Piper PA-32-300 N154DC accident near Prescott Valley, Arizona

Probable cause

Separation of the No. 3 engine cylinder due to a loss of torque on the cylinder base nuts, which resulted in a loss of engine power and in-flight fire. Contributing to the accident was maintenance personnel’s failure to ensure the engine case and cylinder base nuts were properly torqued during the most recent annual inspection.

NTSB analysis

The pilot was conducting a local flight to try to replicate a fuel mixture leaning problem he experienced on the previous flight. The pilot experienced the leaning issues again and recognized unusual exhaust gas temperature (EGT) and fuel flow readings during the flight, so he elected to return to the airport.

During the return to the airport the pilot heard two loud “booms” before the engine lost total power. Shortly after, smoke and flames appeared inside the cabin near the forward right floorboard. The pilot successfully extinguished the fire and performed a forced landing to an open field. The forward right area of the fuselage was substantially damaged.

Examination of the engine revealed the No. 3 cylinder had separated from the engine case; the No. 3 connecting rod had fractured and mechanical damage was found to the engine case, camshaft, and crankshaft. Five of the eight No. 3 cylinder base nuts were not located. The examination revealed signs of fretting between both engine case halves, as well as on each of the six cylinder’s hold-down plates, consistent with a loss of torque of the engine case nuts and bolts and the cylinder base nuts. No other pre-impact anomalies were noted with the engine.

Damage to the engine was consistent with the No. 3 cylinder base nuts losing their torque and the nuts coming off the studs, which enabled the cylinder barrel to come off the cylinder piston, resulting in mechanical damage to the studs, No. 3 connecting rod, engine case, camshaft, and crankshaft. The damage resulted in a fire originating in the engine compartment.

Although the investigation did not determine why there was a loss of torque of the engine case nuts and bolts or the cylinder base nuts, the engine had operated about 72 hours since undergoing an annual inspection. FAA guidance for an annual inspection includes an inspection for visual evidence of excessive oil, fuel, or hydraulic leaks, and sources of such leaks, and directs inspection of engine studs and nuts for improper torquing. Accordingly, any improperly torqued engine studs or nuts should have been identified by the annual inspection.

Text from the NTSB. Open the NTSB docket for WPR25LA030 for the full record.