What happened
On 24 April 1986, a Shorts SC7 Skyvan (registration 7Q-YAY), operated by the Malawi Police Airwing, was conducting a ferry flight from Lilongwe, Malawi, to Nairobi, Kenya. The flight, which included a planned stop in Dar es Salaam, Tanzania, was intended to deliver the aircraft to new owners in Nairobi.
While approaching Moi International Airport in Mombasa, the aircraft experienced a simultaneous loss of power in both engines. Approximately ten nautical miles from the airport, the pilot was forced to perform an emergency landing. During the descent, the aircraft struck a tree and came to rest on a murram road in the Mutunga area of Kwale District. The impact caused significant damage to the airframe, including the destruction of the nose landing gear and the loss of both wings.
The investigation
The investigation focused on the sudden loss of engine power and the condition of the aircraft's fuel systems. Investigators examined fuel samples, which met required specifications, and analyzed the engine fuel nozzles. Testing of the nozzles revealed irregularities; specifically, some nozzles exhibited narrow spray patterns, while one nozzle in the port engine group showed no flow at standard operating pressure of 100 psi, only ejecting a jet when pressure was increased to 400 psi.
Furthermore, the investigation scrutinized the fuel vent system, which is critical for maintaining pressure balance within the tanks during flight.
Findings
- The pilot held a valid Airline Transport Pilot License and was appropriately rated for the flight.
- The aircraft had been maintained according to the approved schedule and possessed a valid Certificate of Maintenance.
- The primary cause of the accident was fuel starvation.
- This starvation was triggered by an unserviceable fuel vent system, specifically caused by the number 1 tank vent valve binding in the up position.
- Additionally, the left wing vent pipe was obstructed by a foreign object, identified as wax.
- The investigation noted that the irregular spray patterns in the fuel nozzles could have been due to dirt ingress, potentially occurring either before the accident or following the crash due to exposure.
Safety action
- Maintenance organizations and engineers are reminded to strictly adhere to recommended maintenance practices regarding the regular inspection and testing of fuel vent system serviceability, particularly for aircraft that have been parked or out of service for extended periods.