Dehavilland TIGER MOTH DH 82A N5300 accident near Lampasas, Texas

Probable cause

The failure of a bolt in the engine valve system which resulted in malfunction of the engine exhaust valve system and a partial loss of engine power. The rough terrain encountered during the forced landing contributed to the outcome.

NTSB analysis

The airplane’s engine lost partial power and the pilot was unable to maintain altitude. He executed a forced landing to a terraced farm field. During the landing, the airplane struck one of the terraced areas and nosed over. The airplane sustained substantial damage to the rudder, engine mounts, and lower right wing. Postaccident examination of the engine revealed that the ball end of the No. 2 cylinder exhaust valve rocker was not seated in the pushrod. Additionally, the head of one of the bolts securing the rocker arm bracket had fractured and the bolt head was found in the rocker cover.

Based on the available evidence it is likely that the fractured bolt head allowed enough movement of the No. 2 cylinder rocker arm bracket for the pushrod to become separated from the ball end of the rocker arm. This prevented the exhaust valve for the No.2 cylinder from opening and thereby reducing the power output of the engine.

Text from the NTSB. Open the NTSB docket for CEN22LA106 for the full record.