Summary
While en route to the destination airport, the pilot activated the cabin heater. Shortly after, occupants detected an unusual smell and noticed the cabin temperature rising. When the pilot turned the heater off, dark smoke entered the cabin, obscuring the occupants' vision. The pilot attempted an emergency landing in a field, but during the approach the airplane's wing struck the ground, causing the aircraft to cartwheel.
Aircraft Examination
Post-accident examination of the heater unit revealed multiple leaks around weld points on the combustion chamber. No evidence of a pre-existing fire or structural failure unrelated to the heater was found.
Maintenance History
Review of the maintenance logbooks showed that the heater was documented as inoperative during the most recent annual inspection. Although a work order indicated that maintenance was completed at a later date, no logbook entry returned the heater to service. Furthermore, there were no records of any testing of the heater or tracking of its operating hours.
Previous Incidents
A flight instructor who had flown with the pilot previously reported that the pilot had used the heater on the accident airplane at least once before the accident flight. During that flight, the heater's overheat warning light illuminated, and the heater shut down without incident. The instructor showed the pilot how to reset the overheat circuit breaker but did not follow up on its status during their instruction. There is no evidence that a mechanic examined the airplane after that event or before the accident flight.
The airplane flight manual states that after an overheat warning, the heater “should be thoroughly checked to determine the reason for the malfunction” before resetting the overheat switch. The pilot's decision to use the heater on the accident flight indicates a lack of understanding of the heater's status and the risks of continued use without the required thorough check.
Regulatory Context
A review of applicable airworthiness directives found no calendar time limit requiring periodic inspection of the accident heater unit, in contrast to some similar combustion heaters. Additionally, there is no guidance or instruction to disable the heater to prevent activation if it is not airworthy.
Probable Cause
The official probable cause was the malfunction of the cabin heater, which resulted in an inflight fire and smoke in the airplane. Contributing to the accident was the pilot’s lack of understanding concerning the status of the airplane's heater system following an earlier overheat event and the risk of its continued use. Also contributing were the inadequate inspection criteria for the cabin heater.