What happened
On a visual flight rules charter flight from St. Andrews, Manitoba, to Berens River, a Piper PA-31-350 Navajo, registration C-GHMK, experienced a significant engine failure. While cruising at approximately 2,500 feet, the pilot heard a loud noise from the left engine, followed by smoke and visible deformation of the engine cowling. The aircraft experienced a leftward yaw, and a portion of the cowling detached during flight.
Despite applying maximum power to the right engine, the pilot was unable to maintain altitude because the left propeller could not be moved into the feather position. After notifying company dispatch of an intended forced landing, the pilot brought the aircraft down in a moss and mossy marsh area. The aircraft struck small trees during its gradual descent and came to rest with its landing gear retracted. While the cabin remained intact and all passengers and the dog evacuated safely through the right emergency exit, five passengers sustained minor injuries. Following the landing, a fire broke out and consumed the aircraft, leaving only the rear empennage remaining.
The investigation
The investigation focused on the mechanical failure of the left engine and the subsequent inability to control the aircraft's descent. Investigators examined the engine components and found that the number two cylinder had separated from the crankcase. This separation was traced back to a missing base nut on the through stud of the number three cylinder, which caused the lower rear base nut of the number two cylinder to lose its clamping force. As a result, the remaining studs on the number two cylinder failed due to fatigue.
Engineers also determined that the physical displacement of the number two cylinder interfered with the propeller governor control, preventing the pilot from feathering the propeller. This created excessive drag that prevented the aircraft from maintaining altitude. Additionally, the investigation noted that the emergency locator transmitter (ELT) was difficult to access because it was mounted behind a fairing that required tools for removal.
Findings
- The primary cause of the engine failure was the fatigue failure of the number two cylinder studs following the loss of clamping force.
- The loss of clamping force was initiated by a missing base nut on the through stud of the number three cylinder.
- The aircraft was unable to maintain altitude due to the high drag produced by the deformed cowling and the unfeathered left propeller.
- The separation of the cylinder likely caused fuel and oil to spray onto the hot exhaust, fueling the post-landing fire.
- The ELT was not easily accessible in an emergency due to its mounting location.
Safety action
- The aircraft operator has relocated the ELT on its remaining fleet to ensure easier access.
- Transport Canada is utilizing its maintenance safety newsletter to emphasize the necessity of following correct torque procedures during cylinder changes.