History of the Flight
The aircraft, a Cessna 152 registered G-OPJC, was engaged in a training flight with an instructor and a student pilot on board. Approximately 15 minutes after takeoff, while flying at 2,000 ft, the instructor felt a sharp stinging sensation on his right ankle and noticed a burning smell. Smoke then began to fill the cockpit. The instructor declared a MAYDAY, switched off both fuel and magnetos, and selected a field on St Mary's Marsh for a forced landing. He recalled that the smoke cleared during the descent. The aircraft touched down successfully, but during the ground roll it entered a drainage ditch at the end of the field, resulting in damage to the nose gear leg and forward fuselage. Both occupants exited normally and were uninjured. The Coast Guard attended the scene in response to the MAYDAY call.
After landing, the instructor noted that the flaps were not deployed, despite having selected full flap during the approach. He also observed burn marks on his shoe and sock.
Aircraft Information
The Cessna 152 is equipped with an electrically operated flap system. A selector switch on the instrument panel activates either the 'flaps up' or 'flaps down' switch, which powers the flap motor in the wing. A mechanical feedback system from the cable linking the left and right flaps moves the flap position indicator. The circuit is protected by a 15 Ampere circuit breaker, designed to open if current exceeds the rated value.
Examination and Findings
Inspection of the flap control system revealed that the flap position indicator was bent upwards, not protruding from its slot. Mechanical scoring was present on the mounting plate, but it could not be determined when this occurred. The wiring was correct, but significant heat damage was found on the 'flaps up' switch, located behind the console in front of the right seat. The circuit breaker had not tripped.
The 'flaps up' switch was disassembled and examined. Damage appeared to have been caused by arcing inside the switch, though the reason for the arcing could not be determined from the damaged components. It was not possible to determine when the switch was manufactured, but it may have been at the time of the aircraft (1979).
Three-dimensional X-ray imaging of both the circuit breaker and the flaps switch assembly revealed no pre-accident mechanical defect. Laboratory testing of the circuit breaker showed it would not trip at 14 Amperes but would trip at greater than 15 Amperes. It was considered unlikely that the bent position indicator contributed to the accident, as the flap selector was not used during the 15 minutes before the smoke appeared. The pilot stated that the flaps were checked and operated satisfactorily prior to takeoff.
Conclusion
The most likely cause of the smoke was arcing within the 'flaps up' switch. The precise cause of the switch failure could not be identified.