acoustic signature of bearings and can detect faults as they develop. It is the primary method for detecting potential bearing faults on rolling-stock travelling on the interstate main line. Recorded data from each train is stored in a database allowing evaluation, trending and maintenance scheduling of rolling-stock based on predicted bearing condition.
BAM uses sensitive acoustic arrays to record the sounds emanating from wheels and bearings passing through the monitoring site. The recordings are processed for the sound characteristics that are unique to specific types of bearing faults. BAM is best at detecting faults on rolling surfaces such as the inner and outer raceways, and rollers in rolling-stock bearings. BAM can also detect looseness or fretting faults and ‘noisy’ wheels (flanging and wheel flats).
Rail Squeal Acoustic Detection (RailSQAD)
Measures acoustic energy to enable comparison of wheel noise trends over time. It measures the energy in the frequency band where wheel squeal and flanging noise are typically found. Wheel squeal is generated when the wheel tread slips on the rail. Excessive levels can be caused by worn wheel profiles and badly tracking bogies. Flanging noise occurs when the taper of the wheel is insufficient to turn the wheelset around a tight corner. The flange then pushes against the inside edge of the rail gauge face to turn the wheelset. A screeching sound is typical on tight corners but increased noise levels may indicate badly tracking bogies, worn wheel / rail profile or a lubrication malfunction.
Truck Bogie Optical Geometry Inspection (TBOGI)
Optically measure the position and angle of passing wheel sets relative to the rail. There are a number of performance parameters, the two most basic are the tracking position and angle of attack. Measurements for each axle are taken, and when parameters are exceeded, an alert is generated. In this way bogie health is monitored which may reduce wheel set replacement, flange wear, rail wear and identify poorly performing bogies.
Weighbridge sites
High and low speed weighbridge sites weigh trains as they pass. Wheel sensors detect the presence of a train, record the number of axles on the train and measure the speed of each wagon. Strain gauge transducers are mounted in the web of each rail with control equipment to store and process the collected data.
Appendix C – Wheel Impact Load Detector flowchart
Appendix D – ONRSR Safety Bulletin
Safety analysis
The investigation determined that the derailment at Pangela was caused by a wheel failure. The R4 wheel fractured from the rim through to the boss causing a loss of interference fit and resulting in the wheel becoming dislodged from its seat and moving inward towards the centre of the axle. The fracture initiated as a transverse thermal crack in the wheel tread and, due to cyclic stressors, progressed by fatigue crack propagation to eventual failure.
This section examines the metallurgical aspects of the wheel and its fracture. It also explores wheel condition management including wayside monitoring, and risk management by the rail operator.
Metallurgical aspects of the wheel failure
All parts of the derailed bogie, wheelset, and the available fractured sections of the wheel recovered from site were examined and photographed by investigators and technical analysts from the ATSB at a Pacific National workshop at Port Waratah. Testing requirements were discussed with specialists from the ATSB, Comsteel, the Office of National Rail Safety Regulation (ONRSR) and Pacific National. Pacific National engaged the technical services of Comsteel to conduct the metallurgical analysis of the failed wheel. The scope and steps for the metallurgical investigation was agreed upon by these specialists before the wheelset and wheel pieces was taken to the Comsteel testing facility in Newcastle. The final metallurgical report[14] was examined by technical analysts from the ATSB who accepted the methodology and conclusions.
The metallurgical report concluded that multiple thermal cracks had initiated in the wheel rim. A magnetic particle inspection revealed six thermal cracks at a number of locations around the circumference of the wheel tread. It also found two cracks in the wheel rim. One of the thermal cracks progressed by fatigue until it extended from the rim to the boss of the wheel and was responsible for the loss of the interference fit between the wheel and the axle, which allowed the wheel to move inwards along the axle and initiate the derailment. The other crack extended 90 mm from the rim into the rim.
The report stated: ‘It is highly suggestive that the fracture was initiated by a transverse thermal crack in the wheel tread. This type of transverse thermal cracking is typically cause by excessive localised thermal load into a vulnerable region of the wheel tread and is generally associated with abnormal braking load.’
The evidence from the metallurgical report also found:
the failed wheel complied with industry specification requirements
it exhibited a high level of steel cleanliness and no material abnormalities
rolling contact fatigue indications were detected toward the flange side of the tread
the inner rim exhibited a rough, blistered and corroded surface
rim profiles from both wheels showed no significant deviation from original profile
residual stress measurement on both wheels was not possible.[15]
The paired wheel on the wheelset was also examined but no thermal cracks were detected on its wheel tread. This suggested that, at some time prior to the crack formation, there was uneven distribution of braking energy between the two wheels on the wheelset. The appearance of the rough/ blistered /corroded surface on the inner rim of the failed wheel supports the likelihood that there was thermal loading on the wheel rim (Figure 9-1). There were indications on the paired wheel of a white etching layer (Figure 9-2), this is usually indicative of a heat-affected area possibly also caused by a braking event. It appears further away from the rim edge than those classified as near rim edge and has no crack indications. No heat-affected areas of this type were found on the failed wheel, but this evidence would likely have been removed due to the gouging of the wheel after the derailment when it was dragging in the four foot.
Figure 9: Heat indications on wheels
Image 1 shows the surface condition that may indicate overheating of the wheel rim of R4. Image 2 shows an area of white etching layer on the paired wheel L4, which was indicative of a heat affected area.
Source: Comsteel
There was no evidence of any problem with the brake mechanism or brake blocks on bogie NECA71. In addition, there was no record of wayside alarms for high wheel temperature for the wagon. This increases the likelihood that any thermal loading event would have occurred under service brake conditions.
Evidence of overhanging brake blocks was found on two other incidents, 6 October 2015 and 7 October 2015. The incident on 4 November 2014 and the subsequent Pacific National report discussed reducing the risk of brake block overhang by modification of the bogies by reducing the lateral float of the brake beam. There is no evidence that this was addressed prior to the derailment on 28 August 2015 Pacific National have recognised that this is an area of concern and are now trialling fitting modified brake beam liners to self-steering bogies on coal wagons.
The metallurgical report found that the failed wheel was approaching the end of its service life with a rim section 25 mm thick. This thin rim section leads to lower rim rigidity and, hence, higher propensity to unusual thermal distortion and would be a significant contributing factor into the thermal crack formation and propagation. Examples of thermal cracks identified by Comsteel during the metallurgical analysis are shown in Figure 10.
The high wheel loading (30 tonne axle loads) and the cyclic thermal braking events are also likely to have influenced the propensity for crack propagation once the thermal crack was initiated. Research into overheating of wheels found: ‘The loss of compressive residual stress and the development of tensile residual stress in the wheel rim due to overheating can contribute to fatigue failure of the wheel. A reduction in rim thickness due to wear and re-profiling also alters the residual stress distribution, and may increase the risk of wheel failure.’[16]
The ONRSR Safety Bulletin, (see Appendix D) identifies many other factors which contribute to these defects: ‘developments such as electronically controlled pneumatic braking (which can increase maintenance intervention periods), steering bogies (which can reduce wheel to rail contact area), micro-alloyed wheels (which are longer lasting wheels), and solid drawbar assemblies (which can increase maintenance intervention periods).’[17]
Figure 10: Thermal cracks identified during metallurgical analysis
Image 1 shows the location of the thermal cracks in relation to the fracture. Image 2 identifies thermal cracks located on the front face of the wheel tread.
Source: Comsteel
Wheel condition management
Pacific National has in place a number of measures to monitor and manage the condition of its wheels and rollingstock. The primary method used to monitor and manage wheel condition is dependent on inspections conducted by Pacific National employees including train crew, maintenance staff and terminal operators.
A number of visual and audible examinations are regularly carried out. These include but are not limited to the following:
Terminal departure and arrival examinations. These are conducted at a low speed, usually about 10km/h. A terminal operator stands near the track and looks and listens for any abnormalities as the train passes.
Roll-by examinations. Conducted by train crew on one train as another train passes by. One train might be stationary or both might be operating at track speed. After the roll-by inspection radio communication occurs between crews to verify that no defect was present. Roll-by examinations also carried out by staff standing trackside at unloading points.
Full examination (FX). These full inspections are conducted while the train is stationary at a maintenance facility or terminal by examining staff at regular intervals (Coal trains at 28-day intervals). Note that the previous inspection for MB520 was on 29 June 2015, 60 days before the derailment.
Unit Train Maintenance (UTM). A more structured static full examination is conducted during UTM at a maintenance facility every 128 days.
Identifying a wheel defect, especially in the early stages, can be difficult. There are many factors that reduce the likelihood of the detection of a wheel defect, these include:
speed of the train as it passes by
environmental conditions such as lighting, noise levels and weather
defects obscured by other components
position of the wheel
experience of the observer
fatigue and motivation level
time pressure to complete task.
Pacific National have a dedicated wheel condition monitoring team with a number of analysts based in Adelaide, Melbourne and Port Waratah. These analysts collect and analyse wheel data to assist them managing the Pacific National fleet. Some of the data collected by the wheel condition monitoring team includes wheel rim thickness, wheel age, and wheel impact loading. They also monitor wheel profile which is available from one location. Most of the condition monitoring is in relation to wheel wear and organising the examination events for the lifecycle of the wheel. It currently does not, and is not able to, identify wheel defects while the wheel is in service. The use of WILD data, which is explained more fully later, is an indirect way of detecting wheel defects.
The condition monitoring team based at Port Waratah operates during office hours on a 5-day roster. On this basis not all wheel defects will be attended to immediately. They receive electronic data and email alerts from ARTC wayside monitors and their own wheel profiler at Greta. Based mainly on past experience, the condition monitoring team develop strategies to minimise the likelihood of wheel defects and maximise the life of the wheel.
Since 2012, Pacific National has used a wheel profile monitor at the Greta maintenance facility in the Hunter Valley. Using laser and optical devices, it measures the wheel profile, wheel flange thickness and other parameters such as back-to-back measurements as the train passes at low speed. The profile parameters are used to predict wheelset optimum removal time and identify poorly wearing wheelsets.
Wheel profile data for the failed wheelset, collected before the derailment, indicated that it was within allowable wear limits, later confirmed by the metallurgical report. The back-to-back measurement for this wheelset was not recorded for the previous two passes prior to the derailment. It was initially postulated that this was caused by the wheel moving on the axle, giving early indication of wheel failure. An examination of wheel profile records found that a null data reading was a common occurrence for back-to-back measurements. Inaccurate measurements result from variation in the wheel chamfer profile. It is more likely that the missing data is a characteristic of the wheel profiler and not related to the progression of the wheel failure. Improvement in the reliability of this device would assist Pacific National in their management of wheel condition.
Wayside monitoring
There is currently no wayside detection system that can detect cracks on in service trains except at workshop maintenance when post wheel turn non-destructive testing is carried out to check a defect has been removed. Following wheel manufacture, once non-destructive testing is completed, the rollingstock operator primarily relies on visual inspections by train crew, terminal operators, and wagon maintenance staff to detect wheel fracture. The WILD will provide information if the defect is of the type that will cause an impact to be recorded. Typically this will be something like a chip or gouge in the wheel flange, out of round wheels, skidded wheels, and cracks where a portion of the wheel has fallen away. Early stage cracks and thin deep cracks are not detected as minimal impact forces are generated.
The network access provider, the Australian Rail Track Corporation (ARTC), operate and maintain the wayside monitoring systems in the Hunter Valley where this incident occurred. There are different types of wayside devices including detectors for hot bearings, wheel impact loads, acoustic wheel monitoring, and weighbridges. These monitoring systems can be split into two generic types, reactive and predictive. A reactive approach requires an immediate action after a serious condition develops or equipment failure occurs, whereas a predictive approach identifies the requirement for future action before a serious condition develops. Reactive condition monitoring, such as hot bearing detectors, are usually used as the last line of defence to protect significant or critical railway infrastructure assets.
For a more detailed description of their function, see Appendix B.
These trackside systems are primarily used by ARTC to prevent damage to the track infrastructure. They detect a variety of rollingstock and track faults and are positioned at strategic locations in the network (Figure 11).
Figure 11: Wayside monitoring equipment system map
Source: ARTC – redrawn by ATSB
The system comprises a variety of wayside devices, which communicate readings to an ARTC data recording and alert system. Each train and wagon is identified using Automatic Equipment Identification (AEI), each wheelset is also counted. The wayside detection system records data, and at set levels, notifications are sent to ARTC and at times to the rollingstock operator. The pathway for a WILD wayside system alert to ARTC and Pacific National is included at Appendix C.
The failed wheel (R4) was initially identified by the WILD at Metford on 28 July 2015. The WILD then recorded passes by the wheelset until the day before the derailment, with a peak of 326 kN recorded on 26 August 2015 (Figure 12). It is only when the impact level reaches 400 kN that ARTC procedures require more immediate action.[18] At this impact level, i.e., 400kN, ARTC train control contact the train crew, and request they bring the train to a stop to inspect the nominated wheel.
Figure 12: Wheel Impact Load Detector readings
WILD readings from the failed wheel (R4) prior to the derailment and Pacific National’s actions.
No.
DATE
WILD
ACTION
1
28/07/15
175 kN
Nil
2
02/08/15
204 kN
Monitor
3
16/08/15
273 kN
Monitor
4
23/08/15
316 kN
Plan for wheelset removal
5
26/08/15
326 kN
Work order issued for wheelset removal by 20/09/15
6
28/08/15
na
Derailment at Pangela
Source: ATSB
A work order for the removal of the wheelset was created on 26 August 2015, two days before the derailment, in accordance with the Pacific National decision matrix table.[19] The table specifies the action when the WILD reading reaches a certain threshold. This alert was triggered once the second reading in the 300-350 kN range was reached in the month. The alert was sent to the Pacific National wheel condition monitoring team, where a work order was raised to remove the wheelset by 20 September 2015. Pacific National classified the earlier impact levels, from 28 July 2015, as requiring monitoring.
Pacific National use the ARTC wayside monitoring system for two main areas (see Appendix B for explanation of terms):
Wheelset condition monitoring using a Wheel Impact Load Detector (WILD) and Bearing Acoustic Monitoring (RailBAM)
Bogie condition monitoring using Truck Bogie Optical Geometry Inspection (TBOGI) and Rail Squeal Acoustic Detection (RailSQAD).
Pacific National has limited access to the WILD data and impose lower impact level thresholds than ARTC. The purpose of these thresholds is to detect bearing failures that are associated with wheel impacts. Pacific National acknowledge that the WILD wayside data is an indirect way of detecting wheel fracture, is a lagging indicator, and not what the system was principally designed for.
Risk management
As required by national rail safety legislation, Pacific National, as an accredited rail operator, has in place a Safety Management System. This system includes a risk register which identifies major operational risks and the control measures which the operator has in place to prevent and manage the risk.
As part of managing risk, Pacific National have developed technical standards that apply to their rollingstock. These standards are consistent with the Rail Industry Safety and Standards Board codes of practice. As part of the Safety Management System process, these standards are monitored and reviewed.
Pacific National, in their Operational Risk Register[20], identified the risk of derailment caused by defective wheels, where the defect was caused by a vertical split or shattered (fractured) rim. Using estimated consequence and likelihood levels for a defective wheel and with the above control measures in place, Pacific National calculated the residual risk level in the high category.
The control measures identified for the identified risk were:
Wagon maintenance manual WMM 09-11
Generic procedures GRP6 16 and GRP6 23
Procedures NSWCO-032 and 033
Ultrasonic testing regime on 100 tonne axles
Physical/ visual inspection regime of axles for strike marks by maintainers during UTM
Network Owner Wayside Monitoring Equipment.
The controls measures listed, with the exception of the wagon maintenance manual, are not specifically aimed at controlling wheel failure. The procedures and ultrasonic testing are for axles not wheels. The wayside monitoring systems are currently not developed to detect early stage wheel fractures. The wagon maintenance manual is limited to describing the typical faults found by visual inspections and the action to take when the defect is detected. These risk management strategies developed by Pacific National are not likely to increase the probability of detection of wheel failure since the first identified wheel incident.
As a result of the first two identified Pacific National wheel incidents in 31 May 2013 and 25 August 2013, ONRSR published a Safety Bulletin in June 2014. It was titled: “An emerging issue: ‘shattered rim’ wheel defects”, and is included at Appendix D.
This bulletin stated that, while not so common in Australia, shattered rims are one of the most significant causes of rail wheel failures in North America. It said that as rail wheels age, the effects of fatigue can cause a defect to develop. The safety bulletin reminded operators to question current maintenance and wagon inspections and enhance wagon maintenance for those wheelsets which are approaching condemning diameter.
Two control measures were suggested for operators to include:
non-destructive testing during manufacturing phase, and
trackside detection and monitoring devices.
It concluded that Australian rail operators, depending on their operating environments, may need to review and revise current risk registers in order to manage to a ‘so far as is reasonably practicable’ level the hazards associated with a wheel failure due to a shattered rim.’
It is likely the low rim thickness was a contributory factor in the previous Pacific National wheel cracks. Thin rim sections were found in the other six reports of fractures detected in wheels (Figure 8). The rim section thicknesses ranged between 22 – 26 mm. The maximum rim thickness in the fleet was 57 mm with the average being 50 mm.
After the first incident on 31 May 2013, Pacific National commissioned a metallurgical investigation and completed an internal investigation[21]. Many aspects of wheel monitoring and procedures were examined; one aspect was the minimum wheel rim thickness. Pacific National operated wheelsets to a 22 mm minimum wheel rim thickness, which is consistent with RISSB code of practice for wheel defects. The Pacific National report made the following observations:
if a 22 mm rim thickness is determined to be a satisfactory minimum rim thickness, the current levels for wheel impact alerts may not be appropriate for 30 tonne axle load wagons, particularly those fitted to self-steering bogies.
the Association of American Railroads (AAR) identifies wheel configuration at low rim thickness being sensitive to large thermal stresses.
Pacific National should undertake a review of wheel stresses for the current wheel design, with particular focus on low rim thickness.
It is unknown if a review of wheel stresses was undertaken by Pacific National following this incident. There was no change to their practice of using wheels with a rim thickness approaching the condemning limit until after the derailment.
As can be seen from Figure 8, most of the defective wheels were running with thin rim thickness. This was because they were not targeted for removal. The age of the wheels ranged from 13 -19 years since manufacture. Although, like the failed wheel from this derailment, and other Pacific National failed wheels since 2013, the wheels were within accepted standards for wheel design, material composition, inclusions, and wear limits.
The change in the Pacific National wheelset rim thickness in the 30 tonne axle load coal fleet from just prior to the derailment in September 2015 to February 2016 is shown in Figure 13. The lower range of rim thickness from 20 mm to 30 mm was selected to be shown as it is the area of most concern, the upper range from 31 mm to 57 mm is not shown. The graph shows the progress of the removal of the bottom range of wheel rim thickness.
Source: ATSB table from Pacific National supplied data
Pacific National acknowledge that increasing the minimum rim thickness would assist with reducing stresses in the wheel and may act to prevent further failures. Pacific National did not review the application of this value until after the derailment. Pacific National advised that they had implemented various strategies to reduce the incidence of wheel failure. These strategies include:
Increase the from-workshop outbound wheel turn size upon re-profiling turn from 25 to 28 mm
Actively remove of wheels with low rim thickness (under 25 mm).
Other proposed actions include:
Establish a process to identify thin rims and wheel impacts and remove from service wheels considered to be at greater risk of fracture
Fit modified brake beam liners to bogies on coal hopper wagons to reduce the incidence of overhanging brake blocks and the subsequent formation of thermal cracks
Conduct a fleet wide inspection of wheels for cracked rims.
On 28 August 2015, at 0210[1], a Pacific National train crew, comprising a driver and second person, signed on for duty at Gunnedah, NSW. They were rostered to drive Pacific National coal train MB520 from Gunnedah to Port Waratah where the coal was to be unloaded. The train had been loaded at Maules Creek and driven by another train crew to the Gunnedah loop. The original train crew waited for the replacement crew to take over.
At 0228, the driver and second person relieved the outgoing crew who reported that there were no issues with the train. Two minutes later the incoming driver entered his identification number into the leading locomotive’s computer, confirmed the headlights were on, and departed Gunnedah loop.
As the train departed the loop, the routine roll-by inspection of the train was not conducted due to concerns about the potential for other trains to be delayed. However, the outgoing second person informed the incoming crew that a roll-by inspection was conducted earlier at Emerald Hill, about 18 km away. During this inspection, no problem with the train was identified.
At 0544, the train came to a stand at Chilcotts Creek to allow bank locomotives to attach to the rear of the train.[2] The gradient leading up to the Ardglen tunnel, a summit tunnel on the Liverpool Range, is 1 in 40 (2.5%) (Figure 1). The grade crests inside the tunnel.
Figure 1: Curve and gradient diagram for incident location
The red arrow shows the direction of travel for MB520
Source: Rail Access Corporation
The bank locomotives were attached and the train departed Chilcotts Creek at 0549. Once the train had reached the summit at Ardglen, the locomotives were detached and MB520 continued through the tunnel. The train exited the tunnel at 0612, travelling at 19 km/h.
The occurrence
As the train was descending towards Pangela at 24 km/h, the front right wheel (R4) failed on the 19th wagon (NHIH97081). This resulted in a loss of the interference fit[3] between the wheel and the axle, which allowed the wheel to move inwards along the axle. The point of derailment was at 360.829 km. The derailment of the R4 wheel caused the bogie to rotate and derail the other three wheels. The other paired wheel on the front wheelset (L4), rotated to the extent that it started rubbing against the electronically controlled pneumatic (ECP) [4] train line brake cable. The abrasion of the cable caused the three locomotives to lose power and return the locomotives to idle.
The derailed bogie continued causing damage to over 590 metres of track including 963 sleepers. Pieces of the wheel detached and were later found on the track (Figure 2).
Figure 2: Wheel pieces from failed wheel found in ballast
Source: ATSB
One significant piece of the failed wheel was not found. This piece was likely the first piece to break away and the reason it was not found may be that it occurred sometime in the days leading up to the derailment. It is possible this missing piece might have been the cause of the increase in impact readings in the lead-up to the derailment.
Figure 3: Damaged Electronically Controlled Pneumatic train line brake cable
During the derailment the L4 wheel rubbed against, and damaged, the ECP cable causing the train to come to a stop.
Source: ARTC
Post occurrence
The train came to a stand at 0618 with the front of the train standing at 359.878 km[5]. At 0622, the train crew contacted Australian Rail Track Corporation (ARTC) Network Control Centre North (NCCN) to inform them there was a problem with the ECP braking system. Shortly afterwards the train crew contacted the Downer Rail helpdesk[6] for advice and then contacted the Pacific National operations superintendent to discuss their next action. The operations superintendent directed they secure the train by applying all of the wagon handbrakes.
The driver remained in the cab of the leading locomotive while the second person climbed down from the cab, walked alongside the train and applied the handbrakes. Once this was completed the crew were advised by Downer Rail to verify all jumper couplings were connected. The second person completed this task and on the return from the rear of the train he noticed all wheels of a bogie had derailed (Figure 4 and 5). He informed the driver of the derailment using his handheld radio. At 0810, the driver informed the operations superintendent that the train had derailed.
Figure 4: Derailed bogie viewed from Down side of track
This shows the derailed bogie and the failed wheel that movedoff the wheel seat towards the centre of the axle.
Source: ARTC
Figure 5: Derailed bogie viewed from Up side of track
This shows the derailed bogie at the point the train came to a stand.
Source: ARTC
Following the notification of the derailment, the Pacific National operations superintendent notified the Pacific National operations supervisor at Werris Creek and requested that he proceed to the derailment site to conduct drug and alcohol testing of the train crew. He then notified Downer Rail’s maintenance depot manager and requested that a representative from Downer also attend the derailment site for recovery purposes.
At 0829, an ARTC Network Controller called the driver to get an update on the ECP fault. He was advised that the train had derailed.
At 0915, a Pacific National supervisor, a Pacific National safety and environmental specialist and a Downer Rail production leader arrived on site to coordinate the recovery operations. The ARTC issued a condition affecting the network (CAN) notice at 0928, and a track occupancy authority (TOA) to provide track safety protection at 0941.
During the site inspection, pieces of the failed wheel were found in the ballast. One piece was found 7 m forward of the point of derailment on the Down side of the track. Another piece from the wheel, brake blocks and brake block keys were found inside the four foot. An unsuccessful search was conducted for another large piece from the failed wheel.
The train crew were breath tested at 1128 with a negative result. This was over five hours after the incident. At 1140 the train crew were relieved of their duties.
Because of the derailment the sleepers that sustained damage were replaced and on 31 August 2015, the TOA was fulfilled, ending the track safety protection.
The incident occurred at 360.829 km on the Main North line at Pangela between Ardglen and Pages River (Figure 6). The train was travelling towards Port Waratah.
Figure 6: Location of incident at Pangela
This map shows the incident location and the path of MB520 (shown in red).
Source: Geoscience Australia
Train information
MB520, was a loaded coal service operating from the Maules Creek coal mine to Port Waratah. The train consisted of three locomotives TT 119, 9307 and TT 1 and 82 fully loaded coal wagons (Figure 7). The train was 1326 m long and had a trailing load of 9840 tonnes.
The wagons in the consist were a hopper type wagon with a 30 tonne axle load. Eighty wagons were NHIH-type wagons[7] and two were RHDH-type wagons. The wagon involved in the derailment was NHIH97081. Programmed maintenance was last carried out on the wagons on 29 June 2015, this is when the wagons would last have undergone a static inspection.
The leading ‘A’ end bogie that derailed was a NECA71 three piece bogie, last overhauled by Bradken, a maintenance contractor, in October 2013.
The wheelset (1G6S981356) was reprofiled (a skim taken off the tread face of each wheel) and installed under the wagon on 18 September 2014; it had since travelled 122,300 km. It was estimated to have travelled over 2 million kilometres since manufacture.
The failed wheel (R4) was manufactured by Comsteel in 1998 as part of the wheelset supplied to Pacific National. The wheel discs were the original discs as fitted to the axle. The serial number of failed wheel was not able to be determined due to derailment damage, but the paired wheel (L4) had a serial number 98 9891 CSC S6479. The maintenance records show that the other wheelset on the same bogie was removed on 23 October 2014 when a bearing replacement was made.
A data logger analysis was conducted on each of the train’s locomotives. The investigation concluded that the management of the train from Maules Creek did not contribute to the derailment.
Figure 7: Loaded coal wagons
This photo shows a number of loaded NHIH-type wagons in the consist of MB520.
Source: ARTC
Train crew
The train was crewed by a driver and a second person based at Port Waratah. The train crew were appropriately qualified and held the required route qualifications.
Track information
The section of track where the derailment occurred was a single, bi-directional, standard gauge railway line. Signals were controlled by ARTC Network Control Centre North at Broadmeadow. The track configuration was ARTC Class 1XC standard heavy haul track with 60kg/m rail directly fastened to concrete sleepers with resilient Pandrol type clips on a ballast formation and capable of 30 tonne axle load operations.
The section of track where the leading bogie of vehicle NHIH 97081T derailed was a steep 1 in 38 falling grade with a relatively tight 240 m radius. The posted track speed was 55 km/h.
The track was controlled and maintained by ARTC. ARTC controls the interstate rail network plus the NSW Hunter Valley rail network as part of a 60-year lease with the NSW State government from September 2004.
The Main North line starts from Sydney extends north towards the Queensland border. It once was the main line to Queensland but is now closed beyond Armidale. The track carries both freight and passenger trains. Coal trains dominate, especially between Port Waratah and the coalfields of the Hunter Valley, Gunnedah and the Narrabri Basin.
According to a report commissioned by Pacific National, the track infrastructure was in a good serviceable condition prior to the derailment and ARTC had maintained the track according to the relevant standards.[8] The investigation found that track condition did not contribute to the derailment.
Related occurrences
In Australia, the number of derailments caused by wheel failure is difficult to determine due to the lack of coordinated data collection. An examination of NSW Independent Rail Safety Regulator’s annual rail safety industry reports from 2007 to 2012 did not mention any incidence of wheel failure in NSW over this period.
On 28 May 2011, there was a failure of a locomotive wheel on an intermodal freight train at Fisher in South Australia. The ATSB investigation found that a fatigue crack had initiated on the wheel rim and then radiated towards the flange and tread regions before the wheel completely failed. The rate of growth of the fatigue crack was influenced by high in-service mechanical loading of the wheel. The initiating mechanism was different from the one in this investigation.
Pacific National wheel incidents
Significantly, there have been seven reports of fractures detected in wheels, within a 3-year period, on Pacific National coal wagons in NSW (Figure 8). Three occurred over a 27-month period before this incident and three within a two-month period afterwards. None of the other incidents resulted in derailment. Five of the defects were found during maintenance, some detected inadvertently when the maintenance team were carrying out other work.
Figure 8: Pacific National wheel incidents
This table summarises the recent spate of Pacific National wheel fractures
No.
INCIDENT DATE
WAGON TYPE
YEAR MADE
RIM THICKNESS
LOCATION DETECTED
HOW DETECTED
1
31/05/13
NHRH
2000
23.9 mm
METFORD
WAYSIDE
2
25/08/13
NHRH
1995
22 mm
GRETA
MAINTENANCE
3
04/11/14
NHRH
1995
23 mm
GRETA
MAINTENANCE
4
28/08/15
NHIH
1998
25 mm
PANGELA
DERAILMENT
5
25/09/15
NHRH
1998
26 mm
GRETA
MAINTENANCE
6
06/10/15
NHRH
2002
23 mm
GRETA
MAINTENANCE
7
07/10/15
NHRH
2002
24 mm
GRETA
MAINTENANCE
Source: Pacific National
The first in this series of incidents occurred at Metford. According to Pacific National, this incident was the first identified wheel failure since the installation of wheel condition monitoring in 1998.[9] On 31 May 2013, a high wheel impact load reading of 465kN was recorded by the ARTC Wheel Impact Load Detector (WILD) wayside equipment at Metford.[10] The train was subsequently stopped at Greta where the train crew inspected it. A partial wheel failure of the wheel tread was identified on a wheel on the 16th wagon. The WILD readings for this wheel had escalated during the weeks leading up to the final reading. The readings over the preceding weeks were: 200kN on 3 May, 236kN on 26 May, 246kN on 28 May, 317kN on 30 May, and 338kN again on 30 May. Interestingly, on 30 May, a senior Pacific National employee reported a possible skidded wheel or spalling defect due to pass-by noise at Greta. A roll-by examination was conducted at low speed at Maitland but no defect was detected. A further roll-by examination at the Newcastle Hunter Bulk terminal did not detect the defect. It was not until 1410 the following day, 31 May 2013, that the high WILD reading caused the train controller to contact the train crew and advise them to conduct an immediate technical examination.
The wheelset was three weeks short of its final scheduled maintenance, when it was to be removed from service. After metallurgical analysis, an investigation found that the failure mechanism was the result of a transverse thermal crack at the edge of the rim front face and tread. This failure is typically caused by excessive localised thermal load into the wheel tread and is generally associated with braking issues on the wagon. This was in combination with the wheel rim having a diminished thickness due to the wheel approaching its condemning diameter. Higher temperatures will occur during braking when the wheel is worn than when it is new. There will also be a tendency to sustain these higher temperatures due to the reduced capacity of the wheel rim to dissipate this heat.
The second incident occurred on 25 August 2013 during non-scheduled maintenance when maintenance staff, while grinding arrisses, incidentally detected a significant wheel fracture on a coal wagon. The defect was identified as a shattered rim where fatigue had initiated at a sub-surface position and then propagated through the wheel’s outer rim (see Appendix A for wheel terms). The cause of the fatigue defect was not identified.
Two days prior, on 23 August 2013, the WILD reading changed from a normal reading of 110 kN to 166 kN. On 25 August it increased to 207kN, well below the 250 kN limit where any preventative action commences. The Pacific National report into the incident made a number of observations about this incident and the previous May incident. It said that while the failure mechanism was different they had a number of similarities, both:
were in the fully worn, minimum rim thickness condition
had fatigue cracking initiating from a sub-surface condition
were fitted to NHRH wagons, operating in the same environment
It stated that they had instituted a program to monitor WILD readings on wheels with a low rim thickness and remove any that see an increase in WILD readings. It was found that 6.5% of wheelsets have a rim thickness of 25 mm or less when returned to the maintainer. It recommended an increase in the minimum rim thickness on 30 t axle load wagons to 28 mm on wheel turning. It stated that a minimum rim thickness of 28 mm will ensure that the stresses in the wheel are lower than that experienced in the two wheel failures (31 May and 25 August 2013). There is no evidence that any of the recommended measures were put in place by Pacific National prior to the derailment on 28 August 2015.
On 4 November 2014, a third wheel fracture was detected during scheduled maintenance, at the maintenance facility at Greta, here a crack was found on the tread and the front inner rim of the wheel. Subsequently a metallurgical examination by Comsteel found numerous cracks at the tread/front face location around the entire circumference of the wheel. The metallurgical report concluded that the thermal cracking[12] may have been initiated by a misaligned brake block causing a high thermal load. The report stated, ‘The rim thickness of the wheel was almost at condemning level. This very thin rim section leads to lower rim rigidity and, hence, higher propensity to unusual thermal distortion and would be a significant contributing factor into thermal crack formation and propagation.’[13]
The fourth incident at Pangela on 28 August 2015, which resulted in a derailment, is the subject of this report.
The fifth incident detected on 25 September 2015 during maintenance found two large cracks and numerous smaller cracks on the circumference of the wheel. A metallurgical analysis of this wheel determined the cracks were typical of thermal cracking likely caused by excessive braking or a brake block overhanging the rim edge.
The sixth incident was detected on 6 October 2015, again during unit train maintenance. Eight large cracks and numerous smaller cracks were found on the circumference of the wheel. No metallurgical analysis was conducted on this wheel.
The seventh incident detected on 7 October 2015 was from the same wagon as the previous incident but from a different bogie. A series of rim edge thermal cracks were found during a targeted maintenance inspection.
The actions taken by the rail operator and the rail regulator as a result of the first three incidents are discussed in the safety analysis section of this report.
Australian Rail Track Corporation (2014) Wheel Impact Load Detection (WILD) Alarm Specifications, Version 2.0, 12 March 2014, p.4.
Comsteel (2015) Evaluation of Pacific National W69 NHRH Coal wagon wheel cracking in the wheel rim. p.13.
Comsteel (2015) Customer Quality Report No. 83582 Examination of Pacific National NHIH97081, Wheel Fracture Wheel set 1G6S981356. p.16.
Liu. X, Saat. M.R, Barkan. C.P.L. (2012) Analysis of Causes of Major Train Derailment and Their Effect on Accident Rates. US Department of Transportation Region V Regional University Transportation Center Final Report.
Mutton and Laczko. (2006) Metallurgical and NDT aspects in the management overheated railway wheels. MATe05 paper.
Office of National Rail Safety Regulator (2014). An emerging issue ‘shattered rim’ wheel defects. Safety Bulletin No. 3 June 2014.
Pacific National (2009). Management of Wayside Condition Monitoring Systems, Wagon Maintenance Manual. 01-12_02.
Pacific National Coal Asset Management (2013) Technical Report Class 5 fractured wheel NHRH50361 31 May 2013. p.5.
Pacific National Asset & Infrastructure services (2014) Incident 73495 Shattered wheel report NHRH 50264J 25 August 2013. p.13.
Pacific National (2015) Operational Risk Register ORR NSW-Vic.
Pacific National (2014) System Safety Accident Investigation into Failure of Wheel Disc No.41574 Class 5 Wheel Fracture Fitted to Wagon NHRH50361M IRS No. 70337 at Greta, date of Incident Friday, 31st May 2013. p.6.
Rail Industry Safety and Standards Board (2010) Glossary of Rail Terminology – Guideline.
Robinson Rail (2015) Derailment of Coal Train ID MB520 Report on condition of track infrastructure.
U.S. Department of Transportation Federal Railroad Administration (2014) Broken Rims in Railroad Wheels.
Submissions
Under Part 4, Division 2 (Investigation Reports), Section 26 of the Transport Safety Investigation Act 2003 (the Act), the Australian Transport Safety Bureau (ATSB) may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. Section 26 (1) (a) of the Act allows a person receiving a draft report to make submissions to the ATSB about the draft report. A draft of this report was provided to:
Australian Rail Track Corporation
Office of the National Rail Safety Regulator
Pacific National
Submissions were received from the Australian Rail Track Corporation, the Office of National Safety Regulator, and Pacific National. The submissions were reviewed and where considered appropriate, the text of the report was amended accordingly.
Findings
From the evidence available, the following findings are made with respect to the derailment of freight train MB520 that occurred at Pangela, New South Wales on 28 August 2015. These findings should not be read as apportioning blame or liability to any particular organisation or individual.
Safety issues, or system problems, are highlighted in bold to emphasise their importance. A safety issue is an event or condition that increases safety risk and (a) can reasonably be regarded as having the potential to adversely affect the safety of future operations, and (b) is a characteristic of an organisation or a system, rather than a characteristic of a specific individual, or characteristic of an operating environment at a specific point in time..
Contributing factors
The derailment occurred due to a wheel failure on the right hand wheel (R4) fitted to the leading axle of the leading bogie of vehicle NHIH97081. This led to the wheel moving off the wheel seat towards the centre of the axle. The bogie then rotated causing the other wheels on the bogie to derail.
The wheel failed due to the initiation and propagation of a transverse thermal crack in the wheel tread.
The wheel inspection processes and systems were not effective in detecting surface damage or cracks on the R4 wheel on wagon NHIH97081 prior to the wheel failure. [Safety Issue]
Despite a number of incidents, Pacific National didnot take adequate action, before the derailment, to reduce the risk of wheel defects, especially in light of previously identified contributors such as low rim thickness.[Safety Issue]
Other findings
The rollingstock operator used the data from Wheel Impact Load Detectors as an indirect method to detect wheel surface damage and cracks. However, despite increasing impact level readings during the previous month, no immediate action was taken to inspect the R4 wheel before it eventually failed.
Metallurgical tests of the failed wheel R4 and the paired wheel L4 found they complied withspecificationrequirements.
Purpose of safety investigations & publishing information
Purpose of safety investigations
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
Ownership of intellectual property rights in this publication
Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia.
Creative Commons licence
With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence.
Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work.
The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau
Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.
On 5 July 2015, the pilot of a Cessna 182 aircraft, registered VH-AHC, conducted a local flight from a private airstrip about 100 km south-west of Bourke, New South Wales. The aircraft took off towards the west. After a flight of about 15 to 20 minutes, the aircraft returned overhead the airstrip. Based on the indicated wind, the pilot elected to conduct an approach to land towards the south.
When on final approach to land, at about 5 ft above ground level, the aircraft sank rapidly. The aircraft landed heavily and the nose wheel detached from the aircraft. The aircraft then bounced into the air, touched down for a second time, and dug into soft ground. The aircraft flipped over and came to rest inverted, resulting in substantial damage (Figure 1). The pilot and two passengers sustained minor injuries.
Pilot comments
The pilot reported that the property had received about 100 mm of rain over a period of 2 weeks, which had stopped about 7 days prior to the incident. Cold weather in the intervening period had prevented the soil from drying. Prior to taking off, the pilot had driven over the runway surface and assessed the surface to be suitable for landing. However, below the runway surface, there was a soft layer of earth. This layer extended about 500 mm down and was not evident during the runway inspection.
The pilot was unsure what caused the aircraft to sink faster than usual. The wind was light and variable. The additional sink and high rate of descent combined with the soft surface led to the aircraft landing gear digging in and flipping the aircraft over.
Figure 1: Accident site of Cessna 182, VH-AHC
Source: Aircraft owner
Safety message
This incident highlights the importance of the identification and management of risks associated with unsealed airfields. Potential hazards such as changes in the runway surface following rain can be hard to detect. Changes in the runway surface can adversely affect the outcome of a hard landing.
The ATSB report regarding a similar incident is available on the ATSB website at AO-2015-038.
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
Ownership of intellectual property rights in this publication
Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia.
Creative Commons licence
With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence.
Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work.
The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau
Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.
On 15 August 2015, the student pilot of a Cessna 172 aircraft, registered VH-MJK (MJK) conducted a solo flight from Emkaytee aeroplane landing area (ALA) to Bathurst Island Airport, Northern Territory (Figure 1). There the student pilot completed touch-and-go circuits for about 30 minutes on runway 15.
Figure 1: Image showing Bathurst Island, Darwin and Emkaytee airports
Source: Google earth – annotated by the ATSB
At about 1210 Central Standard Time (CST), a Cessna 404 aircraft, registered VH-ANM (ANM) and operated by Hardy Aviation, departed from Darwin Airport, Northern Territory, on a scheduled flight to Bathurst Island, with a pilot and five passengers on board. The pilot broadcast when inbound and about 15 NM from Bathurst Island Airport on the common traffic advisory frequency (CTAF) of 126.5 MHz, and did not receive any response. At about 1220, the aircraft joined on the downwind leg of the circuit for runway 15 at 1,000 ft above ground level and broadcast joining the circuit. As the aircraft turned onto base, the pilot sighted MJK also on base, at the same height, closer to the runway and estimated it was about 150 m away (Figure 2).
The pilot of ANM immediately manoeuvred the aircraft to the west to increase separation between the two aircraft. After unsuccessfully trying to contact the pilot of MJK on the CTAF, the pilot of ANM briefly selected frequency 126.7 MHz to try to communicate with the pilot of MJK, but again did not receive a response. The pilot of ANM observed MJK conduct a touch-and-go, and kept that aircraft in sight, while overflying and re-joining the circuit on the crosswind leg.
After the touch-and-go, when upwind of the runway at about 500 ft above ground level, the pilot of MJK sighted ANM. ANM was then to the left, above MJK at 1,000 ft, and turning onto the downwind leg. The pilot of MJK then saw that the radio was selected to frequency 126.6 MHz. The pilot checked their flight plan, noted that the correct frequency was 126.5, and immediately changed the radio to that frequency. The pilot of MJK then broadcast a departure call on the CTAF. The pilot of ANM then contacted the pilot of MJK, who advised that the radio had been on the wrong frequency.
Figure 2: Bathurst Island Airport showing approximate aircraft tracks and relative positions
Source: Google earth – annotated by the ATSB
The pilot of ANM continued the approach, and landed at Bathurst Island, and MJK returned to Emkaytee without further incident.
The radar data provided to the ATSB by Darwin air traffic control, indicated the aircraft came within about 100 ft vertically and 0.6 NM at the closest proximity (Figure 3).
Source: Department of Defence – annotated by the ATSB
Safety action
Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce their safety risk. The ATSB has been advised of the following proactive safety action in response to this occurrence.
Operator of VH-ANM
As a result of this occurrence, the operator of VH-ANM advised the ATSB that they have taken following safety action:
Notice to company pilots
The Chief Pilot distributed a notice to all company pilots advising them of the incident. The notice stated that the Tiwi Islands continue to be a hot spot for traffic, and reminded pilots to be ‘doubly aware’ when operating in the area.
Safety message
The pilot of MJK commented that there were three important learnings from this incident:
crosscheck the selected frequency against the flight planning notes
ensure the selector reaches the detent when selecting a radio frequency
listen for the ‘beep-back’ response from the CTAF to verify the correct frequency has been selected.
An aerodrome frequency response unit (AFRU) identifies correct radio frequency selection at non-towered aerodromes. The AFRU automatically responds to a transmission on the CTAF either with a pre-recorded voice message, if no transmission has been received in the previous five minutes, or with a beep-back.
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
Ownership of intellectual property rights in this publication
Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia.
Creative Commons licence
With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence.
Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work.
The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau
Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.
On 16 August 2014, an Airbus Helicopters AS350B2 Squirrel, registered ZK-HYO, collided with terrain at Mt Alta in the New Zealand Southern Alps. One of the seven occupants of the helicopter was fatally injured and the aircraft destroyed.
The New Zealand Transport Accident Investigation Commission (TAIC) opened an inquiry into the accident on 16 August 2014 (TAIC investigation AO-2014-005). As part of that investigation TAIC recovered the helicopter’s engine for later technical examination. Subsequently, TAIC arranged for the engine examination to be carried out at the engine manufacturer’s facility in Sydney, New South Wales.
On 21 August 2015, TAIC requested the ATSB to appoint an accredited representative in accordance with Annex 13 to the Convention on International Civil Aviation Aircraft Accident and Incident Investigation. The ATSB appointed an accredited representative that day with the task of providing independent oversight of the TAIC engine examination.
To facilitate this support, the ATSB initiated an external investigation under the provisions of the Transport Safety Investigation Act 2003. The ATSB forwarded its report of the engine examination to TAIC on 29 October 2015. This completed the ATSB’s support of the TAIC investigation.
TAIC is responsible for, and will administer the release of the final investigation report into this accident. Any enquiries regarding the TAIC investigation should, in the first instance, be directed to the:
Deputy Chief Investigator of Accidents Transport Accident Investigation Commission Level 16, AXA Center 80 The Terrace PO Box 10-323, Wellington New Zealand
On 16 August 2015, the pilot of a Beech A36 aircraft, registered VH-PAK (PAK), conducted a private flight from Pacific Haven to Southport, Queensland. The pilot reported that the aircraft engine ran normally throughout the cruise. At about 0945 Eastern Standard Time, the pilot joined the circuit at Southport Airport, with the intention to land on runway 19.
About 800 m from the runway threshold, at about 150 ft above ground level, the aircraft’s engine stopped. The pilot broadcast a Mayday[1] and conducted a forced landing (Figure 1). The aircraft collided with trees, resulting in substantial damage and the pilot was not injured (Figure 2).
Figure 1: Southport Airport and accident location
Source: Google earth – annotated by the ATSB
Pilot comments
The pilot provided the following comments:
The pre-flight inspection was normal.
Prior to the flight, there were 10 quarts of oil in the engine, with 12 quarts full oil capacity, and the aircraft operable to a minimum of 6 quarts.
The engine had been running well during the flight.
The pilot was ferrying the aircraft to Southport for its scheduled annual inspection.
Due to the relatively short runway, the pilot conducted a shallow approach, to reduce the landing distance required.
When the engine stopped, as the flaps and landing gear were extended, the aircraft sank quickly.
The aircraft was fitted with an engine analyser (see Engine data section), which would generate a message to the GPS unit if a fuel tank was low on fuel. The pilot did not receive any fuel warnings.
Figure 2: VH-PAK at accident site
Source: Queensland Police
Engine data
The aircraft’s engine data was reviewed for the flight. Figure 3 shows the recorded exhaust gas temperatures (EGT) and fuel flow for the flight. During the cruise, the fuel/air mixture was leaned to achieve a fairly constant EGT around 1,500 ° Fahrenheit (816 °C). Towards the end of the flight, as the aircraft descended, the EGT decreased. About 2 minutes before the engine stopped, the EGT climbed rapidly. The peak EGT, of about 1,552 °F (844 °C), occurred as the engine stopped (Figure 3). Simultaneously, the fuel flow dropped to zero and the engine rpm increased rapidly. This is indicative of an overly lean fuel/air mixture, and is consistent with fuel starvation.
Figure 3: Engine data
Source: Provided to the ATSB
Post-accident inspection
The engine appeared intact, with all connections also intact. There was no evidence of oil loss from the engine.
During the post-accident inspection, the engineer found that only a few drops of fuel remained in the fuel control unit. The engineer drained the right main fuel tank and airframe fuel filter bowl. Less than 2 L of fuel drained from the right fuel tank, and less than 20 ml from the fuel filter bowl. The right main fuel tank was selected in the cockpit at the time of the accident.
The tip tanks and left main fuel tank were ruptured as a result of the collision, and no fuel remained in them. However, there was no evidence of fuel contamination at the accident site, and only a slight smell of fuel.
Aircraft fuel status
The aircraft had a total fuel capacity of 432 L. It had two main fuel tanks, each capable of holding 140 L of usable fuel. The aircraft was also fitted with two wing-tip tanks, each with the capacity to carry 76 L of fuel, all of which was usable.
The pilot provided a fuel receipt, which showed 389 L of fuel had been purchased on 1 July 2015 at Bundaberg. There was no evidence recorded on the maintenance release to show that the aircraft had been in Bundaberg that day. The pilot had recorded four flights from that date, including the incident flight, totalling 3 hours and 55 minutes flight time with four take-offs and three landings (not including the accident).
The pilot reported that the fuel consumption rate used for planning was 60 L per hour for the cruise, and 80 L per hour for take-off and the first hour. The pilot also stated that the fuel gauges indicated that about half fuel remained at the time of the accident.
ATSB comment
The pilot reported that about half of the aircraft’s fuel capacity remained at the time of the accident. However, the subsequent inspection found a total of about 2 L of fuel remained in the intact (and selected) right main fuel tank. The recovered engine data indicated that fuel starvation occurred at the time the engine stopped. The ATSB could not resolve the difference between the pilot’s account and the fuel state found after the accident.
Safety message
The pilot commented that if the aircraft had been higher on final approach, it would have been able to glide to the runway.
The ATSB research publication Starved and exhausted: Fuel management aviation accidents, states that accurate fuel management starts with knowing exactly how much fuel is on board at the commencement of each flight. It also relies on a method of knowing how much fuel the aircraft consumes. The likelihood of fuel starvation is reduced by adhering to procedures, maintaining a record of the fuel tank selections during flight and ensuring appropriate tank selections, particularly for take-off and landing.
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
Ownership of intellectual property rights in this publication
Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia.
Creative Commons licence
With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence.
Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work.
The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau
Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.
On 3 August 2015 at about 0410 Eastern Standard Time (EST), a Cessna 441 aircraft, registered VH-EQU, departed Scone Airport for a positioning flight to Illawarra Regional Airport (Wollongong Airport), New South Wales. The flight was to be conducted as a multi-crew operation with the pilot flying (PF) occupying the left seat, and the pilot monitoring (PM) occupying the right seat.
Prior to departure, the pilots conducted a pre-flight briefing noting that the weather forecast for the Wollongong area included a warning for severe turbulence below 8,000 ft. The departure and cruise were uneventful. After commencing descent into Wollongong, the PF briefed the PM for an area navigation (RNAV) instrument approach and landing to runway 16 (Figure 1). The approach was planned to be hand flown in darkness under clear skies with a westerly wind of 25–30 kt, reducing to a light breeze on the ground.
The aircraft arrived overhead position WOLND (Figure 1) at about 5,000 ft on descent to 3,700 ft. Due to the strong westerly wind, flying conditions were turbulent and the PF reported that it was quite difficult keeping the aircraft’s wings level. Approaching WOLNI, the PF levelled the aircraft and maintained 3,700 ft as planned. The aircraft was then slowed down to permit the extension of flap and landing gear.
Figure 1: Cessna 441, VH-EQU
Source: Aircraft owner, modified by the ATSB
In accordance with the approach, the PF recommenced descent at 2.2 NM prior to position WOLNF. Due to the steeper than normal approach path angle (3.7 degrees as opposed to the usual 3.0 degrees), the PF reported establishing a descent rate of about 1,000 feet per minute (fpm). The PM reported that they were expecting a descent rate of about 900 fpm. Air traffic control surveillance data showed the descent rate established was somewhere between 1,000 and 1,500 fpm.
Shortly before WOLNF and as the aircraft approached 2,700 ft, the PM called ‘two thousand seven hundred’ with the expectation that the PF would maintain that altitude until passing WOLNF. However, the descent continued and a few seconds later, the terrain alerting and warning system (TAWS)[1] generated visual and aural ‘CAUTION-TERRAIN’ alerts. These alerts were generated as the system detected that the flight path was projected to come within 300 ft of an obstacle or terrain.
Figure 2: Wollongong RNAV(GNSS) approach to RWY 16 showing approximate flight path profile (green/red dotted line) based on air traffic control surveillance data
Source: Airservices Australia, modified by the ATSB
Pilot flying comment
The PF’s company duties included aircraft charter pilot and flight instructor. The PF’s flying experience included 230 hours operating Cessna 441 aircraft (16 hours in the last 90 days) and 6,930 total flying hours.
The PF recalled working hard trying to keep the aircraft’s wings level, which may have distracted them from monitoring the descent profile adequately. The PF recalled thinking that they may have been slightly low on profile approaching WOLNF, but was startled when the terrain warning sounded. The PF had not participated in any TAWS training, even though they had flown the aircraft (fitted with TAWS equipment) for the previous 12 months.
Pilot monitoring comment
The PM company duties included aircraft charter pilot and flight instructor. The PM’s flying experience included 350 hours operating Cessna 441 aircraft and 3,400 total flying hours.
The PM believed that the when the TAWS alert activated, the aircraft was at an altitude somewhere between 2,600 ft and 2,500 ft and about 0.5 NM before WOLNF. The time between being below 2,700 ft, having the TWAS alert and an appropriate response was less than 10 seconds.
The PM reported having completed TAWS training with a previous employer. The PM believed that following the TAWS warning, they should have applied full power, climbed at maximum rate and conduct a missed approach in accordance with the operator’s procedures.
Fatigue management
Prior to the flight, both pilots had completed 2 days free of duty. They reported being well rested and obtaining about 8 hours sleep each night.
On the morning of the flight, the pilots reported waking up around 0300. They advised that they generally found it counterproductive to try and get to sleep early and therefore only received about 5 hours of sleep.
Despite the limited amount of sleep, both pilots reported being alert during all stages of the flight.
ATSB comment
The aircraft was fitted with a terrain awareness and warning system (TAWS) designed to reduce the risk of flight into terrain. While a TAWS was not required for the particular type of operation, the lack of TAWS training probably explains the nature of the PFs response to the terrain alert.
Pilots and operators of aircraft are encouraged to be appropriately trained in the use of all equipment fitted to their aircraft.
Safety action
Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce their safety risk. The ATSB has been advised of the following proactive safety action in response to this occurrence.
Aircraft operator
As a result of this occurrence, the aircraft operator has advised the ATSB that all pilots now receive TAWS training.
Safety message
Due to the early start combined with inadequate sleep the night before, despite reporting otherwise, the pilots were likely fatigued. Pilots need to be aware that obtaining less than 7-8 hours sleep a night increases the risk of operating with a level of fatigue known to have a demonstrated effect on performance.
Pilots also need to consider the effect flying in the early hours of the morning has on their performance. Their body is in a circadian low period which could lead to periods of impaired alertness and delayed reaction times.
The ATSB continues to be concerned about flight below minimum descent altitudes. When conducting an approach in instrument meteorological conditions, which includes darkness, pilots must ensure the approach is conducted in accordance with the prescribed procedure. In this case, the pilots were required to manage the vertical profile to ensure the aircraft remained at or above the minimum descent altitude, thereby ensuring clearance from terrain and obstacles.
Descent below the minimum descent altitude compromises the minimum terrain clearance requirements and increases the risk of controlled flight into terrain (CFIT).
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
Ownership of intellectual property rights in this publication
Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia.
Creative Commons licence
With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence.
Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work.
The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau
Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.
On 8 August 2015 at about 0930 Western Standard Time (WST), a Cessna 208B, registered VHLNH, departed Kununurra, Western Australia, on a scenic flight to the Bungle Bungle Range. On board were the pilot and 12 passengers. The weather was fine and clear, with an easterly wind blowing at about 13 knots. The pilot conducted all normal pre-flight procedures, including an engine runup to confirm that the engine and other systems were functioning normally.
The flight departed from runway 12 and made a right turn to the south to follow the standard Kununurra to Bungles departure route. After levelling at about 700 ft above mean sea level (about 550 ft above ground level), the pilot reduced engine power and propeller RPM for a level segment of the departure. The pilot then commenced a commentary for the benefit of the passengers.
Soon after the pilot commenced the commentary, when the aircraft was about 1.5 NM south of the airport, engine oil appeared on the windscreen. The pilot turned back towards the airport but oil on the windscreen soon turned into an opaque film, substantially limiting forward visibility. Additionally, a number of engine instrument indications were abnormal and white smoke began to enter the cabin through cockpit vents. Although the smoke dissipated quickly, the pilot continued to monitor for any signs of a fire in the engine compartment.
The pilot made a MAYDAY[1] call on the Kununurra CTAF,[2] broadcasting that the engine had failed and that the aircraft was returning to land on runway 12. The pilot of another aircraft operating in the area at the time heard the MAYDAY call and relayed relevant information to Air Traffic Control (ATC) (located in Brisbane), who initiated an emergency response.
At around the same time that the pilot made the MAYDAY call, they also attempted to ascertain the extent of the engine malfunction. Advancing the power lever produced a change in engine sound and an increase in engine torque. Although the propeller was turning, there was no audible response and no apparent change in propeller RPM when the pilot advanced the propeller lever. The pilot also noted that the engine low oil pressure light was illuminated, and deduced that the propeller had probably feathered as a consequence of engine oil loss.
The pilot found that the engine/propeller were capable of providing a small amount of positive thrust (but not sufficient to maintain level flight), so established the aircraft in descent, holding a speed of about 85 to 95 knots. The pilot elected to leave the flaps set at 10 degrees, which was the same position that was set at the time the engine problem first became apparent. The pilot assessed that the aircraft could reach runway 12, but with little margin.
Another aircraft operated by the same company as the aircraft involved in the incident was about to depart Kununurra, and had lined up on runway 12. Upon hearing the MAYDAY call, the pilot of that aircraft vacated the runway. The incident pilot advised the pilot in the other aircraft (vacating the runway) of the nature of their predicament, particularly the extent to which forward visibility was affected, and sought their assistance. The incident pilot also advised the Airport Safety Officer on the CTAF that assistance may be required after landing,[3] and advised the passengers that an emergency landing at Kununurra was required.
With forward visibility substantially limited, the pilot maintained orientation using ground based navigation aids, GPS information and the view of landmarks from the side windows. The dark shape of a nearby waterway to the south of the airport was evident, but forward visibility was so poor that the pilot was unable to effectively identify roadways or cleared areas through the windscreen.
With some directional guidance from the pilot of the aircraft that had vacated the runway, and by reference to navigational instruments, the incident pilot was able to position the aircraft on final approach to runway 12. The turn onto final approach was made through about 100 degrees, at around 150 ft above ground level, and required about 50 degrees angle of bank. The pilot elected to leave the flap setting at 10 degrees throughout the approach and for the landing. The aircraft landed firmly, with the pilot unable to effectively judge flare height due to the restricted visibility.
After landing, the pilot found that forward visibility was obscured to the extent that they were unable to taxi safely. The Airport Safety Officer moved alongside the aircraft in a vehicle to guide the pilot to the next taxiway exit. When safely clear of the runway, the pilot contacted ATC and cancelled the MAYDAY.
The time from take-off to landing was about 2 minutes and 15 seconds. From the time the pilot broadcast a MAYDAY call to the time of landing was about 1 minute and 20 seconds. At its furthest point, the aircraft was about 2.3 NM south of the airport.
After exiting the runway and coming to a stop, the pilot completed the engine shut-down checks. No reaction was felt or heard when the pilot selected the propeller lever to the feather position as part of the shutdown procedure (normally, there was an audible change as the pitch of the propeller changed). After exiting the aircraft, the pilot found that the propeller was in the feather position, and had seized. The extent of oil loss was also apparent, with oil smeared over much of the forward fuselage, particularly the engine cowls and windscreen (Figure 1).
Figure 1: Photographs of VH-LNH after the aircraft had landed, with oil visible on the engine cowl and forward fuselage
Source: Aircraft operator
Aside from the apparent engine and propeller system problem, there was no damage to the aircraft and the pilot and passengers were uninjured.
Subsequent on-site inspection of the engine by engineering staff revealed that an oil leak had developed at a join between the oil transfer elbow and oil transfer tube at the forward end of the engine. This assembly was part of a larger assembly that transferred oil under pressure to the reduction gearbox at the front of the engine. One of the oil transfer elbow mounting lugs (that secured the elbow to engine assembly) was found to be fractured, allowing the oil transfer elbow to move and unseat the O-ring between the elbow and the tube (Figures 2 and 3).
Figure 2: Photograph (in situ) showing fractured oil transfer elbow mounting lug and the unseated O-ring seal
Source: Aircraft operator (annotations by the ATSB)
Figure 3: Oil transfer elbow assembly
Source: Engine manufacturer (annotations by the ATSB)
Technical follow-up
Following the incident, the operator submitted a Service Difficulty Report (SDR) to CASA. The engine was removed from the aircraft by the operator and dispatched to a repair and overhaul facility for further examination. The oil transfer elbow (with cracked mounting lug) was removed and sent to the engine manufacturer for detailed analysis. The results of those examinations are summarised in the following paragraphs.
Engine teardown
The engine teardown did not reveal any unrelated engine abnormalities that might have contributed to the oil transfer elbow mounting lug fracture. Among other things, the report noted that there was little or no engine oil remaining in the engine and that the reduction gearbox could only be turned with considerable force (more than the amount of force that would normally be required). The report also noted that the propeller shaft would not rotate.
Inspection of the engine confirmed that an oil transfer elbow mounting lug had fractured and the report noted that the ‘hardware holding it in place was loose’. The lug was found to have completely broken through, and it was noted that a segment of the lug was missing (Figure 4). The report also noted that the O-ring (see Figures 2 and 3) had moved.
Figure 4: Fractured lug (the figure on the right shows that a segment of the lug was missing)
Source: Pacific Turbine (left) and CASA (right)
Mounting lug fracture analysis
The manufacturer’s examination of the fractured oil transfer elbow mounting lug and associated securing assembly allowed them to draw a number of conclusions, including:
The fracture of the lug occurred due to fatigue originating from multiple locations on the side of the mounting lug that meets with the face of the mounting flange. The fatigue mode was found to be high-cycle, under unidirectional bending.
Wear and imprint marks on the securing assembly (bolt, nut and washer) and the fractured lug suggested that the bolt was sitting askew in the bore of the flange to which the elbow mounting lug was secured. The report commented that this wear pattern was consistent with a lack of pre-load on the bolt.
The lack of pre-load on the bolt that secured the mounting lug to the flange was considered to be the initiating factor that led to fracture of the mounting lug. The reason for the lack of preload on the bolt could not be ascertained.
Impact marks were identified on the oil transfer elbow, which may have been caused by interference from tooling used to conduct unrelated maintenance in the vicinity of the oil transfer elbow. This damage was found to be superficial which suggested that it played no part in fracture of the mounting lug.
CASA comments
At the time that this report was prepared, CASA was continuing to consider relevant information, including the manufacturer’s fracture analysis report. Notwithstanding their ongoing consideration of relevant information, CASA did not believe that there was any conclusive evidence of a lack of pre-load on the bolt. CASA commented to the effect that the typical signs of an incorrectly torqued fastener were not evident, and noted that the separated portion of the lug remained in place after the lug had fractured – that was unlikely if there was a lack of pre-load on the bolt. CASA also commented to the effect that incorrect installation of the oil transfer elbow (mounting lug), rather than incorrect installation of the mounting lug bolt, may have been the origin of the problem. CASA reported that this is a scenario that is known to have occurred in practise.
Engine teardown facility comments
Staff from the engine teardown facility suggested that the following may assist in preventing similar occurrences:
Paint-free lug and flange mating surfaces: Paint on the elbow mounting lug and/or the mating face of the flange to which the lug is secured may wear with working and vibration. This wear has the potential to loosen the fastener and amplify the effects of vibration on lug security.
Lug strengthening: The construction of the elbow transfer tube is such that the mounting lug itself is substantially narrower than the stem leading to the lug (see Figure 4). Added thickness/material depth would strengthen the lug.
Expanded and mandated inspection: Mandating inspection of the elbow mounting lug (see CASA Airworthiness Bulletin 72-004 below), and including oil transfer elbow mounting lug inspection as a specific task during relevant engine servicings.
Related background information
Similar occurrences
ATSB investigations AO-2010-005, AO-2008-005 and AO-2010-003 identified the failure of the same oil transfer elbow fitting that fractured in the case of this occurrence. In two cases (AO2008005 and AO-2010-005), the investigations found that oil transfer elbow mounting lug failures were a consequence of other unrelated engine problems (compressor turbine blade failures). In one case (AO-2010-003), both oil transfer elbow mounting lugs were fractured, but the cause of those fractures could not be conclusively determined.
Advice from the engine manufacturer suggested that there have been a number of oil transfer elbow mounting lug failures reported over the past 15 years, including those investigated by the ATSB. In most cases, either a vibration source or mechanically induced damage was found to be a contributor. The manufacturer identified the possibility that tooling could interfere with the elbow fitting during removal and installation of nearby engine components.
Possible sources of oil transfer elbow damage – manufacturer comments
During the course of the investigation (but aside from the specific nature of the mounting lug fracture in this case), the engine manufacturer highlighted possible sources of oil transfer elbow damage that could ultimately lead to mounting lug fracture. While these comments may not be specifically relevant in this case, they are noteworthy and warrant the attention of organisations involved in the logistics of transporting and/or maintenance of PT6A114A and similar engines:[4]
Pratt and Whitney Canada Service Information Letter. In 2008, the manufacturer issued a Service Information Letter (SIL Gen PT6A026 applicable to all PT6A engines) to remind operators of the precautions to be taken when installing, removing and performing maintenance on external engine tubes, lines and fittings. The letter pointed out that:
Investigations of incidents in service (including in-flight shut downs) have determined that mishandling tubes and fittings during normal maintenance activities can subject these components to distortion or stresses beyond normal utilisation, leading to fracture during subsequent operation.
Reduction gearbox chip detector removal/installation. The engine manufacturer commented that the proximity of the oil cooler and reduction gearbox magnetic chip detector may be a factor if tooling used to install or remove those components is allowed to interfere with the oil transfer elbow, and induce mechanical stress on the mounting lug. To that end, the reduction gearbox maintenance manual (in the areas dealing with removal and installation of the magnetic chip detector) included the following caution:
AVOID FORCING OR CONTACT WITH THE OIL PRESSURE TRANSFER ELBOW WHEN YOU REMOVE THE CHIP DETECTOR. DAMAGE TO THE ELBOW MOUNTING LUGS CAN OCCUR RESULTING IN LOSS OF ELBOW RETENTION AND OIL LEAKAGE.
CASA Airworthiness Bulletin 72-004
In 2010, CASA released an Airworthiness Bulletin (AWB) AWB 72004 which identified a possible link between the incorrect installation of an engine mount bracket assembly (vibration isolator), and fatigue cracking in the oil transfer elbow fitting. The AWB identified the possibility that fatigue failure of the oil transfer elbow fitting may be linked to unusual vibration caused by incorrect installation of the engine upper vibration isolator. The AWB went on to recommend that operators inspect the relevant vibration isolators for correct installation, and inspect oil transfer elbow fittings for any signs of cracking.
Safety action
Whether or not the ATSB identifies safety issues in the course of an investigation, relevant organisations may proactively initiate safety action in order to reduce safety risk. The ATSB understands that the following safety actions are being considered in response to this occurrence.
Engine manufacturer
After reviewing the circumstances surrounding this incident (and noting concerns that the oil transfer elbow may be interfered with or damaged during unrelated engine maintenance in the area of the elbow), the engine manufacturer is considering adding information in relevant publications and issuing a Service Information Letter. Publication amendments are likely to draw attention to the need to refer to relevant instructions if the oil transfer elbow is removed/replaced during fuel nozzle replacement. The Service Information Letter is likely to provide information and warnings with respect to the possibility of damage to the oil transfer elbow during engine maintenance. The Service Information Letter is also likely to include information and warnings related to removal and installation (or other disturbance) of the oil transfer elbow.
Civil Aviation Safety Authority
At the time this report was written, CASA was continuing to consider relevant information, which may lead to a re-issue of AWB-72004.
Operator
Prior to this incident, the operator was already in the practice of periodically inspecting the area of the fractured lug (although those inspections did not alert the operator to the problem in this case), In response to this incident, the operator intends to expand the periodic inspection to include removal of the bolt and an inspection of the internal radius of the mounting lug for signs of crack development.
Safety message
The manner in which the pilot handled a very difficult set of circumstances provides some positive examples for other pilots to consider.
The pilot maintained positive control of the aircraft, despite the challenging circumstances. The adage ‘aviate-navigate-communicate’ continues to prove a fundamentally effective prioritisation guide for pilots.
The pilot used available resources including other pilots, ATC, and the Airport Safety Officer, to assist in dealing with the circumstances. The combined efforts of those involved clearly contributed to a favourable outcome. The principles of effective crew resource management extend to all operations, including single pilot operations.
The pilot conducted a pre-flight emergency self-briefing. Even though it may be impractical to consider all possible emergency scenarios during a pre-flight emergency self-briefing, having a general plan in mind may be important, particularly when confronted with a time-critical and stressful situation.
The incident highlights also the importance of care and attention to detail when conducting maintenance on aircraft engines and accessories. This is particularly important where critical components are known to be susceptible to damage through interference from tooling or mishandling. Manufacturers, regulatory authorities, operators and aircraft maintenance organisations all have fundamentally important roles to play in maintenance of the highest practicable standards of airworthiness.
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
Ownership of intellectual property rights in this publication
Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia.
Creative Commons licence
With the exception of the Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this publication is licensed under a Creative Commons Attribution 3.0 Australia licence.
Creative Commons Attribution 3.0 Australia Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided that you attribute the work.
The ATSB’s preference is that you attribute this publication (and any material sourced from it) using the following wording: Source: Australian Transport Safety Bureau
Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.
On 7 January 2015, a Pacific Aerospace Limited 750XL aeroplane, registered ZK-SDT and involved in commercial skydiving operations, departed Taupo aerodrome, New Zealand, with the pilot and 12 passengers on board. On climb through about 2,000 ft above ground level after take off, the single Pratt & Whitney Canada PT6A-34 turboprop engine failed suddenly. The six tandem pairs of skydivers and the pilot parachuted from the aeroplane, landing without injury. The unoccupied aeroplane crashed into the lake.
On 17 July 2015, the New Zealand Transport Accident Investigation Commission (TAIC) requested Australian Transport Safety Bureau (ATSB) assistance with a number of Australian aspects to their investigation. In accordance with paragraph 5.23 of Annex 13 to the Convention on International Civil Aviation Aircraft Accident and Incident Investigation, the ATSB appointed an accredited representative (ATSB investigator) to the TAIC investigation. The investigator assisted TAIC with its engine examination as part of an External Investigation under the provisions of the Australian Transport Safety Investigation Act 2003.
The ATSB has finalised its support of this investigation. TAIC is responsible for the release of the final investigation report into this accident. Any enquiries in respect of the ongoing TAIC investigation or release of the investigation report should, in the first instance, be directed to the:
Transport Accident Investigation Commission Level 16 80 The Terrace PO Box 10-323 Wellington, 6143 New Zealand
________________
The information contained in this web update is released in accordance with section 25 of the Transport Safety Investigation Act 2003.
On the evening of 9 May 2015, a Boeing B737-8FE aircraft, registered VH-YID and operated by Virgin Australia Airlines Pty. Ltd. (Virgin), was on a scheduled passenger service from Sydney, New South Wales to Adelaide, South Australia. During a high-speed descent, the crew responded to aircraft indications that they were approaching airspeeds greater than desired by extending the speed brakes. While the speed brakes were still extended, the airspeed continued to increase towards the aircraft’s maximum speed, the result of which would have been an overspeed. In an effort to prevent an overspeed, the first officer overrode the autopilot by pulling back on the control column until the autopilot entered a secondary mode known as control wheel steering‑pitch mode. This was followed immediately by an abrupt release of the control column, after which one cabin crew member sustained a minor injury.
What the ATSB found
The ATSB found that the crew selected a descent speed of 320 kt, which they routinely used for air traffic control-initiated high-speed descents. However, the increased risk of an overspeed in changing wind conditions had not been adequately considered by the crew.
The ATSB also found that, whereas Virgin’s training included a focus on the management of overspeeds, the crew had not yet completed this training. This increased the risk that the guidance provided through other sources would not be followed correctly.
Additionally, the flight crew had initiated the cabin preparation for landing earlier than usual due to the expectation of turbulence later in the descent. This likely reduced the risk of more serious injury to the cabin crew as they were in the final stages of securing the cabin than had they commenced preparations for landing at the normal time.
What's been done as a result
Prior to this occurrence, Virgin had implemented improved crew training and guidance on managing overspeeds. This included the addition of a cyclic simulator training session that focused on overspeed management on descent.
Safety message
This occurrence highlights the increased risk of overspeed when conducting high-speed descents in conditions of varying winds and any associated turbulence. Identifying and discussing the risks associated with high-speed descent increases the likelihood that crew will select a lower descent speed and/or consider the best way to deal with an impending aircraft overspeed before the descent is initiated.
Safety analysis
High-speed descent and potential overspeed
As part of their consideration of the effect of the ‘fairly strong’ winds on the approach, the flight crew discussed the possible effect of turbulence on cabin safety. Similarly, the crew considered the aircraft’s turbulence penetration speed when accepting the high-speed descent. However, the captain recalled that whilst they were aware of changing wind conditions, there was no forecast of severe turbulence, and therefore a descent at 320 kt was considered by the crew to be appropriate. Although likely influenced by the routine use of 320 kt for high-speed descents, the descent at that speed, when the maximum certified limiting speed was 340 kt, increased the risk of an overspeed.
In considering the influences on crew decision making, Orasanu (2010) stated:
What constitutes an appropriate course of action depends on the affordances of the situation. Sometimes a single response is prescribed in company manuals or procedures. At other times, multiple options exist from which one must be selected.
Poor decisions may…arise when a flight crew is aware of conditions that require a decision, but underestimates the level of risk associated with the conditions…Another arises from pilots’ routine experience. If similar…situations have been encountered in the past and a particular course of action has succeeded, the crew will expect to succeed the next time with the same response.
Likewise, Sitkin (1992) as cited in Orasanu (2010) stated that uniformly positive experiences provide no baseline by which to determine when a situation is becoming more dangerous.
The crew reported that selecting 320 kt for a high-speed descent was routine, indicating that this course of action was expected to be successful. Therefore, the likelihood that the crew would consider the risks of an overspeed in this case were harder to identify.
The Flight Safety Foundation (2014) recommended that the pilot monitoring role should include monitoring the aircraft’s flight path and immediately bringing any concern to the pilot flying’s attention. In this case, the captain was monitoring the aircraft’s speed, before focussing on the first officer (FO) as he extended the speed brake and then other operational tasks associated with the descent. Whilst this precluded the captain’s ability to detect the FO’s reaction to an increasing speed trend vector, it was reasonable that the captain felt that the situation was under control.
Orasanu (2010) outlined that the development of expertise contributes to decision making in different ways. This included the development of ‘stored condition-action patterns’, where decision makers interpret a cue pattern as being of a particular type and match it with an action according to a routine (Klein, 1989 and 1993 cited in Orasanu 2010).
In this case, the FO identified an immediate need to prevent an overspeed, and did so by pulling back on the control column and activating control wheel steering-pitch (CWS-P). This had previously been successful for the FO in addressing an impending overspeed, but the difference in this case was that the force required was larger than experienced by the FO in past situations. The flight data recorded a 41 lb back pressure on the FO’s control column with a resulting 2.14 g loading on reversion to CWS‑P. Given the altitude at the time, reversion to CWS-P was considered contrary to Virgin Australia Pty Ltd’s (Virgin) guidance and training in overspeed management. However, the use of the CWS-P mode was reported common among some pilots and, in this case, had possibly become a stored condition-action pattern.
Surprise is a cognitive-emotional response to something unexpected. It results from a mismatch between the individual’s mental expectations and what actually happens around them. Experiencing surprise is a combination of physiological, cognitive and behavioural responses (Rivera and others 2014). If a pilot is not expecting things to go wrong, then the level of surprise can result in taking no action, or the wrong action (Martin 2012).
In this case, the FO reported feeling a ‘pinch’ when CWS-P activated. This, combined with feeling a high g loading, led the FO to abruptly release the amount of back pressure 1 second after its application.
A combination of a sudden increase, followed by a sudden decrease in g loading would have first pushed the occupants of the aircraft towards the floor, followed shortly after by a feeling of weightlessness. This would have increased the difficulty of moving around the cabin and the risk of injury.
Flight management computer data entry procedures
In this occurrence the QNH variation, temperature deviations and descent winds were insufficient to contribute to an inaccurate VNAV PATH construction. In addition, at the time of the occurrence there was no procedural requirement after pre-flight to enter QNH and temperature deviation data, nor to update descent wind data in the descent forecast page of the flight management computer. There was also no guidance on the benefits of entering that data into the computer to produce a more accurate calculated vertical flight path. In some circumstances, the use of pre-flight data would reduce the accuracy of the calculated vertical flight path and result in increased crew workload in managing the energy state of the aircraft.
Training in the management of overspeeds
At the time of the occurrence, Virgin’s Flight Crew Training Manual and other guidance material included information on how to effectively handle an impending overspeed. However, the simulator training session in which crews practiced overspeed management (including on descent) had not been completed by the crew. This simulator session was subsequently completed by all Virgin B737 flight crew, and additional guidance has also been provided.
Early preparation of the cabin for landing
The procedures for preparing the cabin for landing required their commencement no later than FL 200. In this case, the flight crew identified that turbulence on descent may increase the risk of injury to crew and passengers and therefore decided to initiate the cabin preparation at about FL 270. Flight data for the descent showed that this provided an additional 2 minutes before the occurrence for the crew to secure galley equipment and prepare the cabin, including seating all passengers. This reduced the risk of more serious and numerous injuries to the cabin crew and passengers.
Findings
From the evidence available, the following findings are made with respect to the flight path management occurrence involving a Virgin Australia Airlines Pty. Ltd. B737-8FE, registered VH‑YID, on descent into Adelaide Airport, South Australia on 9 May 2015. These findings should not be read as apportioning blame or liability to any particular organisation or individual.
Contributing factors
Although routinely used for high-speed descents, the selected speed for the descent of 320 kt was close the aircraft’s maximum speed of 340 kt and, in combination with the increased headwind as the aircraft descended through about 8,000 ft, increased the risk of an overspeed.
After identifying the unexpected rapid increase in airspeed, the first officer applied sufficient back pressure on the control column to override the autopilot with the intention of avoiding the impending overspeed. This increased the g loading on the aircraft to an extent where safely walking in the cabin would be difficult.
In response to feeling the increased g loading, the first officer abruptly released the back pressure on the control column, resulting in a sudden pitch change and reduced g loading that led to the cabin crew member losing their footing and sustaining a knee injury.
Other factors that increased risk
The Virgin Australia Airlines Pty. Ltd. procedures did not require flight crew to update the QNH, temperature deviation or the descent winds in the flight management computer after departure. In some circumstances, this would reduce the accuracy of the calculated flight path and increase crew workload in managing the aircraft’s energy state.
At the time of the occurrence, the Virgin Australia Airlines Pty. Ltd. training included a focus on the management of overspeeds. However, the associated simulator training session had not been completed by the crew, increasing the risk that they would not react in accordance with the published operational guidance material.
Other findings
The earlier-than-usual preparation of the cabin for landing meant that, at the time of the occurrence, the cabin crew were in the final stages of that activity. This reduced the risk of more serious and numerous injuries to the cabin crew and passengers.
The occurrence
On the evening of 9 May 2015, a Boeing B737-8FE aircraft, registered VH-YID and operated by Virgin Australia Airlines Pty. Ltd. (Virgin) as ‘Velocity 436’, was on a scheduled passenger service from Sydney, New South Wales to Adelaide, South Australia. The first officer (FO) was the pilot flying.[1]
At about 1915 Central Standard Time[2], the flight crew conducted an approach briefing in preparation for the descent. The crew discussed the ‘fairly strong’ westerly wind forecast in the area that would affect their descent, and decided they would instruct the cabin crew to prepare the cabin for landing at about flight level (FL) 270[3]. This was reported slightly earlier than usual and was intended to reduce the risk of turbulence-related injuries. Soon after, air traffic control (ATC) requested the crew to conduct a high-speed descent into Adelaide, which the crew accepted. The FO changed the planned descent speed from 280 kt to 320 kt, which was reported by the crew to be routinely used during ATC-initiated high-speed descents.
The descent was commenced about 4 minutes later and, passing about FL 270, the captain made the public address ‘cabin crew prepare for landing’. This was the cue for the cabin crew to commence the cabin preparation procedure by securing all loose cabin equipment, ensuring passengers were in their seats with seatbelts fastened and then securing themselves in their jump seats. Seven minutes later the flight crew switched the seatbelt signs on, as a cue to the cabin crew to finalise this procedure and take their seats.
During the descent the FO controlled the aircraft’s vertical profile using various autopilot descent modes. Passing through FL 250, the FO selected the VNAV PATH mode (see the section titled VNAV PATH descent mode).
As the aircraft passed through 10,000 ft, the airspeed started to increase above 320 kt and at 8,400 ft the message ‘drag required’ displayed on the flight management computer (FMC) scratchpad as per system design. In response, the FO extended the speed brake, making an effort to do so slowly and smoothly. The captain observed this action, initially concerned that the FO may extend the speed brake too quickly. However, the captain was satisfied that the speed brake was appropriately extended and that the increasing airspeed trend was being managed. The captain’s attention was then turned to other tasks. Over the next 6 seconds the airspeed continued to increase, and the FO recalled seeing the speed trend vector on the primary flight display (PFD) extend beyond the aircraft’s maximum certified speed (VMO).[4]
In an effort to avoid an overspeed, the FO pulled back forcibly on the control column in order to raise the nose, overriding the autopilot and activating the control wheel steering – pitch (CWS-P). This technique was routinely used to manage overspeeds on descent. On this occasion the FO reported feeling greater than usual resistance when raising the nose due to the already low pitch angle. The FO then recalled feeling a ‘pinch’ as the autopilot reverted to CWS-P, along with a sudden pitch change and a high g loading[5] on the aircraft. In response, the FO abruptly released the back pressure on the control column, rapidly unloading the g loading. At that moment, the cabin crew had almost completed securing the cabin and were about to take their seats. They reported experiencing what they thought to be turbulence, and two cabin crew in the rear galley lost their footing. This resulted in one of the cabin crew impacting the galley floor heavily, sustaining an injury to their knee.
The FO stowed the speed brake, selected the LVL CHG (level change) descent mode and reduced the selected speed to 250 kt. The captain checked the status of the cabin crew and was informed of the injury to the crew member.
The sequence of events during the descent is highlighted in Figure 1.
Figure 1: Sequence of events on descent, including the flight and cabin crew actions plotted against altitude and time
Source: ATSB
Personnel information
The flight crew signed on for duty in Sydney at 0900 Eastern Standard Time[6] and had been on duty for 11 hours at the time of the occurrence.
The captain held an Air Transport Pilot (Aeroplane) Licence and had a total flying experience of 13,500 hours, of which about 8,750 were on the B737 aircraft. The captain commenced flying with Virgin in 2003 and held a valid Class 1 Aviation Medical Certificate.
The FO held an Air Transport Pilot (Aeroplane) Licence and had a total flying experience of about 7,500 hours, of which about 5,000 were on the B737 aircraft. The FO commenced flying with Virgin in 2007 and held a valid Class 1 Aviation Medical Certificate. Although the FO reported feeling tired at the time of the occurrence, there was no additional evidence to indicate a level of fatigue known to affect performance beyond what would reasonably be experienced at the end of the day.
Aircraft information
VNAVPATH descent mode
VNAV PATH is an automated descent mode primarily designed to keep the aircraft on a predetermined descent path using aircraft pitch. During descents in this mode, airspeed is controlled using thrust or displaying messages to the flight crew to increase drag as required. The VNAV PATH mode allows for minor airspeed excursions while prioritising keeping the aircraft on path. If airspeed increases to 10 kt above the selected speed, the message ‘drag required’ appears on the FMC scratchpad. In response, crew extend the speed brake to arrest further speed increases.
Control wheel steering – pitch (CWS-P)
Pushing a CWS engage switch on the mode control panel engages the autopilot (A/P) pitch and roll axes in the CWS mode. At the time of the occurrence, CWS‑P could also be engaged by applying 40 lb back pressure to the control column to manually override the A/P. By doing so, a desired pitch attitude could be set. The ability to manually engage CWS‑P and override the A/P was later removed from the flight control computer’s software to avoid inadvertent and undetected reversion from a descent mode to CWS‑P. With CWS‑P engaged, the A/P manoeuvres the aircraft in response to control pressures applied by either pilot. The control pressure is similar to that required for manual flight. When control pressure is released, the A/P holds the existing attitude.
While the FO’s routine use of CWS-P as a means to manage overspeeds was not part of Virgin training, it was reported used by the FO and other pilots. This was because it was an effective way to handle an overspeed on descent without disconnecting the A/P, particularly when changing weather conditions meant that an overspeed could occur with little time for the crew to react.
Maximum operating indicated airspeed
The maximum operating indicated airspeed (VMO) represents the maximum certified limiting speed of the aircraft. VMO for the B737-8FE is 340 kt. An overspeed warning sounds if the aircraft exceeds VMO.
Airspeed trend vector on the primary flight display
The primary flight display (PFD) presents a dynamic colour display of the parameters necessary for flight path control. This includes (Figure 2):
the manually-selected airspeed
a green speed trend vector, which indicates predicted airspeed in 10 seconds time
the VMO.
Figure 2: B737 PFD, showing the manually-selected airspeed (number 1), the green speed trend vector (number 2) and VMO (number 4). Note VMO gradually reduces below 340kt with increasing altitude above approximately FL250. In this graphic, the representation shows the aircraft at FL380 (38,000 ft), explaining the reduced VMO
Source: B737 Flight Crew Operations Manual and Wikipedia, modified by the ATSB
Effect of g loading
G loading refers to the aircraft occupant experiencing an apparent acceleration equal to the load factor times the acceleration due to gravity. For example, when the pilot performs a pull up with a load factor of two, loose objects fall to the floor at twice the normal acceleration of gravity. A g loading less than one results in feelings of reduced weight and, eventually, loose objects floating upwards once the g loading reduces below zero.
Meteorological information
The forecast meteorological information that was available to the crew included:
an Aerodrome Forecast for arrival into Adelaide that predicted a light south‑westerly wind and showers of rain with intermittent periods of less than 30 minutes of south-westerly winds at 20 kt, gusting up to 30 kt
winds from 264 °M at 54 kt at FL 350, from 250 °M at 38 kt at FL 250, from 258°M at 40 kt at FL 150 and from 270° at 41 kt at 5,000ft.
Recorded data
The flight data was examined to derive the aircraft’s computed airspeed (CAS) and the wind affecting the descent as they pertain to the crew’s actions. This examination showed the following:
By 10,468 ft in the descent the aircraft reached the selected descent speed of 320 kt. Over the next 30 seconds, as the aircraft descended to 8,136 ft (when the speed brake was extended) the headwind increased from 30 to 39 kt, and the airspeed increased from 320 to 333 kt.
Over the next 5 seconds, as the aircraft descended from 8,136 ft to 7,790 ft, the headwind increased from 39 to 48 kt, and the airspeed increased from 333 to a peak of 339 kt. At this point the back pressure on the FO’s control column was recorded as 41.5 lb, and CWS-P engaged with a peak g loading of 2.14 g.
One second later the back pressure on the FO’s control column reduced to 2 lb, consistent with a release of the back pressure, with a minimum g loading of 0.066.
The speed brake was stowed about 10 seconds later at 7,492 ft.
Training and guidance in overspeed management
The following overspeed management training and guidance was provided to flight crew:
B737-NGFlight Crew Training Manual content. ‘Crews routinely climbing or descending in windshear conditions may wish to consider a 5 to 10 knot reduction in climb or descent speeds to reduce overspeed occurrences…During climb or descent, if VNAV or LVL CHG pitch control is not correcting the overspeed satisfactorily, switching to the V/S [vertical speed] mode temporarily may be helpful in controlling speed.’ The cruise section of the manual also stated that ‘there have been reports of passenger injuries due to over‑controlling the airplane during high altitude, high airspeed flight when overriding the control column with the autopilot engaged…’.
High- and low-speed briefing document(valid from February to July 2015). ‘High speed descents need to be managed carefully when operating near areas of steep wind gradients or expected turbulence…Reducing speed prior to entering areas of large wind change is the best technique...It is preferable to accept a temporary overspeed (provided it is not excessive or sustained) rather than a breach of altitude or large abrupt control inputs at high altitude...’. The document also recommended ‘the momentary use of V/S if other modes are not correcting the overspeed.’ and that ‘CWS P may be required but must be applied smoothly with small inputs at high altitude. Speed brake use is also recommended to assist recovery in the cruise.’
Recurrent simulator training program on overspeed events. Guidance to crew for the simulator session on overspeed events stated that ‘use of VNAV PATH with high descent speeds…is not recommended in turbulent conditions or areas of steep wind velocity changes’. This simulator session had not been completed by either flight crew member at the time of the occurrence.
Procedures for FMC updates – wind data, QNH and temperature deviations
The B737 Flight Management Computer System Reference Manual (2015) stated that‘the descent forecasts page [on the FMC] enables the pilot to enter descent wind data to more accurately define the descent path and allow for varying conditions’. It also outlined that ‘ISA DEV’ (the temperature deviation from the international standard atmosphere standard (ISA)[8]) and QNH[9] can be manually entered. The ability to enter QNH and temperature deviation is designed to increase the accuracy of the VNAV PATH construction by the FMC, as it allows the FMC to compensate for deviations from ISA QNH and temperature values at sea level.
Virgin’s Flight Crew Operating Manual documented that during the pre-flight procedure, pilots enter ‘initial data’ and ‘navigation data’ into the FMC. This included entering three of the four available predicted descent winds from the flight plan. These four winds included at FL 350, FL 250 and FL 150 and 5,000 ft. The three selected winds were entered into the descent forecast page.
If an aircraft is fitted with an aircraft communications addressing and reporting system (ACARS), wind data can be automatically uploaded into the FMC once the flight plan has been loaded during pre-flight duties. This also facilitates receipt of updated descent wind information if requested. At the time of the occurrence, Virgin’s procedures did not include updating descent winds in flight or a requirement to enter the temperature deviation and QNH. The captain reported that if there is a ‘really strong’ head or tailwind at the top of descent, the crew may decide to amend the FMC top of descent point (either earlier or later) and use the VNAV SPD vertical mode[10].
In this occurrence, the flight plan wind information was generated 3 hours and 10 minutes before the scheduled arrival time. The aircraft was fitted with ACARS, however updated descent wind data was not requested and the temperature deviation and QNH were not entered before approaching Adelaide. This was not inconsistent with the then Virgin procedures.
Procedures for securing the cabin before landing
The procedures for securing the cabin on descent were documented in the Virgin Flight Crew Operating Manual and Standard Operating Procedures. The following is a summary of those procedures:
Not below FL 200, the pilot monitoring (PM) makes the public address ‘cabin crew prepare for landing.’ In response, the cabin crew commence the ‘galley secure’ actions including stowing all galley items, switching off equipment such as ovens and securing all latches. About 10 minutes is provided for cabin crew to complete these actions.
Around transition (10,000 ft), flight crew select the fasten seatbelt sign ON, which triggers cabin crew to complete the ‘cabin procedure’ actions. This includes ensuring that all passengers have their seatbelts fastened, their seat backs upright and that any cabin baggage is stowed. These actions take about 1 minute.
Related occurrences
Overspeed occurrences on descent
Between 2012 and 2015, 51 occurrence reports were submitted to the ATSB that involved an overspeed in a B737 aircraft on descent. Of these, two occurred during high-speed descents. Of the other occurrences, three reported the use of CWS-P This included investigation AO‑2012‑138 below.
The crew of a Boeing 737-800 aircraft was conducting a flight from Adelaide, South Australia to Canberra, Australian Capital Territory. Just prior to commencing descent, ATC cleared the aircraft to conduct a high-speed descent. The aircraft descended below the 7,000 ft altitude clearance limit and, after being alerted to this by ATC, the flight crew climbed the aircraft back to 7,000 ft and continued the approach to land. The ATSB found that the combination of auto-flight system mode changes (including inadvertent use of CWS‑P) and the management of the airspeed during the descent resulted in a high workload environment. This occurrence highlighted the need to continually monitor descent profiles and airspace limitations in relation to the aircraft’s position.
Purpose of safety investigations & publishing information
Purpose of safety investigations
The objective of a safety investigation is to enhance transport safety. This is done through:
identifying safety issues and facilitating safety action to address those issues
providing information about occurrences and their associated safety factors to facilitate learning within the transport industry.
It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action.
Terminology
An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue.
Publishing information
Released in accordance with section 25 of the Transport Safety Investigation Act 2003
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