Piper PA28R N3790T accident near Moosic, Pennsylvania

Probable cause

The inadequate torquing of anchored-thru and free-thru studs during installation of the No. 2 cylinder, resulting in their fatigue fracture and subsequent total loss of engine power. Contributing to the undershoot landing was the pilot’s failure to divert to closer airports than the original diversion airport after experiencing engine performance issues.

NTSB analysis

During a personal cross-country flight, the owner seated in the right front seat noted the No. 2 cylinder exhaust gas temperature had decreased about 1/2 of its value. The pilot-in-command (PIC) and owner both then felt a vibration and positive control of the airplane was transferred to the owner as was briefed before departure. An emergency was declared with air traffic control while (best glide speed) airspeed was maintained. The PIC reported there were no suitable airports nearby and proceeded to a diversion airport 77 miles away.

While continuing toward the diversion airport, the oil pressure decreased to zero. Realizing the flight would not reach the airport, the pilot maneuvered for a forced landing to a residential street and collided with a powerline. The accident site was located less than 2 nautical miles north-northeast from the approach end of the intended runway at the diversion airport.

Postaccident examination of the engine revealed that all anchored-thru or free-thru studs that secured the No. 2 cylinder to the crankcase exhibited fatigue. The fatigue originated at the roots of the threads and propagated though about 70% to 80% of the diameter of the studs before finally fracturing due to overstress. The fatigue-fractured studs led to the cylinder separating from the engine, the loss of oil supply, and subsequent total loss of engine power.

Although the pilot was not aware of the extent of the problem with the No. 2 cylinder, about the time the problem arose, the flight was about 30 nautical miles west-northwest of an international airport with available runways of sufficient length. As the flight continued in a southerly direction, about the time and location the oil pressure decreased to zero, another airport with a runway of more than adequate length was located 20 nm closer than the intended diversion airport. Thus, had the pilot diverted to the nearest airport immediately after the loss of the No. 2 cylinder, or diverted to an airport about 20 nautical miles closer than his intended airport after the oil pressure decreased to zero, it is likely an uneventful on-airport forced landing could have been performed.

Twelve years after the engine was overhauled, or 3 years and 270 hours prior to the accident, the No. 2 cylinder was removed to replace a fractured anchored-thru stud. After the fractured anchored-thru stud was replaced, the cylinder was installed. Several entries indicated that the hardware that clamped the crankcase halves together on the top portion of the engine was re-torqued. It is likely that during installation of the No. 2 cylinder following replacement of the fractured anchored-thru stud, the anchored-thru and free-thru studs were inadequately torqued, which resulted in fatigue fracturing of all studs followed by cylinder separation, oil exhaustion, and resulting total loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA20LA322 for the full record.