On March 6, 1983, a Cessna 401 (registration N7864F) operated by Doug Bensing was involved in an aviation accident near Bethel, AK. Investigators recorded the probable cause as: A broken bolt on the exhaust clamp securing the exhaust pipe to the turbocharger, and a clamp that had slipped off its bracket, allowed the exhaust pipe to slip down and release hot exhaust gases into the engine nacelle. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During cruise, the right engine emitted sparks from the cowling. The pilot reduced power and landed at Bethel. Inspection found a broken exhaust clamp bolt and a displaced clamp, causing hot exhaust gases to enter the nacelle.
Incident
During normal cruise flight, the aircraft's right engine began emitting sparks from the cowling. The pilot observed a drop in manifold pressure and subsequently reduced power on that engine, which caused the sparks to cease. The aircraft was landed at Bethel without further incident. After landing, the fire department applied a fire extinguishing agent to the engine. Damage was reported as minor.
Inspection and Findings
A subsequent inspection of the right engine revealed a broken bolt on the exhaust clamp that secured the exhaust pipe to the turbocharger. Additionally, the clamp that secured the pipe to the engine nacelle had slipped off its bracket. These conditions allowed the exhaust pipe to slip downward, releasing hot exhaust gases directly into the engine nacelle. The hot gases excessively heated the engine and removed the waste gate from the operational system of the engine. The broken bolt is identified as part of clamp part number MVT 68637-375M.
Probable cause
A broken bolt on the exhaust clamp securing the exhaust pipe to the turbocharger, and a clamp that had slipped off its bracket, allowed the exhaust pipe to slip down and release hot exhaust gases into the engine nacelle.
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