N806RB

On August 20, 2003, at 1015 central daylight time, a Boeing A-75, N806RB, operated by Schwan's Aerial Marketing, sustained substantial damage during a forced landing to a cornfield after a total loss of engine power while on final approach to the La Crosse Municipal Airport (LSE), La Crosse, Wisconsin. The airline transport pilot was not injured. The 14 CFR Part 91 flight departed the Lawrence J Timmerman Airport (MWC), Milwaukee, Wisconsin, at 0845, with LSE as the destination airport. Visual meteorological conditions prevailed. No flight plan was filed.

The pilot reported he was about three miles from LSE when the engine "suddenly began vibrating." He reported the engine seized and the propeller stopped turning 10 seconds after the vibration started. He executed a forced landing to a cornfield, and the airplane nosed over during landing roll. The pilot exited the airplane without injury.

The 450 horsepower Pratt and Whitney R-985 radial engine had 1,150 hours since the last overhaul. It was shipped to Covington Aircraft, Okmulgee, Oklahoma, for inspection and engine teardown. The inspection revealed that the crankshaft was fractured in two pieces, with the fracture between the crankshaft rod journal and the front main bearing. Fatigue stress crack progression was observed on approximately 50 percent of the fracture surface. The cracking appeared to originate in the forward radius of the master-rod bearing journal. The remainder of the fracture surface exhibited conditions typical of tensile overload. The reason for the initiation of the crack in the journal radius of the crankshaft was not identified. (See Covington Aircraft, Radial Division, Engine Investigation Report #REIR9616)

The maintenance manager for the aircraft operator reported that when the accident occurred, they had another airplane with an engine that had about 1,220 hours of operation since the last overhaul. That engine was also shipped to Covington Aircraft for teardown. The results of that inspection revealed that there was no damage to the crankshaft or other engine parts.

The engine manufacturer's recommended time between overhaul (TBO) for the R-985 engine is 1,600 hours. The maintenance manager reported that the operator had voluntarily imposed a 1,400 hour TBO on the engines they operated when they first started using the R-985 in 1984. The maintenance manager reported that as a result of the crankshaft failure to N806RB, they imposed a TBO of 1,200 hours on the engines. He reported that the crankshaft failure was the only crankshaft failure in 90,000 - 95,000 hours of fleet operation. He also reported that the flight team pilots were restricted from performing snap rolls except during air shows and air show practice to reduce the stress placed on the crankshafts.