History of the Flight
An instructor and a student were conducting an instrument training flight in a Piper PA-34-200T Seneca II (registration G-BOCG) with a second student observing from a rear seat. The aircraft was approximately six miles from Birmingham Airport, established on the ILS for Runway 33, when the passenger reported wisps of smoke rising from the cabin heater switches on the centre console. The instructor, who did not see the smoke but detected an acrid smell, immediately took control and instructed the student to remove the instrument flying screens. The rear-seat passenger then reported seeing a flame beneath the console.
The instructor informed Birmingham Tower of the incident and requested to land. He checked the cabin heater circuit breaker (CB), which had not tripped, then switched off the heater and opened the vents. He instructed the passenger to retrieve and use the cabin fire extinguisher, which was stored between the two front seats. The passenger operated the extinguisher while holding it at an angle, noting a poor discharge rate, but the flame was extinguished. The instructor updated the tower that the fire was out, and the aircraft was cleared to land; emergency services met the aircraft after an uneventful landing. Later, the instructor discharged the remaining extinguisher contents onto the dispersal and observed that full discharge pressure was achieved only when the extinguisher was held upright.
Examination of Electrical Connector
The source of the fumes and flames was a partially melted plastic electrical connector located in the centre console, through which electrical power passes to the cabin heater. A pin in the connector that routes power from the CB to the heater selector switch had overheated. Due to the extent of damage, it was not possible to identify why the pin overheated. However, the maintenance organisation that serviced G-BOCG and 12 other PA-34 aircraft informed the investigation that the connector is frequently disconnected to inspect flying control cables. They had, on a number of occasions on different aircraft, found loose and corroded pins and evidence of overheating; the affected parts were replaced each time.
Loose and corroded pins can increase electrical resistance, potentially increasing the voltage drop across the connector. This could cause the connector temperature to rise, further increasing resistance and temperature in a cycle known as 'thermal runaway', continuing until the power supply is disrupted. The 15 amp CB protecting the circuit did not operate during the incident and was subsequently found serviceable.
Fire Extinguisher
The cabin fire extinguisher used halon as the extinguishing agent, propelled by pressurised nitrogen. A label on the extinguisher listed instructions, the first step being 'HOLD UPRIGHT. PULL OUT PIN'. If not held upright, nitrogen escapes with the halon, reducing effectiveness. The manufacturer advised that operating in the horizontal position discharges only 50% of the halon.
AAIB Comment
The maintenance organisation had previously identified that the pins in the connector were susceptible to overheating due to corrosion or working loose. With no other obvious damage, it is likely these were the initiating factors that caused the thermal runaway resulting in the connector melting. The fire extinguisher operated satisfactorily when held vertically; the reported poor discharge rate was probably due to operation at an angle. This demonstrates the importance of fully briefing crew and passengers on the use of any emergency equipment they might need to use in flight.