Incident Summary
A Beech King Air 100, operating a night instrument flight rules (IFR) medevac flight from Fort McMurray to Edmonton Municipal Airport, Alberta, encountered an in-flight upset during descent. While passing through 18,000 feet at approximately 200 knots indicated airspeed (IAS), the aircraft began to yaw and vibrate excessively. The flight crew observed that the upper aft section of the left engine cowling had detached and was lodged against the leading edge of the left wing, outboard of the engine. The crew declared an emergency, reduced speed to 150 knots IAS, and continued the descent. The aircraft landed without further incident, and no injuries were reported. The detached cowling fell to the runway during the landing roll. Subsequent examination of the empennage revealed that the outboard 22 inches of the left elevator had also separated from the aircraft before landing.
Investigation
The Transportation Safety Board of Canada conducted an investigation. Clear skies, smooth flight conditions, and light surface winds were present at the time. The aircraft was dedicated to medevac operations and was normally fueled and hangared for prompt departure. Both crew members held valid licenses. The captain had approximately 2,500 flight hours on King Air aircraft, while the first officer had approximately 80 hours on type.
Findings
It could not be determined if the left upper aft cowling latches were secured before departure. However, it was considered probable that the cowling would have opened sooner if the latches had not been engaged before takeoff, due to the normal pressure differential across the cowling. The rear latch was misaligned after the accident, and wear patterns indicated that the misalignment had existed for some time. This discrepancy would have made the rear latch more difficult to operate and increased the likelihood of improper securing. Testing showed that differential air pressure could disengage the trigger on the forward latch because of a weak trigger spring. The forward latch had reportedly unlatched in flight on at least one previous occasion. The detached cowling lodged on the leading edge of the left wing, forward of the outboard end of the left elevator. The buffeting generated by the displaced cowling was sufficient to excite a destructive vibration in the elevator, causing the tip failure. No pre-existing discrepancies in the elevator failure area were identified on available components.
The aircraft was fitted with early production cowling latches (Part No. H296K854) that have weaker trigger springs than the current version (Part No. H296K1135). The design of the latches results in a force in the opening direction when a pressure differential exists across them.
Probable Cause
It is probable that the left cowling opened in flight because of the combination of weak latch trigger springs and pre-existing damage on the rear latch. The left elevator failed because of buffeting induced by the displaced cowling.
Safety Action
As a result of this occurrence, the operator, Contact Air, made the following change to the Company Standard Operating Procedures: When possible, all night flight walk-arounds are to be completed inside the hangar with all necessary hangar lighting on. This assists the crew in preparing the aircraft for flight and eliminates the need for a flashlight during the walk-around.
The Transportation Safety Board released the report on 04 April 1996.