Incident Summary
At 1245 UTC on 7 October 1998, a Reims Cessna F172M (registration G-GBLP) was engaged in aerial work near Falgunzeon, Dumfriesshire. The aircraft, powered by a single Lycoming O-320-E2D piston engine, was being flown by a 49-year-old commercial pilot with 1,250 total flying hours (1,000 on type). One passenger was on board. During an orbit to allow the passenger to take aerial photographs, the pilot observed that received radio transmissions were breaking up. The ammeter indicated an excessive current supply. Subsequently, a circuit breaker tripped, and the pilot noted a smell of burning and smoke in the cockpit. The ammeter then indicated an excessive current discharge. The pilot decided to make a precautionary landing, advised air traffic control, and successfully landed at a nearby gliding site. Neither the crew nor the passenger sustained injuries.
Technical Findings
Examination of the aircraft by an aircraft maintenance organisation revealed that the over-voltage sensor within the voltage regulator had malfunctioned. An associated circuit breaker had failed to trip due to its contacts becoming electrically welded together in the circuit-made position. This welding of the circuit breaker contacts and the resulting overheating from excess current passage caused the smoke and burning smell in the cockpit. The nature of damage was described as circuit breaker contacts welded.
Conclusion
The incident resulted from a combination of a malfunctioning over-voltage sensor and a circuit breaker that failed to operate correctly due to welded contacts. The pilot's timely decision to land prevented further escalation.