HANA, HI, USA
N8625F
McDonnell Douglas 369D
During flight, the pilot heard a loud noise from the engine compartment, accompanied by the engine out light & horn. He entered an autorotation, & the helicopter rolled over during touchdown on mountainous terrain. Disassembly of the engine revealed the forward splines of the Spur Adapter Gearshaft (SAG), which connected the compressor & turbine module shafts, had fractured, & the compressor module was disconnected from the turbine section. The SAG retaining ring was found to have machined into the compressor impeller, which allowed the SAG to shift aft. Aft movement of the SAG then resulted in heavy wear on the splines of both the Compressor Adapter Coupling (CAC) & the SAG, which resulted in a fatigue failure of the SAG splines. The CAC & impeller were of an old configuration. Allison Commercial Engine Bulletin 1325 recommended replacement of the old CAG, SAG, impellers, & retaining rings with models of a newer design to 'improve spline wear characteristics.' Allison engineers opined that the typical retaining ring to impeller machining was due to the same retaining ring being returned to the engine during overhaul, though evidence of this was not apparent in the engine examination.
On November 12, 1996, about 1450 hours Hawaiian standard time, a McDonnell Douglas 369D helicopter, N8625F, rolled over on touchdown from an autorotation near Hana, Maui, Hawaii. The autorotation was precipitated by a loss of engine power in cruise. The aircraft was operated by Alika Aviation, Inc., dba Alexair, of Kahului, Maui, Hawaii, and was engaged in an island sightseeing flight under 14 CFR Part 135. Visual meteorological conditions prevailed at the time and a company flight plan was filed. The helicopter sustained substantial damage. The certificated commercial pilot and the four passengers were not injured. The flight originated from Kahului on the day of the accident about 1500 as a local for-hire sightseeing flight. In a written statement, the pilot reported that he was in cruise flight when a loud explosion came from the engine compartment. The noise was accompanied by the engine out light and horn. The pilot entered an autorotation and the helicopter rolled over on touchdown. Following recovery of the helicopter, the Allison 250 C20B engine was disassembled at an engine overhaul shop under the supervision of an FAA airworthiness inspector from the Honolulu Flight Standards District Office. Disassembly revealed that the Spur Adapter Gearshaft (SAG), which connects the compressor and turbine module shafts, was fractured. The forward splines of the SAG, which fit into a splined female Compressor Adapter Coupling on the compressor shaft, were fractured off the shaft and the compressor module was therefore disconnected from the turbine section. The engine components were then shipped to the Allison Engine Company in Indianapolis for detailed examination under the supervision of the Indianapolis FAA Manufacturing Inspection District Office. According to the Allison report of the examination, the SAG retaining ring was found to have machined into the compressor impeller, which allowed the SAG to shift aft. The aft movement of the SAG then resulted in heavy wear on the splines of both the CAG and SAG, which resulted in a fatigue failure of the SAG splines. The SAG, CAG, and impeller were examined by the Allison metallurgical laboratory and found to be within chemical and manufacturing specification. During the examination, the CAG and impeller were noted to be of older and superseded part numbers. Allison Comercial Engine Bulletin 1325, dated March 15, 1994, required the replacement of all old series CAG, SAG, impellers and retaining rings with models of a newer design to "improve spline wear characteristics." The new CAG is of a longer design and features an increased mating/contact area between the CAG and SAG splines. Compliance with the bulletin was listed as "no later than the next overhaul when the compressor impeller is replaced, or earlier as a repair." Review of the engine maintenance records disclosed that the compressor assembly was last inspected and repaired on August 12, 1996, in response to an oil over temperature event. At the time, the assembly had accrued 5,902 hours total time and 2,382 hours since last overhaul. The factory time between overhaul period for the compressor is 3,500 hours. The impeller was installed in the compressor assembly at the last overhaul on November 3, 1989. The helicopter daily maintenance logs disclosed that a succession of five engine magnetic chip lights occurred between September 30 and November 7, 1996. Two warning lights came on during flight and three during engine start-up. Flakes and metal paste were found on each occurrence, with the flakes described as "less than 1/32 inch in diameter." The entries note that in accordance with the Allison Overhaul and Maintenance manuals, the plugs were cleaned and the engine test run for 30 minutes in response to each event. According to Allison, the calendar day and flight hour interval between each magnetic chip light was outside of the parameters which would have required a mandatory teardown inspection of the engine.
subsequent failure of the spur adapter gearshaft (SAG), after 'retaining ring to compressor impeller machining' occurred and allowed the SAG to shift aft. The mechanism which induced the ring to impeller machining was not determined. Mountainous terrain in the emergency landing area was a related factor.
Source: NTSB Aviation Accident Database
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