The pilots reported that they flew two visual approaches without incident to Runway 27 at Brodhead Airport (C37) in Wisconsin.
While on downwind for a third approach, the Cessna 150’s engine began to vibrate violently and lost power.
The pilots attempted a forced landing to Runway 9. The flight path continued beyond the end of the turf runway and the airplane nosed over, which resulted in damage to both wings and the vertical stabilizer.
The two pilots sustained minor injuries in the crash.
During the post-accident examination of the engine, the No. 1 cylinder valve rocker cover was removed and the two rocker arms, pivot shaft, and pieces of the cylinder shaft rocker bosses came off freely with the rocker cover.
Examination of the rocker boss fracture surfaces with the aid of a stereomicroscope revealed fatigue crack initiation at and progression from the rocker shaft bore hole.
In 1996, the FAA issued AD 94-05-05 R1 that required inspection of the cylinder rocker shaft bosses at the next engine overhaul or cylinder removal, whichever occurred first. The AD was issued based on a history of fatigue cracking and separation of cylinder rocker shaft bosses of the installed cylinders.
A review of maintenance records indicated the AD was not required to be complied with for the installed cylinders since the engine was last overhauled in 1978 and the No. 1 cylinder was last removed in 1991. A search of previous National Transportation Safety Board reports revealed one accident after the AD issuance that involved failure of rocker shaft bosses. This accident occurred on June 27, 1998.
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.
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This March 2022 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
