Technical Analysis Report " Short Bros. SD360-300, VH-SUM

EXECUTIVE SUMMARY

On 13 August 2000, a Shorts SD-360 aircraft (VH-SUM) sustained an in-flight shutdown of the right engine, which was later attributed to the seizure of the gas generator and power sections of the turbine.

Further examination of the engine by the ATSB and Pratt & Whitney Canada showed the seizure to have stemmed from the distress and severe overheating of the engine number one bearing in the compressor section.

Despite the degree of damage to the bearing races, evidence of electrical arcing damage was found on the inner race (non-thrust side) and was subsequently traced back through the accessory gearbox components to the starter-generator input shaft. Intermittent pitting on the starter-generator drive gear and continuous pitting on the driven gear indicated the primary source of electrical potential was within the starter-generator assembly.

Recommendations for safety action stemming from this examination include the further study and testing of the PT6A-67R starter-generator and engine assembly to determine the conditions under which excessive current leakage could result. It was also recommended that the assembly, as installed within VH-SUM be examined to identify the potential influence of other electrical accessory items fitted.

FACTUAL INFORMATION

Introduction

On 13 August 2000, a Shorts SD360-300 aircraft VH-SUM sustained an in-flight uncommanded feathering of the right propeller. The engine was shut down immediately. Examination of the engine showed the gas generator and power sections of the engine to have seized.

Pratt & Whitney Canada (Australasia) subsequently disassembled the engine. The findings of the teardown examination are presented in Pratt & Whitney report number 25885.

To support the investigation of the occurrence, the ATSB carried out an analysis of the following components from the failed engine.

• Number-1 bearing and debris. The outer race of the bearing remained within the inlet case (figs. 1 & 2).
• Gearbox input shaft and roller bearing assemblies (fig. 3).
• Coupling splined hollow shaft (fig. 3).
• Starter gear and roller bearing assemblies (fig. 3)

In addition, an unused bearing identical with the engine number-1 bearing was supplied, as well as copies of reports prepared by Pratt & Whitney addressing previous engine failures experienced by the operator.

Component history

The history of the engine was reported as follows (Pratt & Whitney Canada report #25885).

Serial Number: 106223
Time Since New: 6410.9 hrs
Time Since Overhaul: 1500.1 hrs
Cycles Since New: 9704
Cycles Since Overhaul: 2463
Previous Workshop Visit: 5 June 1999 for routine overhaul

Figures 1, 2 & 3. Components as received.

ANALYSIS

Visual examination

Number-1 Bearing:

Figure 4 shows the general condition of the bearing as recovered from the engine. All surfaces of the bearing as well as the nearby casing showed heavy discolouration and blackening that is typical of extreme overheating. All the balls and the inner race (fig. 5) showed heavy localised wear and metal flow, signalling the sliding contact of the balls against the inner race.

Figures 4 & 5. Condition of bearing balls and races.

The effects of thrust loads within the operating engine had produced a bias in the wear on the inner race toward the rear inlet casing. Resulting from this loading and the amount of wear experienced, the inlet case of the turbine had come into contact with the adjacent compressor rotor disc. Closer study under the stereomicroscope showed fine pitting towards the inner edge of the non-thrust inner race (fig. 6), although scoring marks obscured much of this. Damage to the outer race was equally severe, with heavy indentation and bruising of the contact surfaces from the ingress of debris (fig. 7).

Figures 6 & 7. Indentation and pitting damage to the inner and outer races.

The bearing cage had experienced multiple radial cracks and cracks around the circumference, with most associated with galling, scoring and other evidence of metal to metal surface contact (fig. 8).

Figure 8. Contact damage and scoring on the external surface of the bearing cage.

Some internal corners of the cage showed shallow fatigue cracks - these had started at areas of heavy surface scoring and metal flow (fig's. 9 & 10).

Figures 9 & 10. Bearing cage fracture surfaces showing evidence of fatigue cracking.

When compared with the new item, the extent of the damage to the bearing parts was clear (figs. 11, 12 & 13).

Figures 11, 12 & 13. Bearing components compared with new items.

Associated components:

The coupling gear, starter gear and coupling shaft each showed varying levels of gear surface and spline pitting damage. Mostly, the pitting was restricted to localised spots at the gear teeth pitch-line (figs. 14, 15 & 16), although the starter gear showed uniform damage over the whole contact surface (fig. 17).

Figures 14 & 15. Pitting on coupling gear teeth faces.

Figures 16 & 17. Fine pitting found on starter gear teeth.

Studying the overall distribution of pitting on each item revealed the large diameter starter gear showed the most significant damage over two or three teeth in four equally spaced regions around the circumference (fig. 18). The smaller coupling gear which mates with this item showed no suggestion of this intermittent pitting, with damage found uniformly around the entire circumference.

The male and female splined connections showed pitting over both the teeth faces (figs. 19 & 20) and end surfaces (figs. 21 & 22), with the damage appearing similar in all cases.

Figure 18. Starter gear, showing four localised areas of pitting around the diameter.

Figures 19 & 20. Pitting found on coupling gear and shaft spline teeth.

Figures 21 & 22. Pitting on coupling gear and shaft end contact faces.

Electron microscopy

Electron microscope imaging of the pitting identified on the internal bearing race and gear teeth showed many of the pits to have a semicircular form, with a shallow concave profile. Observation at higher magnifications found evidence of localised melting, which presented as small balls of metal within the pit confines. Many of the pits showed a distinctive flattened appearance - typical of the effects, produced by normal rolling surface contact. Figures 23 through to 28 show the typical appearance of the surface pits at various magnifications.

Figures 23 & 24. Pitting on coupling gear faces.

Figure 25. Globules of remelted metal within coupling gear surface pits - typical of electrical arcing damage.

Figures 26 & 27. Pitting damage as found on non-thrust side of the inner race.

Figure 28. Closer view of bearing race pitting, also showing remelted globules.

Metallographic examination

Sections for microscopy were taken through a pitted area on the less damaged, non-thrust section of the inner race and prepared using conventional methods. Etching with a 2% Nital solution revealed several regions along the contact surfaces that displayed locally transformed, untempered martensite associated with the pitting and extending to around 60mm in depth (figures 29 & 30). The general microstructure of the bearing race was fine, lightly tempered martensite with areas of globular massive carbide randomly throughout (figure 31).

Figure 29. Transformed regions surrounding pitting on the inner race (X80).

Figure 30. Closer view of a transformed region (X200).

Figure 31. General tempered martensite and carbide bearing race microstructure.

Hardness tests

The parent metal hardness, as measured away from the pitted surface was around 804 - 810 HV30.

CONCLUSION

Material and manufacture

The number-1 bearing inner race (non-thrust section) showed a microstructure and hardness level that was acceptable for the item and contained no obvious deficiencies that could have affected the bearing's service performance.

Engine failure

The breakdown of the PT6A-67R engine (S/N 106223) as installed within the aircraft VH-SUM was attributed to the failure of the number-1 turbine bearing. The bearing had experienced severe frictional damage and overheating, stemming from the seizure of the balls and the sliding contact against the inner race surfaces. Because the bearing carried a degree of rearward compressor axial thrust loading, the front inner race experienced most of the severe wear and scoring, leaving the rear race comparatively undamaged and able to be closely studied.

Development of the breakdown

Electron microscopy of the non-thrust inner race showed evidence of pitting that resembled damage produced by electrical arcing. Subsequent metallographic examination verified the damage; revealing localised thermally affected regions associated with the surface pits. In the presence of such physical and metallurgical discontinuities, rolling contact fatigue cracking developed and larger spalled areas formed over the contact surfaces. Increases in friction stemming from the spalling and the generation of metallic debris would have raised the vibration and temperature levels, eventually resulting in the thermal runaway of the bearing and the seizure as experienced. Such findings concur with the results of previous examinations conducted on PT6A-67R engines by Pratt & Whitney Canada.

Source of the electrical current

Several other components in the accessory gear train showed electrical arcing effects. The damage was found chiefly around the contact points between the gear teeth. The large diameter starter gear showed four evenly spaced regions around the circumference where the pitting was most severe. In contrast, the smaller mating coupling gear showed uniform pitting around the diameter. This observation led to three direct conclusions -

• The electric current flowed from starter gear, through to the coupling gear and into the bearing via the turbine shaft and inner races.
• The electric current was steady or continual and not a transient surge.
• The current was of an alternating or pulsed nature at a frequency equal to four times the rotational speed of the starter gear.

From the general design of the engine, it appeared that the most likely source of the current was from the starter-generator unit, as the unit coupled directly to the large diameter starter gear. Armature leakage current from the unit would have conducted via the frictional coupling; through to the starter gear and into the bearing via the coupling gear, shaft and turbine shaft.

The ATSB carried out a safety deficiency investigation in accordance with powers under section 19CB (1) (d) of the Air Navigation Act 1920.

SAFETY DEFICIENCY

An allegation was made to the ATSB that Australian registered Boeing 747-300 aircraft operating from Bangkok airport in Thailand were failing to meet take-off performance requirements. A 'specified' flight was cited as demonstrating that the aircraft had not complied.

Comprehensive analysis of data from the 'specified' flight, as well as data from other flights departing Bangkok under similar conditions, was undertaken. Documentation provided by the crew of the 'specified' flight was also analysed. The data included brakes release to V_R (take-off rotation speed); V_R to V_LOF (aircraft lift off speed); V_LOF to 35 feet; total distance from brakes release to V₂ (take-off safety speed) at 35 feet.

Appropriate sections of the Boeing and the operator's Flight Crew Training Manuals (FCTM) were reviewed and the ATSB concluded that the actual take-off data correlated with the Flight Crew Training Manual information.

For certification, the Boeing 747-300 aircraft is required to be able to sustain an engine failure at or after a specified speed (V₁) at its maximum take-off weight and safely climb on the thrust of the remaining three engines. V₁ is the decision speed at, and below which take-off can be aborted and the aircraft stopped within the runway confines. It is also the speed at and above which the take-off can safely be continued should the critical engine become inoperative, where the critical engine is the engine that would most adversely affect the performance or handling qualities of the aircraft.

Analysis of the recorded data confirmed that the 'specified' take-off from Bangkok was with four engines operating. No evidence was found to suggest that the aircraft concerned did not meet the certified requirements for take-off performance.

Nothing in the cases examined suggested that the aircraft would not be able to safely climb from the runway on three engines if an engine had failed at or after reaching V₁ speed. Had an engine failure occurred before V₁, by definition the aircraft would have been able to stop within the runway confines.

Formulae from Boeing Jet Transport Methods were used to derive actual take-off distance of the 'specified' take-off. Digital Flight Data Recorder (DFDR) and Quick Access Recorder (QAR) data were used to establish time from brakes release to rotate, time from rotate to lift-off, time from lift-off to a height of 35 feet by radar altimeter, and to calculate take-off distance. Data from the flights analysed showed the aircraft met the certification requirements for take-off performance.

Calculations by Boeing and the operator were assessed by performance engineers from the Civil Aviation Safety Authority. Subsequent independent review of the data, including comparison with the ATSB's calculations, verified the accuracy of those calculations. Again, no evidence was found to suggest that the aircraft did not meet certified performance requirements.

The ATSB was not able to source any information to quantify any in-service experience with the worldwide Boeing 747-300 fleet to suggest that there has been any deviation from the aircraft performance levels indicated in the approved Aircraft Flight Manual.

There is also no evidence that a dangerous situation as described in the report of the alleged safety deficiency has occurred in the 12-month period to October 2000. The ATSB concluded that Boeing 747-300 aircraft are meeting scheduled performance requirements.

Occurrence Date: 14 NOVEMBER 1999
Registration No: VH-EBX
Model: B747-300
Manufacturer: Boeing Co

Executive Summary

The partial wreckage of a Cessna 206 aircraft was recovered from an area in the Gulf of Carpentaria, near where an aircraft of this type disappeared on 24-November 1999 (ATSB Occurrence number 199905562).

Photographs and video footage of the wreckage were supplied to the ATSB and reviewed with a view to gathering further detail regarding the accident. The ATSB subsequently requested that the propeller and attitude indicator instrument from the aircraft be shipped to the bureau's Canberra laboratories for further study and analysis. On the basis of damage to several aircraft articles recovered during the initial search, the original investigation concluded that the aircraft had impacted the water at high speed. The findings of the recent study concurred with this. From the attitude indicator and propeller, it was possible to conclude with good probability that the aircraft impacted the water at high speed in an uncontrolled, inverted attitude. Evidence indicated that the propeller was rotating at impact, although it was not possible to determine whether the engine was developing power.

N.R. Blyth
Senior Transport Safety Investigator
Technical Analysis

The reporting of aviation safety occurrences enables the Australian Transport Safety Bureau (ATSB) to investigate accidents and serious incidents and monitor safety through the analysis of any trends. On 1 July 2003 the Transport Safety Investigation Act 2003 came into effect, introducing the terms immediately reportable and routine reportable matters (IRMs and RRMs, respectively).

This report examines trends in IRMs that involved regular public transport operations and provides a context for interpreting any changes over time. The aim is to inform the aviation community of any important safety trends, and to provide the travelling public with a better appreciation of the types of occurrences that are reported to the ATSB.

The study found that high-capacity regular public transport operations dominated air transport activity, and consequently dominated the reports of IRM occurrences. Furthermore, activity for high-capacity air transport operations, measured by flying hours and movements, increased over the period studied.

The IRM categories examined were either stable or trended downwards between mid 2001 and mid 2006. Violations of controlled airspace reduced over the period while occurrences involving a fire, explosion or fumes and crew injuries or incapacitation also decreased, but only marginally. Other IRM categories such as contained engine failures and fuel exhaustion events were rare, or absent. The exception was breakdowns of separation (BOS) and airprox events, where occurrence numbers went up. However, the rate did not increase relative to the number of movements, suggesting that the increase was largely linked to increased activity.

This review highlighted the consistent reporting culture of the air transport sector and the air traffic service provider, and provided encouraging data concerning the general state of safety in regular public transport operations.

The purpose of this publication is to provide a national overview of serious non-fatal injury in Australia due to transport accidents involving a railway train in the period 1999-00 to 2003-04, including level crossing accidents. The definition of transport injury used in this report excludes injuries given an external cause of intentional self harm, assault or undetermined intent (terms that are defined in the report).

This report includes all injuries that were serious enough to require hospitalisation but did not result in death.

Publication available from the Australian Institute of Health and Welfare website(Opens in a new tab/window)

Reciprocating-engine powered low-capacity transport aircraft (8 to 10 passengers) provide an important public transport connection throughout regional Australia. In the period January 2000 to December 2005, twenty powertrain structural failures of high-power (300 to 375 brake horsepower) horizontally-opposed, reciprocating engines were associated with air safety occurrences reported to the ATSB. These occurrences ranged in severity from; in-flight engine shutdown; engine failure and forced landing; engine failure combined with in-flight fire and fracture of both upper engine mounts; to the fatal accident of a regular public transport flight following the structural failure of both engines to ditching at night. It is evident that the reliability of high-power reciprocating engines is an important requirement for the safe operation of this class of aircraft. This research investigation is a study of the factors that affect reciprocating engine reliability.

The study found that powertrain structural failure was not restricted to one engine model, one engine manufacturer, or one powertrain component. The events that initiated sequences that led to engine in-flight failure could be grouped into three categories: combustion chamber component melting; bearing breakup; and powertrain component fatigue cracking. Analysis of the factors that were associated with each category of initiating event revealed that powertrain component reliability is affected by the development of shockwaves during combustion, the response of bearings to boundary lubrication and out-of-plane alternating loads, the increase in component alternating stress magnitudes, and creation of stress-concentrating features in components during engine operation. These factors may act singly, but on many occasions it is the synergistic effect of the presence of multiple factors that result in a sequence of events ending with engine in-flight failure.

The recurrence of powertrain component structural failure events suggests that the corrective actions that are a part of the airworthiness assurance system may have been ineffective. Corrective action is dependent on accurate analysis and feedback. It is evident that analysis is affected by the complexity of reciprocating engine systems and feedback requires a broad view of the interaction of systems and a detailed view of the components of a system.

This report was tabled in the Australian Parliament on 23 November 1999

Class G airspace (or uncontrolled airspace) has the lowest level of service and the fewest restrictions on aircraft operations. In Australian Class G airspace, third-party directed traffic information is provided to pilots of aircraft operating under the instrument flight rules.

There have been a number of attempts to change the operation of Class G airspace since its introduction in 1995. As part of the Airspace 2000 program, the Civil Aviation Safety Authority (CASA) decided to conduct a 'Class G demonstration' featuring:

• implementation of a national advisory frequency;
• provision of a conditional radar information service;
• cessation of directed traffic information.

The demonstration commenced on 22 October 1998 in the airspace between Canberra and Ballina below 8,500 ft. An end date was not specified; rather, the Authority intended that the demonstration airspace procedures should be extended throughout Australia in June 1999.

The demonstration was conducted in the highest traffic density area of Class G airspace in Australia. The timing and location of the demonstration placed significant pressures on the Civil Aviation Safety Authority to ensure that consultation, safety analysis and education activities were comprehensively addressed.

Following receipt of over 70 air safety incident reports BASI concluded that a safety deficiency existed and commenced an investigation on 5 November 1998 into the systemic issues associated with the development and operation of the Class G airspace demonstration.

The Bureau identified a number of operational deficiencies that contributed to an increased safety risk for users of the demonstration airspace. Following an interim recommendation issued by BASI on 8 December 1998, the demonstration was terminated by the Authority on 13 December 1998.

In addition to the operational deficiencies already noted, a number of organisational factors adversely affected the ability of CASA to effectively manage the Class G airspace demonstration project. Moreover, the division of roles and responsibilities between CASA and Airservices Australia regarding the design and regulation of airspace was not clearly defined.

Safety deficiencies identified during the course of the investigation formed the basis for safety recommendations developed by BASI. The recommendations called for a review of program management policies and procedures for current and proposed changes to the aviation system; a review of corporate governance issues; and clarification of the roles and responsibilities of respective organisations in relation to the regulation, design and management of airspace to ensure the safety integrity of the aviation system.

A full description of these safety actions can be found in Part 4 of the complete report.

This study provides an overview of accidents involving private aircraft operations between 2001and 2005. With approximately 400,000 flying hours conducted annually, private flying accounts for around a quarter of general aviation activity. Within private operations, rotary-wing activity now contributes about 10 per cent of all hours flown.

The accident rate in private aviation activities generally declined over the five-year study period, but the fatal accident rate for fixed-wing aircraft remained generally stable. There was an apparent increase in the rotary-wing fatal accident rate.

The pattern of accident types showed similarities for both fixed-wing and rotary-wing aircraft. Most accidents can be classified against a small number of accident types: collisions, loss of aircraft control, airframe, and powerplant issues. Additionally, collision accidents and those involving a loss of aircraft control account for most of the fatal accidents.

Differences between fixed-wing and rotary-wing aircraft occurrences are more apparent when accidents are examined by phase of flight. More than half of all fixed-wing accidents occur in the landing phase of flight, but manoeuvring and cruise are among the most common phases of flight for accidents involving rotary-wing aircraft. These phases of flight are also associated with fatal accidents.

There is increasing recognition that methods which proactively monitor airline safety may be useful in preventing air safety occurrences. Proactive rather than reactive safety programs are particularly important, considering the high social and economic costs of airline accidents to the community. However, in the aviation industry, there are currently few formal proactive safety management systems in use, and none that reliably demonstrate the desirable goal of improving safety performance. This paper outlines a new proactive safety method for the airline industry, called INDICATE (Identifying Needed Defences In the Civil Aviation Transport Environment).

INDICATE is an airline self-management safety tool which encourages regular passenger transport operators to critically evaluate and continually improve the strength of their safety system. INDICATE also provides a formal communication channel for airline operators to regularly identify and report current weaknesses in aviation regulations, policies and standards to the Bureau of Air Safety Investigation (BASI), before they result in an accident. A major Australian regional airline is currently trialling INDICATE, so that an evaluation of its effectiveness and application to the wider aviation industry can be established. Preliminary results from this trial are presented.

Controlled flight into terrain (CFIT) has been identified as one of 'aviation's historic killers', claiming the lives of more than 35,000 people since the emergence of civil aviation in the 1920s. The purpose of this report was to provide an overview of CFIT from an international perspective, to examine current and potential CFIT preventative strategies, and to specifically identify those characteristics associated with CFIT in Australia.

A search of the Australian Transport Safety Bureau's (ATSB) aviation safety database identified 25 CFIT accidents and two CFIT incidents in the period 1996 to 2005. General aviation accounted for the greatest proportion of CFIT accidents, fatal accidents and fatalities. Only one CFIT occurrence over the reporting period (VH-TFU, Lockhart River, Queensland, 7 May 2005) involved regular public transport operations, but this accident accounted for nearly one-third of all CFIT fatalities. This highlights the catastrophic impact one CFIT accident involving passenger operations can have.

In line with international experience, nearly two-thirds of CFIT accidents and incidents in Australia occurred in the approach phase of flight, of which half of these were during an instrument approach.

When compared with the total number of accidents recorded by the ATSB over the 10-year period, the results of the study indicate that CFIT in Australia is a rare event. However, when CFIT does occur, the likelihood of it resulting in fatalities is high.