What happened
On 27 July 2011, a Socata TB10 aircraft, registration G-RIAM, was conducting a private local flight from Leicester at 2,500 ft. During the flight, the occupants noticed smoke entering the cabin from the base of the windscreen. The pilot in command handed control to a front-seat passenger, an experienced professional pilot, who declared a MAYDAY and decided to divert to Coventry Airport.
As the aircraft descended, the smoke intensified, causing the occupants to choke and significantly reducing visibility. The handling pilot performed emergency procedures from memory, including turning off the alternator field switch, closing cabin air vents, and turning off the battery master. To mitigate the smoke, the pilot also opened the left-hand door and used a dry powder fire extinguisher. The aircraft completed a rapid descent and landed safely at 90 kt on the Coventry Airport apron.
The investigation
Investigators examined the aircraft and identified that the smoke originated from a layer of cabin insulation located near the alternator regulator. The regulator, situated behind the engine compartment firewall but on the cabin side, had suffered an internal failure. This failure generated enough heat to cause extensive charring to the insulation and leave residue on the inside of the windscreen.
Maintenance checks revealed that the 60 amp alternator circuit breaker had tripped and the battery had boiled. While the regulator's internal circuitry was encased in resin—making it impossible to determine the exact failure mode—the depth of the charring indicated high temperatures. The investigation also noted that while the aircraft's wiring loom was unaffected, the regulator's location placed it in close proximity to combustible cabin materials.
Findings
- The primary cause of the smoke was an internal failure of the alternator regulator.
- The regulator was positioned behind the engine firewall, placing it in a location where heat could easily ignite adjacent cabin insulation.
- The intensity of the smoke was exacerbated by the proximity of the faulty component to combustible insulation material.
- The experienced handling pilot's decisive actions and use of the fire extinguisher were critical in preventing a more serious outcome.