Sunday, July 1, 2007
Rotorcraft Report: NTSB Blames Air Tour Crash on PMA Part
The "inadequate design" of a key PMA engine part caused the June 2003 crash of an air tour McDonnell Douglas 369D in Hawaii that killed four, the U.S. National Transportation Safety Board has concluded.
The Tropical Tour Helicopter aircraft crashed June 15, 2003 onto a rugged, hardened lava flow following a power loss during cruise. According to the safety board, other helicopter pilots heard the pilot make a mayday call with the clear "engine out" audio warning in the background. A post-crash fire consumed most of the wreckage, the NTSB said; an examination of what remained of the airframe revealed no evidence of a preimpact malfunction.
The board ruled May 29 that the crash of the followed the fatigue fracture and separation of the compressor coupling adapter, which disconnected the compressor from the turbine section of the Rolls-Royce 250 engine. The adapter, which the board said was produced under an FAA parts manufacturing authority (PMA) "by a company other than Rolls-Royce," failed due to "the inadequate design of the coupling and the coaxial misalignment of the spur adapter gear, compressor-coupling adapter, and compressor impeller during recent engine maintenance where the gearbox was removed and replaced." The board’s inspection of the engine and metallurgical examination found the compressor-coupling adapter had failed above the shear point due to fatigue cracking initiated by fretting on the pilot diameter.
The board added that Rolls-Royce had records of "12 other compressor-coupling adapters that had fractured and failed, with all instances of the failure/fracture occurring in this specific new coupling design, which was significantly different than the previous design."
The nature of the fretting and fractures indicated that the newly designed couplings have a small amount of longitudinal movement that is occurring between the outer diameter of the compressor-coupling adapter and the inner diameter of the impeller, which was not a factor in the previous design, the NTSB said. If the spur adapter gear, compressor-coupling adapter, and compressor impeller (all coaxial spline joints) were in alignment, there would be no significant longitudinal movement of the pilot diameter. "However, any axial misalignment of these components during engine buildup, for example, could induce a misalignment that would result in relative motion between the components with each engine rotation. The misalignment would also induce bending stresses into the compressor-coupling adapter in addition to torsion stresses that could result in fatigue." One month prior to the accident, company maintenance personnel had changed out the engine gearbox due to finding a loose stud on the gearbox to turbine section mounting. Changing the gearbox would involve disturbing the coaxial spline joints between the spur adapter gear, compressor-coupling adapter, and compressor impeller.