NTSB Identification: WPR12LA275
14 CFR Part 91: General Aviation
Accident occurred Saturday, June 23, 2012 in Fontana, CA
Probable Cause Approval Date: 12/19/2012
Aircraft: DEHAVILLAND TIGER MOTH DH 82A, registration: N523R
Injuries: 2 Uninjured.

NTSB investigators may have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

During cruise flight, the engine lost partial power. The pilot performed a forced landing into a field, and the airplane nosed over. Postaccident examination of the engine revealed that a rocker bracket assembly clamping bolt had failed in fatigue, causing the assembly to partially detach from the cylinder head. The detachment prevented the exhaust valve from actuating. Peening damage sustained to the bolt revealed that it most likely failed during a prior flight, at an undetermined time before the ultimate displacement of the bracket assembly.
No airworthiness directives existed for the antique engine, which was overhauled 46 years prior to the accident. However, a “modification news sheet” was issued by the manufacturer 16 years before the overhaul, which detailed a design change intended to preclude the failure of the clamping bolts. The modification was not performed on the failed cylinder; however, the three other cylinders exhibited an undocumented modification to their rocker valve assemblies, which appeared to perform the same function.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Partial loss of engine power during cruise flight due to a failed engine cylinder rocker assembly clamping bolt. Contributing to the accident was the failure of unknown maintenance personnel to perform a modification to the assembly that would have prevented the failure.

Full narrative available

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