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Factual Information


Factual Information

At 1100 Central Standard Time on 16 April 2004, a British Aerospace Plc, J32, Jetstream aircraft registered VH-OAE, with 2 pilots and 19 passengers on board, was on descent, during a scheduled passenger flight from Melbourne, Victoria to Mount Gambier, South Australia. As the aircraft passed through flight level (FL) 140, approximately 37 NM from Mt Gambier, the crew reported hearing a bang from the right engine. Simultaneously, the aircraft yawed to the right and they heard something impact the right side of the fuselage. Some smoke was evident in the cockpit.

A check of the aircraft's engine instruments confirmed a problem with the right engine and the crew shut down the engine and feathered the right propeller in accordance with the operator's quick reference handbook drills. The crew then advised air traffic control of the situation and continued for a landing at Mount Gambier Airport.

An inspection of the aircraft by the operator revealed that there had been an uncontained failure of the propeller reduction gearbox on the right TPE 331-12UHR-702H turboprop engine, serial number P66338C. There was also evidence of impact damage on the right side of the fuselage, below the co-pilot's side window area. That impact had not breached the aircraft's pressure hull.

An examination of the engine, supervised by the Australian Transport Safety Bureau (ATSB), found that a section of the spur gear teeth from the outer rim of the reduction gearbox bull gear, had detached during engine operation (See Figure 1). Spur gear teeth are radial, uniformly spaced around the gear's outer periphery and parallel to the shaft axis1. The detached section of gear had penetrated the diaphragm housing (intermediate gearbox housing) and the gearbox accessory case, before exiting the engine through the compressor air intake.

Figure 1: Cutaway diagram of TPE 331 reduction gearbox

Figure 1: Cutaway diagram of TPE 331 reduction gearbox

The hole in the accessory case had allowed engine oil to escape and flow over the engine cowling, with metallic debris and oil entering the engine's compressor intake. Once inside the compressor, the oil was able to enter the aircraft's compressor bleed air system that supplied the aircraft's air conditioning and pressurisation air. There was also significant associated damage to the diaphragm housing, the high speed pinion and compressor/turbine main shaft, with metallisation2 observed on the turbine and exhaust sections.

An ATSB Technical Analysis report (Appendix A), on the mode of failure of the bull gear, part number 3108295-1, found that the gear had failed as a result of a mechanism known to the manufacturer. The report indicated that the progressive propagation of high cycle fatigue cracking within the gear web and rim transition region, had caused a section of the gear rim to separate from the gear.

The engine manufacturer had introduced several changes to the bull gear design to address 'reliability and reparability issues' that had occurred in the TPE 331 engine type. Among those were changes in gear relief, gear tooth roots had been ground and shot peened to improve fatigue life, the gear rim inside diameter was shot peened to increase fatigue resistance and a coating was added to the gear web to dampen gear vibrations. The engine manufacturer reported that despite those actions some of the re-worked and coated gears had a higher failure rate than non-reworked gears.

The engine manufacturer also investigated TPE 331 engine diaphragm housings, in which gears had failed, to ascertain if distortion of the housing could cause bull gear to pinion gear misalignment. Several problems were identified with those housings that may have contributed to the gear failures. These included bull gear to pinion gear centreline growth and misalignment, growth between diaphragm to gearbox alignment pins and out-of-round bearing bores.

In October 2001, the engine manufacturer issued Service Bulletin (SB) A72-20873 in response to 16 in-service bull gear rim separations and 13 high speed pinion torque shaft failures. Four of those failures resulted in gearbox debris being ejected from the engine. One failure resulted in the penetration of the right side of an aircraft's pressure hull by a gear fragment. The bulletin indicated that high tooth loading on the bull gear to high speed pinion mesh, bull gear tooth profile, and distortion of the intermediate gearbox housings, had resulted in abnormal wear and subsequent failure of the assemblies.

Service Bulletin A72-2087 required replacement of the bull gear and high speed pinion with new, zero-time components, at intervals not to exceed 3,600 hours in service. It also required the inspection and the rework/overhaul of some gearbox components such as the diaphragm housing, plus a more stringent periodic inspection of specified gearbox components to ensure an optimum operating environment for the bull gear. At the time of failure, the bull gear assembly in this engine had accrued 1,199.55 hours and 1,523 cycles since installation. The engine had accrued a total of 10,755.7 hours and 12,295 cycles since new.

In Australia, the Civil Aviation Safety Authority (CASA) issued Airworthiness Directive (AD) AD/TPE 331/57 to require compliance with SB A72-2087. That AD became effective on 31 October 2001. Amendment 1 to that AD was issued in January 2002. The AD actions had been incorporated into the occurrence engine at the manufacturer's German maintenance facility on 20 December 2002.

Information received from the engine manufacturer following this occurrence, indicated that there had been three bull gear failures in post SB A72-2087 engines. One of those failures was the subject of a UK Air Accidents Investigation Branch (AAIB) investigation, published in AAIB Bulletin number 7/2005. Information from the AAIB on that failure indicated that the bull gear had failed in a similar manner to the gear in this occurrence.

The engine manufacturer reported that a spectrometric oil and filter analysis program (SOAP)4 was used to monitor an engine's in-service condition and to reduce the possibility of a premature mechanical failure. That program monitored the type and quantity of the deposits in the engine oil and oil filters over a specified period. A trending feature within that program could highlight an engine with a rapidly increasing filter 'weight' and indicate that further maintenance action was required. A high filter weight quantity of Carbon Steel in a sample could indicate a problem with the bull gear assembly. In November 2000, the engine manufacturer issued Alert Service Bulletin TPE 331-A79-00345 that changed the SOAP interval periodicity to a fixed 100+/- 20 engine hours to minimise variability. On 25 January 2001, CASA issued AD/TPE 331/55 that required Australian compliance with that Alert SB.

The operator had complied with the engine manufacturer's and CASA's SOAP requirements, forwarding samples to the manufacturer's approved venue for testing. The operator reported that they had become concerned about a SOAP report for the occurrence engine that had been received on 3 February 2004. That report, although still within the manufacturer's 'normal sample' guidelines, had a significantly higher filter weight result than had been previously noted for the engine. When queried, the manufacturer confirmed the results of the sample and indicated that a higher reading may be seen following an engine oil change. The engine oil had been changed 101 engine hours prior to that sample being taken. In the subsequent SOAP sample taken 60 engine hours later, on 24 February 2004, the filter weight had returned to a similar level to that of the pre-3 February 2004 samples. The final sample taken prior to the occurrence, on 1 April 2004, was higher than usual and all of the samples had traces of carbon steel.

1. ASM International, Materials Information Society Handbook, Volume II.

2. Metal pulverised by the compressor becomes molten or burned in the combustion chamber and flows rearward, attaching to the turbine and exhaust assemblies (US Department of the Air Force.(1987). Safety Investigative Techniques (AF Pamphlet 127-1, Volume II). Washington DC: Author).

3. Honeywell Alert Service Bulletin - TPE 331-A72-2087, ENGINE - REDUCTION GEAR AND SHAFT SECTION - Replace Gearshaft (Sun and Bull Gear) Assembly, Part No. 3107037-9/10, 3107122-1, 3107162-1, or 3108222-1, or 3108294-1 with Part No. 3108294-1, issued October 2001 and revised 16 November 2001.



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