Piper PA28R N4363F accident near Gainesville, Georgia

Probable cause

A partial loss of engine power due to inadequate thru-bolt and stud preload tension by undetermined maintenance personnel, which resulted in fretting between the engine crankcase halves, and the subsequent separation of the No. 2 cylinder due to the fatigue failure of the No. 2 cylinder stud/thru bolts.

NTSB analysis

The commercial pilot receiving instruction and a flight instructor were conducting a local instructional flight when the engine lost partial power and oil obscured the front windscreen. The instructor took control of the airplane and maneuvered toward a highway on which to conduct a forced landing; the airplane struck power lines and a vehicle before veering off the road and down a steep embankment, where it came to rest inverted.

Examination of the engine revealed that the No. 2 cylinder had separated from the cylinder mounting deck. The No. 2 cylinder rocker box cover and a pushrod tube were protruding from the left forward side of the cowling. The No. 2 cylinder connecting rod was protruding through the top of the cowling. The No. 2 cylinder base studs and thru bolts remained in the crankcase and were fractured at their threaded section. Metallurgical examination revealed that each of the fractured surfaces exhibited evidence of crack arrest marks consistent with fatigue cracking and microvoid coalescence features typical of overstress separation. The crankcase web mating surfaces at the Nos. 2, 3 and 4 bearing journals exhibited pitting consistent with fretting, which is typically a result of inadequate preload tension or loss of preload tension to the fasteners that secure the cylinder to the engine. The inadequate or loss of preload resulted in fatigue cracking of the No. 2 cylinder studs and thru-bolts, and the subsequent separation of the cylinder.

According to maintenance records, the engine underwent a major overhaul about 4 years before the accident and had accrued 1,071.11 hours since the last overhaul. At the time of the overhaul, the crankcase was reassembled, and all four cylinders were installed. It could not be determined if the thru-bolts and studs were improperly tightened by maintenance personnel at the time of the overhaul or during an undocumented maintenance action that was performed after the overhaul.

Toxicology testing of the flight instructor revealed an unquantifiable amount of doxylamine, a sedating antihistamine that was well below that considered to cause significant effects. Therefore, it is unlikely that the pilot's use of doxylamine contributed to his inability to successfully perform a forced landing on a highway.

Text from the NTSB. Open the NTSB docket for ERA18FA137 for the full record.