What happened
During a night instrument flight rules (IFR) medevac mission from Fort McMurray to Edmonton Municipal Airport, a Beech King Air 100 experienced a sudden mechanical failure while descending through 18,000 feet. While traveling at approximately 200 knots, the aircraft began to vibrate intensely and experienced a yawing motion. The flight crew identified that the upper aft section of the left engine cowling had detached and was resting against the leading edge of the left wing.
In response to the emergency, the crew reduced their airspeed to 150 knots and continued the descent. The aircraft landed safely with no injuries to the crew. During the landing roll, the detached cowling became dislodently and fell onto the runway. A subsequent inspection of the aircraft's tail revealed that the outboard 22 inches of the left elevator had also broken away from the airframe during the flight.
The investigation
Investigators examined the engine cowl latch assembly and the structural integrity of the elevator. While it was impossible to confirm if the latches were fully secured prior to departure, evidence suggested the rear latch was misaligned, a condition indicated by wear patterns to have existed for some time. This misalignment likely made the latch harder to secure properly.
Testing of the hardware revealed that the aircraft was equipped with older-style latches featuring weaker trigger springs. These springs were susceptible to being disengaged by the pressure differential across the cowling during flight. The investigation also looked into the failure of the elevator, finding that the physical buffeting caused by the displaced cowling was the source of the destructive vibration.
Findings
- The left engine cowling opened during flight due to a combination of weak latch trigger springs and a misaligned rear latch.
- The detachment of the cowling created aerodynamic buffeting against the left wing.
- This buffeting induced severe vertical bending vibrations, which caused the failure of the left elevator tip.