AVRO-504K N4929 accident near Rhinebeck, New York

Probable cause

A loss of engine power due to the in-flight separation of the number 3 cylinder valve rocker arm attachment hardware. Contributing to the accident was the improper maintenance inspection of the engine.

NTSB analysis

The pilot was performing an aerial demonstration for spectators. Shortly after takeoff, he heard a “thump,” the engine lost power, and he performed a forced landing in trees. Post-accident inspection of the wreckage revealed a keeper and cotter pin that connects the push rod to the number 3 cylinder valve rocker arm was missing. The high-tension spark plug lead to the number 5 cylinder was severed. The pilot, who also maintains the aircraft, believes the flailing spark plug lead flew into the commutator ring, shorting out other cylinders and leading to a total power loss. The missing hardware from the number 3 cylinder was never recovered. According to maintenance records, the airplane received a condition inspection on September 22, 2009. During that inspection, the number 5 cylinder was removed for repair and reinstalled, and the other rocker arms and valve springs were inspected.

Text from the NTSB. Open the NTSB docket for ERA10LA021 for the full record.