What happened
On 7 October 1998, a Reims Cessna F172M, registration G-GBLP, was conducting an aerial work flight near Falgunzeon, Dumfriesshire. While the pilot was orbiting the area to facilitate aerial photography for a passenger, the aircraft experienced several simultaneous electrical issues. The pilot first noted that radio communications were becoming intermittent. This was accompanied by the ammeter displaying an excessive current supply.
Shortly thereafter, a circuit breaker tripped, and the cockpit was filled with smoke and the smell of burning. The ammeter then indicated an excessive current discharge. Following these developments, the pilot notified Air Traffic Control of his intention to land and successfully performed a precautionary landing at a nearby gliding site. There were no injuries to the crew or the passenger.
The investigation
An investigation into the incident involved reviewing the aircraft accident report and conducting inquiries with an aircraft maintenance organisation. Technical examinations of the aircraft's electrical system were performed to determine the source of the smoke and the electrical instability.
Findings
Investigators determined that the primary cause of the incident was a malfunction of the over-voltage sensor located within the voltage regulator. This failure led to an electrical surge. Furthermore, the investigation established that an associated circuit breaker failed to function correctly because its contacts had become electrically welded in the 'circuit-made' position. The passage of excessive current through the circuit breaker unit caused the unit to overheat, which produced the smoke and burning odor observed in the cockpit.