What happened
On 15 February 2012, a Piper PA-34-200T Seneca II, registration G-BOCG, was conducting an instrument training flight near Birmingham Airport. While established on the ILS approach for Runway 33, a passenger noticed wisps of smoke and an acrid smell emanating from the cabin heater selector switches on the centre console. The passenger further reported seeing a flame beneath the console.
The instructor took control of the aircraft and directed the student pilot to clear the instrument flying screens. To address the fire, the instructor switched off the heater, opened the vents, and directed the passenger to use the cabin fire extinguisher. Although the discharge rate of the extinguisher was noted as poor, it was sufficient to suppress the flame. The instructor notified Birmingham Tower of the emergency, and the aircraft was cleared to land. Emergency services met the aircraft upon its uneventful landing.
The investigation
Investigators examined the electrical connector through which power flows to the cabin heater. They discovered that a pin within the connector had overheated, causing the plastic to partially melt. While the specific cause of the overheating could not be determined due to the extent of the damage, the investigation looked into the maintenance history of the aircraft.
It was revealed that the maintenance organisation frequently disconnects this specific connector to inspect flying control cables. During previous maintenance on G-BOCG and several other PA-34 aircraft, technicians had found pins that were loose or corroded, necessitating replacement. The investigation also noted that the 15 amp circuit breaker protecting the circuit remained intact and was found to be serviceable after the event.
Findings
- The fire was caused by an overheated electrical pin within the cabin heater power supply connector.
- It is likely that loose or corroded pins increased electrical resistance, leading to a voltage drop and a subsequent temperature increase.
- This process created a cycle of thermal runaway, where rising temperatures further increased resistance until the connector melted.
- The cabin fire extinguisher's effectiveness was reduced because it was operated at an angle; the manufacturer noted that the extinguisher must be held upright to ensure the nitrogen propellant properly disperses the halon.