Accident Overview
Visual meteorological conditions prevailed for the planned cross-country flight. The pilot obtained a weather briefing, filed an IFR flight plan, and received an ATC clearance. Approximately 8 minutes after takeoff, radar indicated the airplane was at 11,200 feet msl, heading 241 degrees, with a ground speed of 180 knots. No distress calls or additional communications with the pilot were recorded, and radar contact was subsequently lost. The airplane impacted the ground in an uncontrolled descent.
Wreckage and Component Examination
Examination of the wreckage revealed that the right wing tip tank separated from the airplane and was found 0.18 nautical miles from the main wreckage. The landing gear and flaps were in the retracted position. A teardown and examination of both engines disclosed that the type and degree of damage indicated engine power section rotation and operation at the time of impact. There were no complete systems intact due to the impact sequence and post-impact fire, which consumed the aircraft.
The portion of the right propeller shaft coupling found at the site was fractured through 360 degrees. Metallurgical examination revealed that the propeller shaft coupling failed in fatigue. The presence of fatigue cracks indicated the coupler fractured in fatigue in service, not from ground impact. The circumferential fracture intersected the ends of several internal spline teeth. The origin of the fatigue crack could not be determined due to severe corrosion damage on the fracture surface. Fatigue propagation was in the aft direction and from the inside to the outside of the coupling. The engine core rotating components would have been free to rotate when uncoupled from the propeller shaft.
Maintenance records indicated that the failed coupling had accumulated approximately 4,000 hours since new and 1,250 hours since engine overhaul in 1989. Since 1990, the manufacturer introduced several design changes for the propeller shaft coupling via optional Service Bulletins, to be accomplished at the next access or hot section inspection (HSI). Impact and thermal damage of the right propeller precluded a determination of the in-flight blade angles. Calculations by the airplane manufacturer indicated that "the [intact] airplane was capable of continued flight" with the right propeller feathered, and that the "airplane can keep attitude, but cannot climb and cannot maintain altitude" with the right propeller in the flat pitch or windmilling positions, respectively.
Metallurgical examination of the component brackets and associated bolts from the right tip tank revealed that the separation resulted from a single-event overstress fracture of both the forward and aft tank attachment fittings. Calculations showed that a 3.763 radians per second (35.9 RPM) spin rate would cause the failure of the forward wing fuel tank attachment fitting. There had not been a previous in-flight separation of a wing tip fuel tank on this model airplane.
Probable Cause
The official probable cause of the accident was the pilot's failure to maintain airplane control following a loss of right engine power, which resulted in impact with terrain in an uncontrolled descent. A contributing factor was the loss of right engine power as a result of the fatigue failure of the propeller shaft coupling.