What happened
On March 7, 2007, a Piper PA-28/180 Cheroke C, registration CS-AIC, was conducting a navigation training flight from Cascais to Évora. While flying near Fonte da Telha at 1,000 feet AMSL, the crew engaged the autopilot, which immediately triggered an electrical short circuit within the autopilot control unit. This caused a strong odor of burning plastic and smoke to enter the cockpit.
The flight instructor took control of the aircraft and declared a "PAN-PAN" urgency signal to Lisbon Approach. Following the procedures outlined in the aircraft's operating manual, the instructor isolated the faulty circuit and activated the ventilation system to clear the smoke. During the subsequent troubleshooting, the crew discovered that the alternator was no longer providing electrical power to the system.
Due to the reliance on battery power alone, the instructor aborted the mission and returned to Cascais. The aircraft landed safely at 10:50 UTC, and all three occupants (one instructor and two student pilots) were uninjured.
The investigation
Upon inspection of the aircraft after landing, investigators examined the engine compartment and the instrument panel. The investigation established that the short circuit originated at one of the terminals of the autopilot control unit, which had become slightly loose from its base. Vibrations during flight likely caused this terminal to move closer to an adjacent terminal, creating an electrical arc that melted the protective insulation of the wires.
Investigators also found that the alternator belt had snapped. It is believed that the electrical discharge between the terminals caused a momentary surge in the alternator's load, creating enough mechanical stress to break the belt. Although the circuit was protected by a 5-amp fuse, the fuse did not blow, which allowed the electrical fault to persist and sustain the heat.
Findings
- The primary cause of the incident was inadequate maintenance and inspection of the autopilot control unit, which allowed a terminal to become loose.
- The electrical arc caused the melting of wire insulation, resulting in smoke and odor in the cabin.
- The resulting electrical load surge contributed to the failure of the alternator belt.
- The aircraft's airworthiness certificate and maintenance program were found to be in order.