What happened
On a day with visual flight conditions, a de Havilland DHC-3 Otter, registration C-GGON, departed from a water base at Lac du Bonnet, Manitoba, destined for George Lake. The aircraft, operated by Whiteshell Air Service Ltd., was carrying one pilot and three passengers.
During a shallow climb over the shoreline following takeoff from the Winnipeg River, the engine emitted a loud backfire and suffered a complete loss of power. The pilot attempted a forced landing straight ahead, but the aircraft struck several large trees before coming to rest in a swamp. The impact, which occurred on the aircraft's left side, caused both wings to detach and the engine to become buried in the ground. The accident resulted in one serious injury and two minor injuries.
The investigation
Investigators examined the engine's recent maintenance history, noting that the No. 1 cylinder had been replaced the previous day due to excessive blow-by. While a run-up prior to the accident showed no anomalies, the investigation focused on the mechanical state of the valve train.
Technical analysis revealed that a spacer had been incorrectly installed during the assembly of the No. 3 cylinder exhaust push rod. This error led to progressive wear on the cam roller and cam ring. Furthermore, the investigation found that the valve adjustment screw protrusion had been set beyond the limits specified in the aircraft flight manual.
Regarding the aircraft's equipment, the investigation established that the emergency locator transmitter (ELT) had been removed for re-certification and was not installed in the aircraft at the time of the crash. Additionally, the pilot, who also served as the company's maintenance engineer, had been performing inspections with an expired AME licence.
Findings
- An incorrectly installed spacer in the No. 3 cylinder push rod tube caused increased wear that eventually prevented the exhaust valve from opening.
- The accumulation of hot exhaust gases in the induction system caused the backfire and subsequent engine power loss.
- The operator failed to perform a required field barometric power reference check following the No. 1 cylinder replacement, which would have helped identify engine issues.
- The valve adjustment screw protrusion was set outside of allowable limits, and the company failed to follow the required 400-hour progressive inspection schedule for valve clearances.
- High deceleration forces during the impact caused the passenger seat structures to fail.