What happened
On January 31, 2001, at approximately 17:00 local time, a Piper PA-31-350, registration EC-CJL, was performing a commercial air taxi flight from Málaga to Melilla. The aircraft was cruising at 5,000 feet with two crew members and four passengers on board.
While approximately 25 NM from the destination, the pilot observed the upper cowling of the left engine lifting backward until it reached a nearly vertical position. The component then detached and fell away. The detached cowling struck the aircraft fuselage, breaking the second left-side passenger window. This impact caused serious injuries to two passengers, while the remaining two passengers sustained minor injuries.
Following the event, the pilot reduced power to the left engine to mitigate potential damage. Although vibrations were noted, engine parameters remained within normal limits, and the crew decided to continue the flight. Upon approaching Melilla, the crew notified Air Traffic Control and requested medical assistance. The aircraft landed safely on runway 3/33 and taxied to the apron.
The investigation
The investigation examined the structural integrity of the engine cowlings and the maintenance history of the aircraft. The engine cowlings are composed of upper and lower fiberglass halves, secured by six fasteners on each side, four quick-release screws at the front, and fifteen quick-release screws at the rear. Additionally, four non-original hinge-type safety closures had been installed to reinforce the connection.
Post-flight inspections revealed significant damage to the aircraft, including a dent in the upper fuselage and various impacts and erosions on the left wing area between the engine and the wingtip, consistent with debris from the cowling. The propeller cone showed fiber delamination and the propeller blades showed erosion, likely caused by vibrations or contact with the cowling during its partial detachment. The investigation also found that the central left-side fastener had been violently torn from the upper cowling.
Findings
- The detachment of the upper left engine cowling was likely caused by the failure of a single fastener holding the upper cowling to the lower cowling.
- The failure of this initial fastener likely increased the aerodynamic loads on the remaining fasteners, leading to a sequential failure of the remaining attachment points.
- The presence of supplemental hinge-type closures indicates a known or suspected deficiency in the original cowling connection that had been previously addressed through reinforcement.
- It is possible that improper tightening of a fastener during maintenance allowed it to loosen over subsequent flight hours, initiating the failure chain.