FACTUAL INFORMATION

At 1435 Eastern Standard Time on 10 August 2004, a Boeing Company 717-200 aircraft, registered VH-VQA, was climbing to cruise altitude on a scheduled passenger service from Melbourne, Vic. to Hobart, Tas. with six crew and 52 passengers on board. As the aircraft passed through flight level (FL) 110, the crew heard a loud bang, with a corresponding increase in indicated left engine vibrations. The left engine began to spool down and the turbine gas temperature (TGT) indications began to increase significantly.

The crew initially brought the left engine power lever back to idle. However, the TGT continued to increase, indicating a maximum of 1,149^oC, before they shut the engine down and discharged a fire bottle into the cowling area in accordance with the operator's procedures. They then notified Melbourne air traffic control (ATC) of the engine failure and returned to Melbourne.

The operator examined the left engine and found metal fragments in the exhaust area and some metallisation¹ of the exhaust duct.

At the time of the failure, the BR700-715 engine, serial number 13148, had completed 10, 321 hours and 8,888 cycles since new, and 6,474 hours and 5,417 cycles since repair.

Engine investigation

The operator removed the engine and forwarded it to the engine manufacturer in Germany for detailed investigation, under the supervision of a representative of the German Federal Bureau of Aircraft Accident Investigation (BFU²), on behalf of the Australian Transport Safety Bureau (ATSB).

The manufacturer conducted a visual inspection of the engine's exterior, noting a bulge around most of the circumference of the high-pressure turbine (HPT) casing (Figure 1), in line with the Stage-1 HPT (HPT 1). A borescope examination of the engine interior showed that one HPT 1 blade was almost completely missing, with the remaining HPT 1 blades separated just above the blade platforms (Figure 2). There was also significant damage to the subsequent HPT and low-pressure turbine stages. Examination of the engine's compressor assembly revealed no significant damage. All of the high energy debris from the failure had been fully contained³.

A detailed examination of the engine revealed that the reason for the engine failure was the release of a single HPT 1 blade. The blade failed following the development of low-cycle fatigue⁴ (LCF) cracking in its internal cooling passages. All other engine damage was considered to be a consequence of the initial HPT 1 blade failure.

Figure 1: Bulged HPT casing

Figure 2: Damage to HPT 1 and HPT 2 rotors

Blade design considerations

The failed HPT 1 blade (Figure 3) was a life improvement package⁵ (LIP) blade. The blade was a shrouded-tip aerofoil design, with multi-passage internal cooling (Figure 4). There was a vapour aluminised coating on the blade's external aerodynamic surfaces and internal cooling passages.

The manufacturer indicated that there have been four similar failures of LIP HPT blades in the BR700-715 engine type, with another engine failure still under investigation. One failure occurred prior to this event in November 2003. The remainder occurred after this incident.

Figure 3: The failed HPT 1 blade (position 21)

Following those failures, the manufacturer conducted additional computer stress modelling on the LIP blades. That modelling found that there were stress levels in the larger trombone radius feature, within the blade's cooling passages (Figure 4) that were potentially in excess of the manufacturer's original design intent. The manufacturer also found that the thickness of the vapour aluminised coating inside the blade's internal cooling passages was variable and difficult to predict. In certain operational conditions, dependent upon high strains in areas of stress concentration and local temperature, the coating could crack with the possibility of subsequent growth into the coated (parent) material. The area from which the failure occurred was confirmed to be the most susceptible to this behaviour (Figure 5).

Figure 4: Intact HPT 1 blade (left); blade internal cooling passage showing trombone feature (right)

Figure 5: Computer generated stress diagram from the manufacturer indicating the point of potentially excessive stress and crack origin

Flight data recorder information

The ATSB's examination of the aircraft's flight data recorder (FDR) for the occurrence flight found that the left engine had surged as the aircraft passed through 10,240 ft. The engine pressure ratio (EPR) and engine rotational speed indications decreased abruptly, while the turbine gas temperature (TGT) for the engine began to increase. HPT vibration values for the engine increased from a level of 0.5 units before the failure to a maximum of 6.3 units over a three-second period. The manufacturer's high-limit for vibrations was 4.0 units.

The FDR readout indicated that the TGT for the engine continued to increase following the engine failure and remained at an indicated maximum of 1,149^oC for 1 minute and 46 seconds before decreasing (Figure 6). It is likely that the maximum TGT reached during the failure was higher than 1,149^oC, however the aircraft systems do not record above that temperature.

The FDR report indicated that there were no anomalies observed in the performance of the left engine prior to the failure.

Figure 6: FDR data plot of key engine parameters at the time of the failure

1 Metal pulverised by the turbine becomes molten and flows rearward attaching to the subsequent turbine and exhaust assemblies (US Department of the Air Force (1987). Safety Investigative Techniques (AF Pamphlet 127-1, Volume II. Washington DC: Author).
2 Bundesstelle für Flugunfalluntersuchung (BFU).
3 FAA AC 33-5, paragraph 5.c. definitions state '…Contained means that no fragments are released through the engine structure, but fragments may be ejected out of the engine air inlet or exhaust'.
4 Fatigue that occurs at relatively small numbers of cycles. Brooks, C. (1993). Metalurgical Failure Analysis. USA: McGraw-Hill, Inc.
5 The Life improvement Package 3 (LIP3) was a suite of HP Turbine modifications that included the HPT blade P/N BRH20351. The manufacturer introduced the package by SB-BR700-72-100801.

Sequence of events

On 26 July 2004, as the Boeing 717 (717), registered VH-VQA, was being prepared a scheduled passenger service between Brisbane and Hamilton Island Qld, the cabin crew noticed water spilling from the hand basin in the aircraft's aft left lavatory, and notified the flight crew. The leakage was cleaned up, and the aircraft was dispatched from Brisbane with the aft lavatory locked and placarded to prevent its use during the flight, in accordance with the provisions of the CASA-approved aircraft minimum equipment list.

During the cruise at flight level 320, when the aircraft was about 60 NM south of Mackay, the cabin crew noticed a faint `electrical smell' in the aft area of the passenger cabin. The cabin manager notified the flight crew, and about 1 minute later advised the flight crew that the aft cabin interphone hand piece located on the outside wall of the aft left lavatory was very hot, and that the smell was becoming stronger. Shortly after, the cabin manager notified the flight crew that the aft cabin interphone handset flexible-wiring loom was `melting'.

The flight crew conducted the aircraft quick reference handbook (QRH) `Electrical Smoke/Fumes of Unknown Origin' non-normal checklist. They both donned their oxygen masks, and selected emergency electrical power, as per the checklist. The 717 QRH non-normal checklist procedure dictated that a flight crew should plan to land at the nearest suitable airport. At that stage, the aircraft was about 30 NM south of Mackay. The flight crew broadcast a PAN¹ to air traffic control, and advised the controller that they required a clearance for an immediate descent and diversion to Mackay. The flight crew notified the cabin crew of their intentions, and the cabin crew prepared the passenger cabin for the landing at Mackay.

The controller in receipt of the PAN call notified the Mackay tower controller, who activated the aerodrome emergency plan and notified the emergency response services. The aircraft landed safely at Mackay about 9 minutes after the flight crew first notified air traffic control of the problem. After the aircraft landed, the flight crew confirmed with the aerodrome rescue and fire fighting service that there was no smoke at the rear of the aircraft. The flight crew also confirmed with the cabin manager that the electrical smell had abated and that the cabin interphone had cooled down. The aircraft was taxied to the terminal, and a normal disembarkation was conducted through the forward door of the aircraft. Eight of the occupants were treated for smoke/fumes inhalation after they had left the aircraft. There were no other reported injuries.

Following the occurrence, the flight crew reported to the operator that during the landing approach at Mackay, the landing gear `down' indication in the aircraft was different from that indicated in the 717 flight simulator during training sessions for flight on emergency electrical power. The operator conducted an investigation into the matter, and determined that the simulator landing gear `down' indication for flight under those conditions was incorrect. That was confirmed by the aircraft manufacturer.

During the subsequent engineering inspection of the aircraft, the B1-778 `Miscellaneous Cabin & Lavatory Occupied Aft' electrical circuit breaker was found to have popped, and could not be reset. The fault was traced to a short circuit in a connector plug located under the aft left lavatory hand basin. Water from the overflowing hand basin had seeped into the connector plug. The short circuit within the connector plug resulted in several pins within the plug becoming welded together. As a consequence, the aft cabin interphone handset flexible-wiring loom was overheated from a 115 VAC over-current within the loom and resulted in the in-flight `electrical smell' and overheating of the handset. The damaged components were replaced, and the aircraft was returned to service.

¹ PAN is a radio broadcast indicating uncertainty or alert. It is a general broadcast to the widest area, but not yet the level of a MAYDAY, which is the international broadcast for urgent assistance.

The Australian Transport Safety Bureau did not conduct an investigation into this occurrence. The report produced below is derived from an investigation conducted by the Department of Defence-Royal Australian Air Force (RAAF)¹.

Reported information

On 20 July 2004, at approximately 1514 central standard time, a de Havilland Dash 8-200 (Dash 8) was on a visual approach for a landing on runway 36 at Darwin and was operating under the instrument flight rules (IFR). At the same time, an Australian Army Bell 206B-1 (Kiowa) helicopter was departing Darwin and was operating under the visual flight rules (VFR). The helicopter crew had been cleared to depart the Darwin control zone via Wickham Point. Wickham Point was a visual fix located approximately 3.5 NM south of the threshold of runway 36. According to the RAAF report, the two aircraft came within 2.6 NM of each other when there was 500 ft vertically between them. There was an infringement of separation standards.

The RAAF, as the airspace administering authority, was responsible for the provision of air traffic control services at Darwin. Local 'low level helicopter release procedures' authorised the aerodrome controller (ADC) to depart VFR helicopters not above 1,000 ft, tracking direct to one of six visual fixes around the Darwin control zone, whilst separating the VFR helicopter from all traffic operating under the IFR.

According to those procedures the approach controller became aware of a departing helicopter once the aircraft taxied and the surface movement controller entered the aircraft details into the automated air traffic control system in use at Darwin. The approach controller would again be alerted to the departing helicopter when the radar detected the departing aircraft and the aircraft's track symbol appeared on the radar display. The approach controller was also required to advise the ADC of any inbound aircraft so that the ADC could separate a departing helicopter with other relevant aircraft.

The RAAF report found that:

• the team of controllers that were operating in the control tower at the time of the occurrence had been controlling for 2.5 hours, in high density and complex traffic levels, and may have been fatigued
• the controllers in both the tower and in approach had difficulties coordinating with each other due to the high traffic levels
• the approach controller did not receive notification that the Kiowa was taxying
• the approach controller coordinated the inbound Dash 8 with the ADC when that aircraft had left 2,000 ft on descent and the helicopter was level at 1,000 ft
• the approach controller became aware of the helicopter when that aircraft's track symbol appeared on his radar display.

¹ For further information on the RAAF report contact The Directorate of Flying Safety - Australian Defence Force (FS5), Campbell Park Offices, Canberra ACT 2600.

The Australian Transport Safety Bureau did not conduct an investigation into this occurrence. The report produced below is derived from an investigation report produced by the Department of Defence-Royal Australian Air Force (RAAF)1 which was the administering authority for the Darwin Control Zone.

Reported information

On 21 July 2004, at 0955 central standard time, a Cessna Aircraft Company 206 (C206) departed Darwin for Croker Island, NT. The aircraft was being operated under the visual flight rules (VFR). Another aircraft, a Cessna Aircraft Company 210 (C210) departed Darwin at 0956 for Snake Bay, NT, and was also being operated under the VFR. A third aircraft, a Cessna Aircraft Company 404 (C404) departed Darwin at 0958 for Croker Island. That aircraft was being operated under the instrument flight rules (IFR). All three aircraft were instructed by the aerodrome controller (ADC) to fly a heading of 060 degrees after take off.

The RAAF investigation report found that the pilot of the C404 had been instructed to sight and maintain visual separation with the C210. However, the C404 also caught up with and eventually overtook, the C206. Neither the pilot of the C206 nor the pilot of the C404 received traffic information on the other aircraft, and neither pilot was assigned the responsibility for separation with the other aircraft. According to the RAAF investigation report, the C404 passed within 0.2 NM of the C206 while both aircraft were at the same altitude. There was an infringement of separation standards.

The Manual of Air Traffic Services (MATS) section 4.5.1.4 authorised the assignment of responsibility for separation to the pilot of one aircraft if that pilot reported 'sighting the other aircraft and is instructed to maintain visual separation with, or to follow that aircraft'. The MATS Section 4.5.1.2 also stated that, when applying visual separation, 'controllers shall consider aircraft performance characteristics, particularly in relation to faster following aircraft'.

Darwin was operating on auto-release procedures at the time of the occurrence. The MATS Part 10, section 1 defined auto release as a procedure whereby the ADC must ensure that the spacing between successive departing aircraft is sufficient to enable the Departures controller to establish and maintain the required separation minima. In this occurrence, the responsibility for establishing and maintaining the required separation minima between the C210 and the C404 had been assigned to the pilot of the C404. The ADC did not ensure that the departures controller could establish and maintain separation between the C206 and the C404, and the responsibility for establishing and maintaining separation had not been assigned to either pilot.

1 For further information on the RAAF report contact The Directorate of Flying Safety - Australian Defence Force (FS5), Campbell Park Offices, Canberra ACT 2600.

1. The pilot conducted a low-level return transit to the replenishment point that was outside the pre-planned safe transit route.
2. The pilot did not see the powerline, or did not see the powerline in time to avoid a collision.

The damage to the right door frame corroborated the nature and distribution of the damaged perspex windscreen fragments along the wreckage trail and indicated that the helicopter struck the powerlines. The wirestrike was at a position consistent with that at which the property manager estimated he observed ‘glitter’ emanate from the helicopter.

It was likely that following the initial contact with the powerlines, the cockpit windscreen deformed sufficiently to capture the powerlines around the cockpit door hinge area. That would have prevented the powerlines from sliding up the windscreen towards the mast, or down towards the landing gear skids, before severing, and would explain the lack of any powerline damage or markings to the remainder of the helicopter.

The investigation considered whether a helicopter or associated system fault might have been implicated in the circumstances leading to the wirestrike. In that regard, there was no evidence of any technical or other failure of the helicopter or its associated systems prior to the ground impact, and the engine performed normally under test throughout the normal power range. That and the quality and quantity of the fuel remaining onboard the helicopter indicated that it was unlikely that any airframe, engine or system fault had contributed to the accident.

Risk management options for application during an aerial application task include reducing the consequence and/or likelihood of adverse events, such as the helicopter striking a powerline. Those options having the potential to decrease the adverse consequences of a wirestrike include:

• the use of helmets, as worn by the pilot
• if appropriate to the helicopter structure, the installation of wire strike protection systems
• the installation of advanced safety harnesses, such as that in the accident helicopter
• appropriate flight following and search and rescue procedures, as in this case due to the ongoing interaction between the pilot and loader/driver at the replenishment point.

However, as was the case in this accident, the consequence of a helicopter striking a powerline can generally be expected to be severe to catastrophic. As a result, a large investment is generally made by involved parties in order to decrease the likelihood, and therefore risk of a wirestrike. In this instance, that included adherence by the pilot, company operations manager and property manager to those regulatory and company requirements affecting the operation, and application by that group of the relevant content of the Aerial Application Pilot’s Manual to the task.

However, by electing to return to the replenishment point via a direct track, and not via the safe transit zone, the pilot unwittingly altered the context or parameters on which his pre-spray application risk assessment was based. The result was that none of the risk treatments applied to the spray operation and safe transit route were applied to the pilot’s approach to the replenishment point.

In that case, the pilot’s return transit was via an unsurveyed route. The dull surface of the powerlines, the nature of the vegetation and topographical background, the location of the supporting poles and long span of the powerlines would have reduced the likelihood that the pilot might have visually acquired the powerlines in sufficient time to have allowed him to avoid striking them.

The investigation was unable to quantify the effect that the installation of high visibility devices on the powerlines, which were not mandated in this case, might have had in preventing this accident. Similarly, the investigation was unable to assess the potential risk reduction capability of the possible installation in the helicopter of an advanced technology wire detection system.

While the reason for the pilot returning to the replenishment point via a direct track could not be determined, the investigation considered it most likely that the pilot had sought to optimise the efficiency of his operation. For example, if the pilot had exhausted the chemical supply at a position where he could observe the location of the replenishment point on the onboard global positioning system equipment, it might have been natural to ‘pop up’ the hill and fly direct to the truck, rather than conduct a long right turn and fly back through the valley. Had the pilot followed the pre-planned safe transit route, or included the direct route from the treatment area to the replenishment point in his risk assessment, he may not have collided with the powerlines.

CONCLUSIONS

Findings

1. The pilot was licensed and qualified for the flight in accordance with existing regulations.
2. The maintenance records indicated that the helicopter was equipped and maintained in accordance with existing regulations and approved procedures.
3. There was no evidence that engine or airframe failure or system malfunction influenced the circumstances of the occurrence.
4. The helicopter’s weight and balance were within prescribed limits.
5. There was no evidence that meteorological conditions influenced the circumstances of the occurrence.
6. The location of the powerlines was outside the safe transit zone between the designated spray area and the replenishment point
7. The powerlines were not marked, nor were they required by the relevant Australian Standards to be marked, with high visibility devices.
8. The position of the powerlines was not depicted on the relevant aeronautical or topographical charts.
9. The company Operations Manual and Aerial Application Pilot’s Manual included advice on the pre-flight planning, briefings and survey necessary before commencing aerial agricultural operations.

Sequence of events¹

On 19 July 2004, the owner-operator pilot of Bell Helicopter Company 47G-3B-1 Soloy² helicopter, registered VH-RTK, was contracted to spray herbicide on a property near Wodonga, Victoria.

Early in the day, the pilot and the company operations manager met with the property owner to discuss the proposed work. The discussion included the identification of known powerlines, other hazards, and sensitive areas likely to affect the operation. Shortly before the discussion, the operations manager showed the pilot the known primary powerlines in the general area where the spraying and loading was to be conducted.

Later that morning, the loader/driver of the chemical and re-fuelling truck arrived at the proposed replenishment point. The pilot positioned the helicopter at the replenishment point at about midday and commenced the final preparations for the day's spraying operations. The preparations included an aerial survey, in the company of the property manager, of the areas to be sprayed. During that flight, the property manager indicated to the pilot the areas that were free from powerlines, including a valley that he considered a safe transit zone between the replenishment truck and the proposed spray area.

The pilot commenced spraying operations on the occurrence property at about 1458 Eastern Summer Time. Shortly before 1512, the property manager heard the helicopter returning to the replenishment point from an unexpected direction and not from the direction of the identified safe transit zone.

The property manager reported that he observed some 'glitter' emanate from the helicopter, before it then descended and disappeared into heavily timbered terrain. No radio broadcasts were made by the pilot to air traffic services, or to the loader/driver to signify that an emergency had occurred.

The helicopter impacted terrain about 860 m to the south-west of the replenishment point, and was destroyed by impact forces. The pilot, who was the sole occupant, was fatally injured. There was no fire.

The loader/driver notified the company operations manager, then commenced to search for the helicopter. The company operations manager notified emergency services, and they arrived at the accident site about 20 minutes later.

A two-cable electric powerline was found severed at a point approximating that at which the property manager reported seeing 'glitter' come from the helicopter. There was no other property damage.

The powerlines struck by the helicopter during the approach to the replenishment point consisted of two parallel three-strand lightweight high-tensile steel cables, each of 2.75 mm diameter. The powerlines had been erected several years previously, and were located on the north-eastern side of a ridgeline, strung across the direct track from the last treatment area to the replenishment point.

The powerlines were not depicted on the relevant aeronautical or topographical charts. They were not connected to the power supply grid and had developed a dull oxidised finish. No high visibility devices were attached to the powerlines, and nor were they required to be.

Personnel information

The pilot was reported to have been fit, well and adequately rested prior to the flight, and he was appropriately qualified and endorsed for the agricultural operation in the helicopter.

Helicopter information

The investigation found that the helicopter was certified, equipped, and maintained in accordance with existing regulations and approved procedures. It was not, nor could it have been, fitted with a wire-strike protection system, nor was there any regulatory requirement for such a system.

No evidence was found of a defect in the helicopter or its systems that may have influenced the circumstances of the occurrence. The weight and balance of the helicopter were within prescribed limits.

The helicopter's perspex windscreen was clean and there was no pre-existing damage, or any obstruction that might have adversely affected visibility from the cockpit. Based on the available evidence, the aircraft was certified, equipped and maintained in accordance with the regulations and approved procedures.

Meteorological information

No evidence was found to suggest that the weather or environmental conditions influenced the circumstances of the occurrence.

Wreckage and impact information

The powerline that was struck lay approximately perpendicular to the helicopter's flightpath from the spray area to the replenishment point (see Figure 1). The powerline was supported by two poles, 669 m apart, which were located on heavily timbered terrain. The point at which the powerline was severed by the helicopter was estimated to be about 34 m above the tree tops, and 54 m above ground level.

Figure 1: Aerial view of powerline and approximate aircraft track

The wreckage distribution and damage to the surrounding trees was consistent with the helicopter impacting the ground with significant vertical, but low forward speed.

A trail of shattered perspex commenced in a position approximately underneath the point at which the powerlines had been severed, and extended for approximately 180 m to the point where the helicopter impacted the ground. The perspex corresponded to that used in the construction of the helicopter windscreen. No other helicopter components were found along the perspex trail.

The ground impact point and wreckage were located on steeply sloping and heavily wooded terrain that was covered with dense vegetation and large rocks (see Figure 2). The area was covered with small to medium sized trees, some approximately 12 to 20 m tall. The ground impact scars indicated that the helicopter was in a relatively level attitude at the time of impact, before it slid approximately 13 to 15 m down the sloping ground and collided with a tree.

Figure 2: Helicopter wreckage

Impact damage to the helicopter was extensive. The helicopter lay on its deformed left side with the cockpit left side panel, the firewall, and the seat assembly compressed in a rearward/inward direction. Damage to the cockpit right side panel was less severe and the panel had retained its shape. A limited amount of the shattered perspex windscreen remained with the main wreckage.

There was severe disruption to the cockpit, and the instrument pedestal had been partially severed from the floor and was pushed towards the pilot's seat. The engine, fuel, hydraulic, and flight control systems' components were dislodged from their installed position and damaged, but remained in their respective locations. The engine was recovered from the accident site and transported to an overhaul facility for technical examination under Australian Transport Safety Bureau (ATSB) supervision. The examination revealed that the engine was capable of normal operation at the time of the accident.

The main rotor blades and mast, main transmission, and the remainder of the helicopter's major components, systems, and controls were also accounted for at the accident site, and in close proximity to the main helicopter wreckage.

Both fuel tanks and their respective firewalls dislodged from their installed position and were damaged. That included impact puncture damage to the fuel tanks. While fuel leakage was evident on the ground around the main wreckage, approximately 20 litres of fuel was recovered from the helicopter's fuel tanks for sampling purposes. A fuel sample was also taken from the re-fuelling truck. The results of independent laboratory tests of both samples were consistent and indicated that the Jet Fuel was bright, free from water and considered suitable for use.

The remains of the tail boom structure was bent slightly upwards, and the tail boom assembly had detached from the rear of the centre fuselage frame about 2 m back from that frame. Sections of the tail boom and the tail rotor drive shaft had separated at impact, but remained oriented in the correct position relative to each other. That was consistent with the helicopter having negligible yaw at impact. A small section of the tail boom structure was located down slope from the main helicopter wreckage.

All major components and extremities of the installed agricultural spray equipment were accounted for with the main helicopter wreckage.

The left and right cockpit door frames were deformed just above the upper door hinge, corresponding to about 15 to 20 cm above the top of the instrument pedestal. That included a deep, circular indentation to the right door frame and a partial tear and bulging of the left door frame. The indentation in the right door frame was 'married' to a length of the severed power cables, and the damage was found to be consistent with the cables having impacted with that door frame (see Figure 3). There were no similar deformations or markings to the instrument pedestal, cyclic or other helicopter controls, or any other part of the helicopter.

Figure 3: The cut into the perspex bubble frame matched with the conductor

Medical and pathological information

Based on the autopsy, toxicology, and medical reports, there was no evidence to indicate that the pilot's performance was degraded by physiological factors.

Survival aspects

A four-point restraint harness, with inertia reel shoulder straps was fitted to the pilot's seat. Video footage of the helicopter departing on the accident flight revealed that the pilot had fastened the harness. Examination of the harness at the accident site revealed that it remained attached to the firewall and that there was no evidence of failure of the locking mechanism.

The pilot was wearing a helmet at the time of the occurrence. That helmet exhibited significant impact damage, but remained intact. The helmet visor was found in the raised position.

Agricultural spray computer disk

The hard disk unit from the on-board agricultural spraying computer was recovered. The unit was assessed by the manufacturer's distributor and an external independent forensic expert. Those examinations confirmed that the damage to the unit precluded extraction of the data.

Organisation

Company Operations Manual

Section A19.5 of the company Operations Manual (manual) contained the en-route procedures affecting low flying operations by company pilots. In part, those procedures included that:

…due consideration shall be given to the dangers of unseen wires, effects of turbulence and other low flying hazards. Prior to the commencement of low level operations, the pilot in command shall carry out an aerial survey of the area to establish an optimum path and aircraft manoeuvring plan for the safe conduct of the operation.

An overview of those procedures affecting the conduct of aerial agricultural operations by company pilots was included at Section D 6.1 of the manual. That included that:

Normal aircraft operating procedures are detailed in the relevant Pilot Operating Handbooks, the Aeronautical Information Publication, as well as CAO 20.21, and the Agricultural Pilots Manual (AAAA [Aerial Agricultural Association of Australia Limited] Pilot and Operations Manual). And these should be followed unless specified otherwise below.

Section D 6.5 of the manual reinforced the regulated requirement for an agricultural pilot to conduct an aerial inspection of a proposed treatment area prior to commencing aerial agricultural as follows:

It is a CASA requirement that prior to commencing agricultural operations the pilot shall carry out an aerial inspection of the proposed treatment area - where practicable, a ground inspection will also be conducted as per sub-section D 6.4.

The manual continued that the pre-treatment aerial inspection was to include the examination of:

…Wires - their location in relation to the boundaries of the area to be treated, height and position of, and distance between poles and guy wires…

Additional information

Previous wirestrike occurrences

A review of the ATSB's accident and incident database revealed that there were six single-fatality agricultural helicopter accidents in Australia during the period 1 January 1995 to 20 June 2005. Those accidents represented 43% of all fatal helicopter wirestrike accidents during the period.

Regulatory framework

Civil Aviation Regulation (CAR) 206 lists agricultural operations as operations conducted for aerial work purposes. Agricultural operations are defined as:

…the broadcasting of chemicals, seeds, fertilizers and other substances from aircraft for agricultural purposes of pest and disease control.

Civil Aviation Order (CAO) 40.6 defines the requirements of the Agricultural Pilot Rating Grades 1 and 2. Those requirements include completion of a period of ground training and a written exam, before carrying out initial and operational flying training. The operational flying training includes operational planning, such as: ground and aerial inspections of the treatment area; assessment of wires; and route selection to and from the treatment area, including the height to fly during such transit flights. That training is to be followed by a period under supervision, before the newly rated agricultural pilot is able to conduct unrestricted agricultural operations.

Aerial Application Pilots Manual³

Chapter 5 of the Aerial Application Pilots Manual contains information pertaining to the identification of powerlines and other low flying hazards with the potential to affect agricultural operations. Chapter 8 of that manual outlines the information specific to helicopter agricultural operations. Those chapters elaborate on the recommended pre-flight planning and inspections that should be carried out by a pilot prior to the conduct of agricultural operations, including that:

If the pilot is obliged to rely on a farmer briefing regarding wire hazards, a thorough interrogation of the farmer is necessary to establish the position of concealed wires…

They are seldom aware that wires outside the treatment area can be hazardous to aircraft approaching for a spraying run. Farmers may forget that they recently connected a shed to the power, ran a new overhead wire to a pump or accidentally hooked a powerline with a farm implement and pulled it over 'a bit'.

Considerable effort is needed to extract vital information from the client. Remember there are some wires that you may strike unless the farmer makes you aware of their location.

In addition, sometimes the background to the wires - trees, hills etc. - provides a poor contrast, while poles may be concealed by intervening obstacles or by being located so far to the periphery of the pilot's visual field that they are not noticed.

During climb and cruise to and from the treatment area the aircraft should track as directly as possible consistent with avoiding nuisance areas and poor terrain.

Don't come below possible powerline height until you are certain all wires have been safely identified.

The property manager reported that he was aware of the powerlines that were struck by the helicopter, but that he had not passed that knowledge on to the pilot because they were located outside both the area to be treated, and the safe transit route between the replenishment truck and the proposed spray area.

Risk management

Australian/New Zealand Standard AS/NZS 4360:2004 Risk Management (the Standard) defined risk as:

the chance of something happening that will have an impact upon objectives.
NOTE 1: A risk is often specified in terms of an event or circumstance and the consequences that may flow from it.
NOTE 2: Risk is measured in terms of a combination of the consequences of an event … and their likelihood…
NOTE 3: Risk may have a positive or negative impact.

The Standard described risk management as 'the culture, processes and structures that are directed towards realizing potential opportunities, while managing adverse [or negative] effects'. The risk management method described by the Standard includes that initially the basic parameters or context affecting the assessment of risk should be identified. That can include 'defining the extent of the project activity or function in terms of time and location'. Residual risk is that 'risk remaining after implementation of risk treatment'. Options for modifying or treating identified risks with negative outcomes included:

• influencing the likelihood of a risk, in order to reduce the probability of a negative outcome
• changing the consequence(s) of an event to minimise the extent of any losses.

Detection of powerlines

The requirements for the mapping and marking of power cables and their supporting structures are published in Australian Standards AS 3891.1 1991 Part 1: Permanent marking of overhead cables and their supporting structures, and AS 3891.2 - 1992 Part 2: Marking of overhead cables for low level flying. The general requirements of those standards were discussed in ATSB  investigation report 200404286 and include that, in general, there is no requirement for the marking of power cables with a height above terrain, or obstacles of less than 90 m. The power cable that was struck by the helicopter did not require marking in accordance with either standard.

Technical committees are formed by Standards Australia to develop and review relevant standards, and comprise a balance of interested and affected parties that are nominated by generally national organisations. The aim is that the standards should include consideration of the views of large, common interest groups. Organisations that consider they represent a valid, previously unrepresented interest group are able to nominate for consideration for inclusion in a committee.

A number of aviation industry associations and other bodies were involved in the development of the Australian Standards affecting the marking of overhead power cables and their supporting structures. That did not include some of the groups and associations normally associated with a number of agricultural and other low-level operations.

There are currently a number of engineering solutions available, with the potential to assist pilots identify overhead powerlines. While their suitability or cost-effectiveness may not prove acceptable for all helicopter types or operations, those engineering solutions include, but are not limited to:

• laser-based systems that alert a pilot of approaching electrified powerlines, or that scan the environment for wires and other obstacles
• enhanced ground proximity-based warning systems that include relevant software and an onboard powerline database in order to identify approaching powerlines.

¹ Only those investigation areas identified by the headings and subheadings were considered to be relevant to the circumstances of the occurrence.
² The designation 'Soloy' indicates that the helicopter had been modified and fitted with a turboshaft engine.
³ Version 6 prepared by the AAAA. A significant upgrade of the Aerial Application Pilots Manual was sponsored by the Civil Aviation Safety Authority (CASA).

The Aviation Safety Investigation Report 200402669 on the fatal accident involving a Bell 47G-3-B-1 helicopter registered VH-RTK which occurred 12km west of Wodonga, Vic. on 19 July 2004. The aircraft was on an aerial agriculture mission when it collided with a powerline. There have been related safety actions from CASA, the Aerial Agriculture Association, and the upcoming ATSB research report on wirestrike accidents.

SIGNIFICANT FACTORS

1. The ADC did not communicate to the crew of the 737 the requirement to pass behind the 717.
2. The crew of the 737 did not perceive the potential threat presented by the 717 until the crew of that aircraft acted to ensure that their flight path did not place them in a near collision situation.

ANALYSIS

The Hamilton Island Aerodrome Controller (ADC) had a plan to separate the aircraft, but did not clearly communicate the plan to the pilots and consequently it was not executed correctly. Having the 737 pass behind the 717 was going to present some difficulties due to the intended tracks of the aircraft, and required the ADC to advise the crew of the 737 as early as possible of that tracking requirement. While traffic information about the 717 was provided to the crew of the 737, the ADC did not communicate an important aspect; that is, that after reporting seeing the 717 they would have to pass behind it. Had that been the case, the crew of the 737 probably would have been able to turn right in sufficient time to safely pass behind the 717. Alternatively, they may have requested another means of separation as the position of the aircraft may have prevented them from passing behind it.

The use of visual separation, either by controllers or pilots, increases the likelihood of an apparent traffic alert and collision avoidance system (TCAS) alert between aircraft. Apparent alerts result from aircraft being within the TCAS alert parameters while complying with an air traffic control (ATC) clearance. In this occurrence, if the 717 crew had not turned away, it is possible that one or both of the aircraft's systems would have issued a traffic alert (TA) or a resolution advisory (RA) In the case of the latter, the pilots would have had to comply. That would have increased crew workload, particularly for the departing 717 crew, when the aircraft was in a critical stage of flight. As it was, the crew of the 717 had to descend to avoid the 737.

The limitations in using visual separation, highlighted in the Manual of Air Traffic Services, applied similarly to the ADC and both crews. The fact that the aircraft were tracking on almost reciprocal tracks, with little or no divergence when viewed by the ADC or the crews, made it difficult for those involved to obtain adequate cues about the situation. However, the crew of the 717 was able to appreciate the potential for conflict by using available visual and system information.

Despite the regulations stating that in a situation where aircraft are approaching head on a pilot shall alter the aircraft's heading to the right, the actions by the crew of the 717, in turning left, could be seen to be reasonable in the circumstances. A turn to the right may have increased the risk of collision. Similarly, if the crew of the 737 had turned their aircraft to the right in accordance with procedures, as the 717 turned left, the risk of a collision may have increased.

Once the ADC was under the impression that responsibility for separation had been transferred to the crew of the 737 and had issued instructions to the crews to climb/descend, the protection afforded by the vertical separation standard was lost. From that point on, the only defences available to the crews to prevent them being in close proximity were their awareness of the other aircraft and the use of TCAS. As they could clearly see the 717, the crew of the 737 did not perceive that there was a problem. The crew of the 717 were concerned at the developing situation and turned away from the 737. That action ensured that sufficient spacing was maintained between the two aircraft and probably prevented a subsequent TCAS RA.

Had the ADC maintained the 1,000 ft vertical separation standard between the aircraft until they had definitely passed, or else confirmed that one of the crews could maintain separation with the other aircraft, it is likely that the occurrence would have been prevented. The use of vertical or lateral separation standards instead of a visual standard would have also limited the possibility of a TCAS alert.

The occurrence highlighted the importance of using unambiguous radiotelephony phraseology to avoid misunderstandings and the need for pilots and controllers to remain vigilant at all times, especially when the dynamics of a situation require action to be implemented early to ensure that the safety of aircraft is not compromised.

FACTUAL INFORMATION

On 17 July 2004, at about 1619 eastern standard time, a Boeing Company 737-476 (737), registered VH-TJH, was inbound to Hamilton Island from the south-east for a landing on runway 14. The Hamilton Island Aerodrome Controller (ADC) instructed the crew to descend to 4,000 ft above mean sea level (AMSL) due to the pending departure of a Boeing Company 717-200 (717), registered VH-VQB, from runway 14. The crew of the 737 requested and were approved by the ADC to track for a left downwind to runway 14. The ADC instructed the crew of the 717 to maintain 3,000 ft AMSL, to make a right turn to track to Mackay and that they were clear for takeoff. The weather was visual meteorological conditions (VMC) and the crew of the 737 reported to the ADC that they could see the 717. The ADC instructed the crew of the 737 to make a visual approach to left base that was amended to a right base after the crew requested that change. Shortly after intercepting the outbound track at about 2,000 ft, the crew of the 717 received a traffic alert and collision avoidance system (TCAS) traffic advisory (TA) and saw the 737 crossing from left to right on descent. The 717 crew's perception was that the expected track of the aircraft would place them on, or close to a collision course so they turned left and descended to avoid the 737 by passing behind it.

Analysis of air traffic control recorded radar data and aircraft flight data revealed that at 1619:15, after the 717 had turned left, the lateral and vertical distance between the aircraft was 1,112 m and 700 ft (737 above the 717). Both aircraft were fitted with a traffic alert and collision avoidance system (TCAS). The flight data recorder (FDR) in the 717 was only capable of recording TCAS resolution advisory (RA) parameters while the 737 FDR did not record any TCAS parameters. Data from the 717 revealed that there was no TCAS RA. The crew of the 717 changed the aircraft's heading by about 35 degrees and descended to 1,500 ft during the manoeuvre, before returning to their assigned track and climbing to 3,000 ft.

Hamilton Island Air Traffic Control

The ADC was responsible for providing air traffic control services in Class D airspace from ground level to 4,500 ft AMSL. In Class D airspace, air traffic control (ATC) is required to separate aircraft operating under the instrument flight rules (IFR) from other aircraft operating under the IFR or special visual flight rules (VFR). In addition, ATC is required to provide the crews of aircraft operating under the IFR with traffic information about aircraft operating under the VFR. The 737 and 717 were both operating under the IFR.

The circumstances were not related to any national airspace changes as both aircraft were IFR and in airspace being managed by ATC.

Clearances and separation

The pilot of the 737 had been issued with a clearance by the Brisbane Centre controller to track inbound to Hamilton Island via the 143 radial of the Hamilton Island VOR¹. The ADC issued a clearance to the pilot of the 717 to track via the 157 radial of the Hamilton Island VOR to Mackay and then the planned route to Sydney. The Aeronautical Information Publication En Route Supplement, Hamilton Island special procedures, nominated a right circuit for operations to runway 14. The use of runway 14 and the issued clearances would result in the aircraft tracks intersecting at some stage (see Figure 1).

Figure 1: Hamilton Visual Terminal Chart with aircraft tracks and times.

The ADC was aware of this and assigned altitudes to the crews that provided the 1,000 ft vertical separation standard required between two aircraft operating under the IFR. The application of that standard was necessary until an alternative separation standard was in place.

When a pilot of an arriving aircraft has been approved by ATC to make a visual approach, they are required to track in accordance with the assigned track clearance until within 5 NM of the aerodrome. From 5 NM, the pilot can diverge from the inbound track to join the circuit as directed by ATC for an approach to the nominated runway.

Air traffic control (ATC) visual separation standards and procedures

The Manual of Air Traffic Services (MATS) details the standards and procedures to be used by controllers to separate aircraft. Part 4, Section 1, Separation Standards stated that in the provision of separation, controllers shall place greater emphasis on traffic planning and conflict avoidance than on individual conflict resolution being achieved. This is to enable separation assurance to be achieved through planning traffic to ensure separation, executing the plan to achieve separation and monitoring the situation to ensure that the plan and the execution are effective. Section 5, Visual Separation stated that visual separation could be achieved by the use of visual procedures (by controllers) or by assigning visual separation responsibility to a pilot. Controllers are to consider aircraft performance characteristics when applying visual separation.

The application of visual separation by the ADC or either crew would have been an appropriate alternative to the vertical separation standard. When aircraft are operating at or below flight level (FL) 125² and will continue to do so during the application of visual separation by a pilot, the pilot of one aircraft is required to report sighting the other aircraft and has to be instructed by a controller to maintain visual separation with, or to follow, that aircraft. Also, if a pilot has been instructed to maintain separation from, but not to follow an IFR category aircraft, then the controller is required to provide traffic information to the pilot of the IFR category aircraft. That information should contain as much detail as possible including the aircraft type, altitude or flight level, position and intentions or direction of flight. If there is any doubt about a pilot's ability to either keep another aircraft in sight or to maintain separation, a controller shall issue alternative instructions to provide separation.

The ADC's reported expectation was that the aircraft tracks would cross such that the 737 would pass behind the 717. At 1615:15, when the 717 was lining up on the runway, the ADC advised the crew of the 737 that the 717 was due to depart direct to Mackay on climb to 3,000 ft and that they could expect to maintain 4,000 ft until the 717 was sighted [by the 737 crew]. The ADC did not advise either crew of the intention, after the 717 was seen by the crew of the 737, to assign separation responsibility to that crew and have them pass behind the 717 (see Appendix A.

While the 717 was taking off, the crew of the 737 reported approaching 4,000 ft and requested a clearance to track for left downwind. The ADC instructed the crew to track for left downwind. At 1618:26, the crew reported to the ADC that they could see the 717. The ADC issued a clearance for the crew to make a visual approach and to report turning a left base. The crew acknowledged the instruction and requested to track to a right base for the runway. The ADC instructed the crew to track for and to report on right base. At 1619:00, the ADC instructed the crew of the 717 to climb to FL310. A pilot approved to make a visual approach can descend as required to establish an aircraft on base or final to the assigned runway. The ADC's instruction to the crew of the 737 to make a visual approach and the instruction to the crew of the 717 to climb, cancelled the separation assurance provided by the application of the vertical separation standard.

Controllers can separate aircraft by visual observation of aircraft position and projected flight paths. The MATS advised that in providing visual separation, controllers should rely primarily on azimuth and if visual separation by judgement of relative distances or altitude is used, then it should be with such wide margins that there is no possibility of the aircraft being in close proximity. The MATS notes that 'experience has shown that a controller's visual determination of the relative distance of aircraft in close proximity can be in error, even to the extent of an apparent reversal of the positions of the two aircraft.' The convergence of the aircraft's respective tracks meant that the lateral spacing and the distance between them, from the ADC's perspective, was reducing.

At 1619:10, approximately 5 seconds before the crew of the 717 elected to turn to avoid the 737, the ADC queried the crew of the 737, 'just confirm that you will be passing behind the 717'. That query was the first time the ADC had mentioned to them that they were required to pass behind the 717. As they replied that they could pass behind, the pilot rolled the 737 right to a maximum bank angle of 26 degrees and 2 seconds later reversed the roll to the left to a maximum bank angle of 15 degrees in conjunction with a left turn when they saw that the 717 was turning away.

At the time of the occurrence, the ADC was managing the two jets and two other light aircraft operating remote from the Hamilton Island Airport area.

Rules of the air

The Civil Aviation Safety Authority Regulation 161 - Right of Way, states that 'an aircraft that is required by the rules in this Division to keep out of the way of another aircraft shall avoid passing over or under the other, or crossing ahead of it, unless passing well clear' and that 'the pilot in command of an aircraft that has the right of way must maintain its heading and speed, but nothing in the rules in this Division shall relieve the pilot in command of an aircraft from the responsibility of taking such action as will best avert collision'.

Regulation 162 - Rules for Prevention of Collision, states that 'when 2 aircraft are on converging headings at approximately the same height, the aircraft that has the other on its right shall give way' and 'when two aircraft are approaching head-on or approximately so and there is danger of collision, each shall alter its heading to the right'.

717 flight crew

The copilot was the pilot flying (PF) and the pilot in command (PIC) was the pilot not flying (PNF). They reported that they were both aware of the inbound 737 and understood the ADC's application of the vertical separation standard. The crew thought that the 737 was probably inbound from Brisbane. Prior to departure, the crew set both cockpit navigation displays³ to 10 NM range in accordance with company procedures.

After take-off, the crew saw on the 717's TCAS display that the 737 was presented as other traffic at about the 11 o'clock position⁴ at approximately 7 NM At that stage they could not see the 737. Shortly after retracting the aircraft's flaps from the take-off position of 18 degrees, they saw the 737 as proximate traffic, slightly above them, on the TCAS. They then saw the 737. The crew became concerned because:

• the 737 was stationary in their windscreen
• the vertical distance between the aircraft was reducing
• the track of the 737 did not appear to be changing
• the 737 was unlikely to be able to pass behind them.

They were also not sure of what action, if any, the crew of the 737 was taking. The copilot had previously reduced the rate of climb because he thought the rate of closure between the aircraft may cause a TCAS alert. The level of concern was such that the PIC instructed the copilot to turn left immediately, and he complied. During the turn the copilot descended the 717 and during that phase the crew received a slats 'overspeed exceedance warning'. The crew reported that the exceedance was 8 kts.

The time between the crew being issued with a clearance to take off and turning left from their outbound track was about 2 minutes.

737 flight crew

The PIC was the PF and the copilot was the PNF. They reported that they understood the effect of the ADC's altitude limits. They saw the 717 back track along the runway and line up as the visibility was 'quite clear.' The crew were initially unconcerned with the departure of the 717 as it was in plain view.

The copilot requested a clearance to track for left downwind in error and 25 seconds later amended it to a request to track for a right base. As the aircraft turned left to track for right base, the 717 was almost stationary in their windscreen where previously, while they were established on the 142 radial, it had appeared to be moving slowly from right to left. They later reported that they eventually realised the potential for conflict after the 717 turned left as they attempted to comply with the ADC's instruction to pass behind it. They received a TCAS TA after the 717 had turned away.

During the period that the crew was requesting approval to track for downwind, the aircraft's flight data indicated that its airspeed was 250 kts and that it had turned left 10 degrees. At that time, it was 9 NM from the aerodrome.

Radiotelephony procedures

Pilots are required to notify receipt of the current terminal information on first contact with ATC, either when taxiing for departure or when inbound for landing. If that advice is not provided, a controller is required to either confirm receipt of the information by the pilot or else provide the current terminal information. Part of the information normally provided is the local altimeter setting. That setting is required by a pilot to enable an aircraft's altimeter to provide the height above mean sea level. It is also needed to ensure the correct application of vertical separation standards between two aircraft. The crew of the 737 did not report receipt of the Hamilton Island terminal information. The ADC did not confirm with the crew that it had been received, nor did the ADC provide the information.

When a pilot is assigned and required to maintain separation with a sighted aircraft, a controller is required to instruct that pilot to, 'maintain separation with (or pass behind or follow) and include details of the aircraft type or identification and any restrictions'⁵. The ADC did not instruct the crew of the 737 to maintain separation or to pass behind the 717 after they reported that they could see that aircraft.

Pilots are required to read back some clearances and/or instructions issued by a controller. Readback items include any altitude or level assignments. If a required readback is not provided by a pilot, a controller is required to challenge the pilot to read back the necessary item. Following the initial inbound report by the crew of the 737, the ADC instructed the crew to descend to 5,000 ft. The crew did not read back that altitude and the ADC did not challenge the lack of a readback. The crew also did not read back the subsequent clearance to make a visual approach. The ADC did not challenge the lack of that readback.

Studies conducted by the US National Transportation Safety Board found that controllers have a tendency to relax their level of alertness in a low workload environment, which makes them vulnerable to operational errors and omissions. Similarly, pilots have been known to relax attentiveness and vigilance when under ATC control.⁶

Traffic alert and collision avoidance system

The Honeywell Incorporated, TCAS II Pilot's Handbook⁷ describes TCAS as:

...an independent on onboard collision avoidance system. It is designed as a backup to the ATC system and the "see-and-avoid" concept⁸. The [TCAS] system is designed to provide safe separation between aircraft predicted to be on collision trajectories while minimising ATC clearance deviation or excursions.

The TCAS does not replace the ATC system. TCAS II continually calculates and tracks the projected positions of air traffic control radar beacon system transponder equipped aircraft within 20 NM and within altitudes 10,000 ft of the aircraft's altitude. The system then generates Resolution Advisories (RA) and Traffic Advisories (TA) against intruder aircraft with ATC transponders.

The level of traffic information displayed is subject to the limits of TCAS, the aircraft's cockpit display and pilot display selections.

A TA is generated for aircraft which are predicted to be within 20 to 48 seconds of the aircraft's collision area⁹, while an RA is generated for an aircraft that is predicted to be within 15 to 35 seconds of the collision area. The timing for an alert is subject to sensitivity levels and altitude layers set in the system. These layers and levels prevent the system from initiating a descent RA when below 1,100 ft above ground level (AGL). For the altitudes of the aircraft at the time of the occurrence, a TA would be generated 25 seconds before the collision area and an RA would be generated 15 seconds before the collision area.

During an RA event TCAS should provide between 300 ft and 800 ft of vertical spacing between the aircraft involved.

¹ VOR - VHF omnidirectional radio range navigation aid.
² 12,500 ft.
³ The displays include details of navigation aids, TCAS and the route to be flown.
⁴ The numbers on a clock are commonly used by a pilot to refer to the relative position of another aircraft. For example, an aircraft observed abeam to the left would be said to be at 9 o'clock.
⁵ Manual of Air Traffic Services, Annex 6-12 Radiotelephony Phrases.
⁶ Shari Stamford Krouse, PhD Aircraft Safety, 1996, ISBN 0-07-036026-X.
⁷ Honeywell Inc, TCAS II Pilot's Handbook, 1.2 Introduction.
⁸ See the ATSB website for further information on see-and-avoid.
⁹ A volume of three dimensional airspace surrounding a TCAS equipped aircraft that varies in size depending upon the rate of closure of a conflicting aircraft.

On 17 July 2004, at about 1619 eastern standard time, a Boeing Company 737-476 (737), registered VH-TJH, was inbound to Hamilton Island from the south-east for a landing on runway 14. The Hamilton Island Aerodrome Controller (ADC) instructed the crew to descend to 4,000 ft above mean sea level (AMSL) due to the pending departure of a Boeing Company 717-200 (717), registered VH-VQB, from runway 14. The crew of the 737 requested and were approved by the ADC to track for a left downwind to runway 14. The ADC instructed the crew of the 717 to maintain 3,000 ft AMSL, to make a right turn to track to Mackay and that they were clear for takeoff. The weather was visual meteorological conditions (VMC) and the crew of the 737 reported to the ADC that they could see the 717. The ADC instructed the crew of the 737 to make a visual approach to left base that was amended to a right base after the crew requested that change. Shortly after intercepting the outbound track at about 2,000 ft, the crew of the 717 received a traffic alert and collision avoidance system (TCAS) traffic advisory (TA) and saw the 737 crossing from left to right on descent. The 717 crew's perception was that the expected track of the aircraft would place them on, or close to a collision course so they turned left and descended to avoid the 737 by passing behind it.

At about 1921 CST, on 13 July 2004, the left engine of a Fairchild Industries Inc. SA227 DC aircraft, registered VH-KEX, failed during the climb phase after take-off. The aircraft was being operated on a scheduled passenger service from Ceduna to Adelaide, South Australia with 2 crew and 17 passengers.

A subsequent examination of the engine, a Garrett TPE331-12UHR, by the operator, in the presence of Australian Transport Safety Bureau (ATSB) investigators, found that the turbine rotating airseal had failed transversely through the cooling holes around the centre of the cylindrical section. That resulted in the liberation of the seal flange and substantial damage to the downstream turbine module (refer Figures 1 and 2).

Since 1995, the engine manufacturer had received four reported failures of the rotating airseal for the TPE331-12 engine. These failures were attributed to progressive thermal cracking. As a result, the manufacturer introduced several service bulletins related to modifications to improve core engine airflow and the introduction of a Fluorescent Penetrant Inspection (FPI) to the rotating airseal. The requirements of these service bulletins had been complied with on this engine during manufacture and FPI was performed at scheduled intervals as required by the manufacturer.