Accident Narrative
During an engine start prior to flight, the left engine backfired two or three times. One witness described the first backfire as "the loudest I have ever heard." When the aircraft took off, two persons living near the departure end of the runway noted that the aircraft—believed to be the same—sounded different and had a "cluttering sound." While climbing, the pilot declared an emergency and reported a fire in the left engine. He was vectored toward Altus AFB while descending in instrument meteorological conditions (IFR). He descended into visual flight rules (VFR) conditions and saw the airport about three miles away. A witness observed smoke and fire trailing from the vicinity of the left engine. The witness stated that the aircraft's nose came up, the plane rolled to the left, then it entered a steep, nose-down, inverted attitude just before it crashed and burned. Both occupants were killed.
Investigation Findings
Examination revealed that the engine exhaust manifold header assembly had failed, allowing hot exhaust gases to escape into the engine nacelle in the vicinity of fuel and oil lines. Corrosion had progressed through the header assembly. Airworthiness Directive (AD) 75-23-08 requested a general inspection but contained a note stating "do not remove clamps." No record of replacement of the header assembly existed in the last 6,100 hours of operation.
Probable Cause
The official probable cause findings are presented as multiple occurrences:
- Occurrence #1: Airframe/component/system failure/malfunction (phase of operation unknown) — The exhaust system manifold/pipe was corroded; maintenance inspection was inadequate; procedure was inadequate (manufacturer); surveillance of operation was inadequate (FAA); the exhaust system manifold/pipe experienced total failure and a leak.
- Occurrence #2: Fire (phase of operation: climb to cruise) — A fire occurred, attributed to the exhaust system failure.
- Occurrence #3: Loss of control in flight (phase of operation: approach VFR pattern final approach) — The reason for the loss of control was undetermined; weather condition included clouds; the pilot in command initiated a precautionary landing attempt; aircraft handling was not maintained.
- Occurrence #4: In-flight collision with terrain/water (phase of operation: descent uncontrolled) — The aircraft descended uncontrolled and collided with terrain.