The US Federal Aviation Administration has issued an emergency airworthiness directive (AD) which requires inspections of CFM International CFM56-7B turbofans that are used to power newly delivered Next Generation Boeing 737s.
The move follows two inflight engine shutdowns on 26 June, both caused by failures in the accessory gearbox (AGB).
One incident involved Russian airline Transaero's first 737-700, which is on lease from Bavaria Fluggesellschaft, while flying near Moscow. The other occurred on a Braathens 737-700 shortly after beginning a descent into Oslo, Norway, following a flight from Trondheim. Both aircraft landed safely and there were no injuries, although they were grounded for several days while replacement AGBs were fitted.
In each case, the accessory gearbox starter-shaft spur gear failed because of high stresses introduced during production. The FAA says that a lack of shotpeening, a manufacturing process that relieves stress and prevents fatigue crack propagation, was the primary cause. CFMI subsequently modified the manufacturing process at the Paris plant of Snecma subsidiary Hispano-Suiza, which produces the AGBs.
Airlines are required to inspect magnetic chip detectors on the engines every other operating day and replace the starter gearshaft if abnormal levels of magnetic particles are found. This takes 12h and costs about $10,000. All No 2 engine starter gearshafts must be replaced by 1 August, or within 350 flight hours, whichever comes first, while No 1 engine gearshafts must be substituted by 1 September, or within 725 flight hours, whichever occurs first.
The FAA says the AD affects 47 aircraft, including those of Continental Airlines (nine), Eastwind Airlines (one) and Southwest Airlines (13). Other operators include Denmark's Maersk Air, one of whose aircraft suffered an unrelated, one-off engine failure in early May - the first major failure of a -7 engine. CFMI says the replacement of affected AGBs will be complete by early 1999.
A similar AGB problem had been identified in December 1997 affecting a limited number of CFM56-3s, powering 737-300/ 400/500s and some -5s, which power Airbus A320 family aircraft and the A340.
This problem was caused by the use of over-concentrated acid during cleaning and prompted the FAA to issue an AD which required airlines to replace or modify affected engines.
Source: Flight International
