What happened
On 30 May 2008, a Piper PA-31-350 Navajo Chieftain, registration G-VIPP, was climbing through 7,000 feet after departing from Boscombe Down, Wiltshire, when the crew detected a burning smell and smoke in the cockpit. Flames were observed emerging from the overhead panel.
In response, the commander deactivated all non-essential electrical systems, leaving only the radio, transponder, and navigation equipment active, and initiated an immediate return to the departure airfield. While the co-pilot utilized a BCF fire extinguisher to combat the flames, the fire continued to smoulder throughout the descent. The aircraft landed safely at Boscombe Down with fire services in attendance. Although the two crew members and one passenger escaped without physical injury, they were transported to a medical centre as a precaution.
The investigation
Investigators examined the aircraft's air recirculation fans, located in the lower fuselage. Upon removal, the right fan unit was found to have seized due to an impeller that had detached from its central boss, a failure attributed to material fatigue in the fan back plate. This mechanical failure caused an electrical overload in the circuit.
Examination of the cockpit overhead panel revealed significant heat damage to a 10 Amp circuit breaker and associated wiring. Laboratory testing of the wiring suggested the circuit had been subjected to currents as high as 40A—four times the rated capacity. The investigation also noted that the circuit breaker had failed to trip despite this massive overcurrent. Furthermore, the operator discovered that a separate landing gear circuit breaker on the same panel had also failed in the open circuit position due to internal corrosion.
Findings
- The primary cause of the fire was a mechanical failure of the right air recirculation fan impeller, which led to an electrical overload.
- The circuit breaker failed to interrupt the current despite the load reaching approximately 400% of its rated capacity.
- Excessive current caused the wiring to overheat, resulting in fire damage to the circuit breaker and nearby electrical components.
- The investigation highlighted the risk of dormant faults in passive components like circuit breakers that are not regularly exercised through manual switching.