AAIB: Incident highlights value of SB compliance

By Malcolm Payne

October 1, 2008
Accidents

A Jan. 20, 2007 incident over the English Channel resulted in the UK’s Air Accidents Investigation Branch (AAIB) recommending the FAA mandate operators adhere to a service bulletin Honeywell issued subsequently in late 2007. Falcon 900B G-HMEV had departed Farnborough en route to Tel Aviv. When the aircraft reached FL130 over the English Channel, there was a loud noise from the rear of the aircraft. Shortly afterward the engine fire aural warning sounded and the number-three engine fire warning light and the ITT warning light illuminated. Declaring an emergency, the crew carried out the engine fire procedure and after the first extinguisher shot the warning ceased. Ten minutes later, the crew landed the airplane at London Gatwick, the nearest airport with full emergency services.

The AAIB investigation found that the low-pressure turbine assembly on the TFE731-5BR-1C turbofan had suffered a major break-up, with debris penetrating the engine casings and cowlings, while causing light damage to the horizontal stabilizer. The fire warning that was recorded soon after the break-up was triggered by hot engine gases escaping onto the firewire element through a gap caused by the break-up. A substantial number of the failed parts were lost overboard into the sea. The remaining components indicated that fracture of a low-pressure turbine blade had led to further internal damage and there was indication of a casting defect in one of the Stage 2 blades. This probably initiated the failure.

Previous Fan Blade Failures
There have been 65 reported failures of Stage 3 blades in the TFE731, some 66 percent being on the -5B. The IN100 alloy used to manufacture the blades is sensitive to any degree of surface damage, such as stone chipping during ground maintenance runs. In addition, the low-pressure turbine blades are cast using molten IN100 to fill a mold in a vacuum furnace, the rate of cooling being controlled by the mold insulation and the environmental temperature. Occasionally, unsatisfactory cooling of the casting develops internal cracking, which should be detected by inspection. Experience gained from in-flight failures suggests that, on occasion, Stage 3 blade failure could initiate a “domino effect” with other blades in the stage and, in turn, Stage 2 and Stage 3 stators could spin up and burst, causing damage to the interstage turbine transition duct (ITTD).

The Garrett (later Honeywell) TFE731 was first certified in 1972 as the -2 version and development produced the -3,-4 and -5 versions, these being known as the “Classic” models. Further development produced the -20, -40, -50 and -60, known as the NG (Next Generation) models.

In September 2007, engine manufacturer Honeywell issued a service bulletin recommending the modification of the Classic ITTDs by replacing the rear portion with an Inconel heat-resisting alloy section as already in service with the NG series.
In its Safety Recommendation, the AAIB suggests that the FAA “require the timely incorporation” of Honeywell’s service bulletins regarding the ITTDs on the Classic engines.

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