Accident Overview
On August 5, 2020, at approximately 1245 eastern daylight time, a Piper PA-28-180 airplane, registration N7213W, sustained substantial damage during a forced landing near St. Agatha, Maine. The flight instructor and student pilot on board reported minor injuries. The aircraft was being operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.
Flight Sequence
The flight instructor and student pilot had planned a refresher flight to prepare the student for a checkride. After completing various maneuvers and basic navigation procedures, the student pilot demonstrated flight at less than cruise airspeed. The instructor noted that the airplane had slowed to about 90 to 100 mph, engine power was reduced to approximately 2300 rpm, and carburetor heat was applied. The instructor then directed the student to increase power to maintain about 3,000 ft mean sea level. As the student applied power, a "pop" was heard and felt from the engine, and power dropped to idle. The instructor described the engine power as insufficient to maintain altitude, though the engine ran "perfectly" at idle with all engine instruments in the normal range.
The instructor took control and checked all systems. Switching fuel tanks, adjusting mixture and power settings, checking the primer, and adjusting carburetor heat did not restore engine power. The instructor identified an open field and executed a forced landing. During the landing roll over rough terrain, the landing gear collapsed, causing substantial damage to the fuselage.
Engine Examination
An examination supervised by a Federal Aviation Administration inspector found that the crankshaft rotated when the propeller was turned by hand, but the crankshaft gear did not engage. As a result, the engine had no compression, and components in the accessory gear box did not rotate. The crankshaft gear bolt was found loose by about 2 to 3 threads, the lock plate holding the bolt was loose, and the alignment dowel had sheared. No other anomalies were observed in the engine.
The National Transportation Safety Board Materials Laboratory examined the crankshaft's aft section, including the drive gear, attachment bolt, lock plate, and a fractured dowel remnant. No significant anomalies were noted with the gear, its teeth, bolt, or lock plate. The fractured dowel remnant exhibited banded crack arrest marks, consistent with crack propagation, and ratchet marks consistent with multiple crack initiation sites. These features indicated fatigue crack propagation. Two opposite-facing fatigue cracks initiated at the dowel surface and propagated inward, with a thin middle region showing overstress fractures.
Maintenance Records
In January 2003, Lycoming published Mandatory Service Bulletin No. 475C, Crankshaft Gear Modification and Assembly Procedures, applicable to the accident engine. The bulletin required compliance during overhaul, after a propeller strike, or whenever crankshaft gear removal was needed. It stated that improper assembly techniques or reuse of worn parts could cause damage to the gear and dowel, leading to complete engine stoppage. Review of the airplane's maintenance records showed the last engine overhaul was in October 2001, with about 660 hours since overhaul. Records from the overhaul through the most recent annual inspection on February 26, 2020, contained no entries referencing work on the crankshaft or compliance with the service bulletin.