Incident Summary
On 7 March 2007 at 10:40 UTC, a Piper PA-28/180 Cherokee C, registration CS-AIC, operated by Aero Clube da Madeira and based at Cascais Municipal Aerodrome, was conducting a navigation training flight from Cascais to Évora and return. The aircraft was at an altitude of 1,000 ft AMSL near Fonte da Telha when the autopilot was engaged. Shortly after, the crew detected a strong odor of burnt plastic and observed smoke emanating from the instrument panel.
Sequence of Events
The flight instructor took control, declared PAN-PAN-PAN, and informed Lisbon Approach Control of an electrical fire on board. He then executed the procedures specified in the airplane’s operating manual for an electrical fire. After identifying and isolating the faulty circuit, electrical power was restored, but the alternator was found to be inoperative. With the electrical system limited to battery capacity, the instructor decided to abort the mission and return to Cascais. The aircraft landed at 10:50 without further issues.
Occupants and Damage
Three persons were on board: the instructor, a student pilot, and an observer. All were uninjured. The aircraft sustained no damage beyond a fractured alternator belt and a short circuit in the autopilot control unit. No third-party damage occurred.
Investigation Findings
Examination revealed that one terminal on the autopilot control unit was slightly loose and allowed electrical current to arc to an adjacent terminal, which had more exposed wire than normal. The resulting short circuit melted the protective plastic coating on some wires. Despite a 5-ampere fuse in the circuit, the fuse did not blow, allowing continued electrical arcing. The fire was extinguished when electrical power was disconnected. The alternator belt was found broken; however, the belt fragments were not available for further analysis. The alternator and engine appeared clean and well-maintained, having recently undergone an overhaul.
Conclusion
The incident was caused by a short circuit in the autopilot control unit, which resulted from a loose terminal and exposed wiring. The broken alternator belt prevented normal electrical generation, prompting the crew to return early.