What happened
A charter flight departed Cochenour, Ontario, heading toward Sandy Beach Lodge. The aircraft, a DHC-3 (Otter), was carrying the pilot and six passengers when the pilot noticed a loss of engine power accompanied by a popping sound and grey smoke entering the cabin. While the engine instruments appeared normal and the fire warning system remained inactive, a passenger noticed flames appearing near the cockpit floor.
In response to the emergency, the pilot attempted to use a hand-held extinguisher, but thick black smoke quickly filled the cabin, causing respiratory distress for the occupants. The pilot performed an emergency landing on McNeely Bay. Although the aircraft remained upright on its floats after the hard landing, the fire spread rapidly, and the aircraft was consumed by flames within minutes. The pilot sustained second-degree burns to the face and arm, and one passenger suffered burns to the leg, but all seven occupants escaped the wreckage.
The investigation
The investigation focused on the mechanical failure of the Pratt & Whitney R1340 radial engine and the path of the fire. Investigators examined the No. 2 cylinder and found evidence of significant overheating. Analysis of the engine components revealed that the exhaust valve guide had suffered interdendritic cracking, leading to the failure of the exhaust valve. This failure allowed a combustible fuel-air mixture to escape through the exhaust port.
Investigators also examined the aircraft's heating system, specifically the fibreglass ducting used for cabin heat. Testing confirmed that this material could ignite and produce dense black smoke when exposed to high temperatures. While the fire warning system was functional, it was not positioned to detect the specific path of the flames, which traveled along the engine mount and toward the augmentor tube.
Findings
- The primary cause of the incident was the continued operation of the engine following an exhaust valve failure in the No. 2 cylinder.
- This failure created a path for flaming gases to escape into the exhaust manifold, significantly increasing temperatures within the upper right exhaust augmentor tube.
- The intense heat from the escaping gases was sufficient to burn away the carbon and oil deposits normally present in the augmentor tube.
- The resulting fire path moved toward the firewall and ignited the fibreglass heat ducting, which then contributed to the thick smoke in the cabin.
- Evidence of heat discoloration suggested that overheating in the cylinder's exhaust ear had been occurring for some time, potentially undetected during the 500-hour inspection completed just days prior.
- There were no established procedures in the aircraft flight manual for handling a partial loss of engine performance or for operating a rough-running engine.