Incident Description
An Aerospatiale AS 350 B3 helicopter (registration C-FRPQ, serial number 3636), operated by the Royal Canadian Mounted Police, was en route from London, Ontario to Windsor, Ontario at 2000 feet above sea level. At approximately 0755 eastern standard time, the helicopter yawed sharply to the right, rotor rpm dropped, the engine chip and governor light illuminated, and the warning horn sounded. The engine, a Turbomeca Arriel 2B, had failed and the pilot commenced an autorotation landing into a farm field. During the descent, the pilot transmitted a mayday call and activated the emergency locator transmitter. The helicopter landed without further incident. There were no injuries and the helicopter was not damaged. The pilot completed the shutdown checklist and switched off the battery.
Weather and Pilot Information
Weather in the area at the time of the incident was reported as good with scattered clouds. The visibility at 2000 feet was approximately 6 miles; winds were out of the north at approximately 22 knots and the temperature was −3°C. Records indicate that the pilot was certified and qualified for the flight in accordance with existing regulations. He had been on duty for 1 hour prior to the occurrence following 13 hours off duty.
Engine Examination
An examination of the engine determined that the 41-tooth bevel gear (part number 0292127330) had fractured due to high-cycle fatigue cracking. The gear was installed during engine manufacture and had accumulated a total of 1644 hours since new. Metallurgical examination (TSB Laboratory report LP 005/2008) of the bevel gear revealed numerous fatigue cracks radiating from the roots of many of the gear teeth. Circumferential fatigue cracks were also observed in the rim of the gear. There were no relevant manufacturing flaws found in the gear that could contribute to the failure.
Accessory Gearbox and Failure Mode
The 41-tooth bevel gear is located in the accessory gearbox module 1 (MO1) and has a 20-tooth idler gear mounted to its shaft. The MO1 is composed of the accessory gear box and the transmission shaft. The starter-generator output shaft is coupled to the compressor through a series of bevel gears that includes the 41-tooth gear. During the starting sequence, the starter-generator drives both gears, which in turn drive the compressor and accessories. Once the engine is running, the compressor drives the 41-tooth bevel gear which, in turn, drives the accessories and starter-generator. Failure of the 41-tooth bevel gear will decouple the engine from the accessories and result in an uncommanded engine shut down. Once the engine is shut down, it cannot be restarted.
Other Failures and Contributing Factors
There have been eight other reported failures of the 41-tooth bevel gear. The initial five failures occurred on low-time engines and were attributed to improper shimming. The remaining four failures were all on engines equipped with APC starter-generators. Discrepancies were found with the dampening system in three of the examined starter-generators, indicating they were not operating in accordance with their design specification. Vibration tests indicated a vibration spike at approximately 9300 rpm for the occurrence starter-generator. Torsional oscillations were observed when using an APC starter-generator with an over-tightened dampening system, as in the occurrence helicopter. These oscillations affected the 41-tooth bevel gear and can have a detrimental effect on its fatigue life. No metallurgical anomalies were found that would have contributed to the high-cycle fatigue failure. Though there was a discrepancy found with the torque dampening system of the starter-generator, it did not affect the operational ability of the starter-generator nor would it have presented itself as an identifiable vibration during normal operation.
Safety Actions
Eurocopter issued alert service bulletins AS 350 No. 80.00.07 and EC 130 No. 80A003, both dated 19 December 2008, to check for proper adjustment of the torque dampening system on Unison starter-generators. Turbomeca issued Service Bulletins No. 292 72 0325 (Arriel 1) and No. 292 72 2090 (Arriel 2) to introduce a 41-tooth bevel gear with a thickened rim, aimed at improving tolerance to dynamic stress from high electrical power. However, Turbomeca concluded that these modifications do not resolve failures related to incorrectly adjusted dampening systems. The European Aviation Safety Agency issued Airworthiness Directive 2009-0004 requiring mandatory compliance with the service bulletins.