What happened
During a forest fire suppression mission near Tobin Lake, Saskatchewan, a Bell 205A-1 helicopter, registration C-FMQN, experienced an engine fire while flying at approximately 250 feet above the ground. The crew, operating on behalf of Campbell Helicopters Ltd., was en route to replenish a water bucket when the pilot observed smoke emerging from the engine cowling and an engine fire warning light illuminating.
As the situation progressed, the pilot noted a drop in oil pressure to zero, alongside the activation of the master caution and engine chip detector lights. The pilot performed an emergency landing in a nearby farmer's field. Although the crew discharged the onboard fire extinguisher during the descent, the fire persisted. Following the landing, the crew shut off the fuel and evacuated the aircraft. The fire was eventually extinguished by a secondary helicopter and a ground fire truck. There were no injuries to the crew, though the aircraft suffered substantial damage to the engine, engine bay, and baggage compartment.
The investigation
Investigators examined the engine compartment and the recent maintenance history of the aircraft. The helicopter had undergone significant engine deck and firewall repairs roughly six weeks prior to the event. Additionally, the starter/generator had been removed and reinstalled 13.5 flight hours before the accident to replace a seal.
Technical analysis of the starter/generator revealed that three hold-down nuts at the 12, 10, and 8 o'clock positions were not secured. This lack of fastening allowed the starter/generator to vibrate excessively. This vibration caused a fuel line to become pinched and subsequently chafed through, allowing fuel to leak onto the starter/generator's commutator. The heat and sparks from the component acted as an ignition source for the leaking fuel.
Findings
- The primary cause of the fire was a fuel line failure caused by chafing, which resulted from the vibration of an improperly secured starter/generator.
- Three hold-down nuts on the starter/generator flange were left unfastened, which was reportedly done to facilitate the recent removal and installation of the component.
- The vibration from the unfastened component caused the fuel line to be pinched between the diffuser housing and the cooling fan housing.
- The continuous supply of fuel through the breached line prevented the fire from being permanently extinguished by the fire bottle while the engine was still producing fuel pressure.