Cessna 150M N63246 accident near Brodhead, Wisconsin

Probable cause

A loss of engine power due to the fatigue fracture of the No. 1 cylinder’s rocker shaft bosses, which resulted in a forced landing into uneven terrain.

NTSB analysis

The pilots reported the engine began to vibrate and lose power while on the downwind leg in the traffic pattern. The pilot in the right seat took control and attempted to conduct a downwind forced landing to the runway, but the airplane’s flight path continued beyond the end of the runway into uneven terrain and the airplane nosed over. The airplane sustained substantial damage to both wings and the vertical stabilizer.

Postaccident examination revealed the No. 1 cylinder rocker shaft bosses were fractured. Further examination of fracture surfaces with a stereomicroscope revealed fatigue crack initiation at and progression from the rocker shaft bore hole.

In 1996, based on a history of fatigue cracking of cylinder rocker shaft bosses, the Federal Aviation Administration issued an airworthiness directive (AD) that required inspection of the cylinder shaft rocker bosses at the next overhaul or cylinder removal. Since the engine was last overhauled in 1978 and the No. 1 cylinder was last removed 1991, compliance with the AD was not required for the accident airplane’s No. 1 cylinder.

Text from the NTSB. Open the NTSB docket for CEN22LA141 for the full record.