What happened
On October 15, 1998, a Beech 100, registration F-GHHV, operated by Air Normandie, was performing a scheduled passenger flight from Paris Orly to Montluçon. While cruising at FL130, the flight crew noticed white smoke and a strong burning odor emanating from the pedestal area of the cockpit.
The crew identified the smoke as being of electrical origin and immediately implemented the "electrical smoke or fire" procedures from the aircraft's operating manual. The crew set the transponder to code 7700 and diverted the aircraft to Châteaurast. As the aircraft descended through FL60, the smoke dissipated, and the aircraft landed at Châteauroux without further incident.
The investigation
Technical investigators examined the aircraft upon its arrival at Châteauroux and later at Le Bourget. Ground inspections revealed that the circuit breaker for the left engine inlet anti-ice system was charred, with its push button fused in the extended position. Further examination of the electrical system revealed that a short circuit had occurred within the power supply circuit for the left engine anti-ice heating element. The resulting overheating had caused heat to conduct through the wiring, destroying several cable sheaths.
Detailed laboratory analysis of the circuit, reconstructed from the thermal breaker to the firewall connector, identified several points of damage. Investigators found that the insulation on wire H79B12 had been compromised, leading to a short circuit to the aircraft structure. Additionally, wire H81A12 had been severed by fusion due to a short circuit approximately 2.5 meters from the contactor relay. The investigation also focused on the 25A thermal circuit breaker, which had been destroyed by overheating and failed to interrupt the current.
Findings
- The smoke in the cockpit was caused by the overheating and destruction of the insulation on the electrical circuit supplying the left engine inlet anti-ice heating element.
- The primary cause of the incident was insulation wear on an electrical cable caused by friction against the internal fuselage structure, which led to a short circuit to the airframe.
- The thermal circuit breaker failed to perform its protective function, likely due to either internal contact welding caused by aging or a mechanical blockage of the external push button.
- The investigation noted that because of the cramped cockpit space, it is common practice for crews to place flight documentation against the circuit breaker panel, which could potentially obstruct the mechanical operation of the breakers.