Synopsis
Occupants of a Socata TB10 Tobago, registration G-RIAM, were on a local flight at 2,500 ft when smoke entered the cabin around the base of the windscreen. The pilot handed control to the front seat passenger, who held an Airline Transport Pilot’s Licence and had over 7,000 flying hours. A MAYDAY was declared and the aircraft diverted to Coventry Airport, which it had just passed. The smoke increased in intensity, affecting visibility, but the aircraft landed safely. The smoke originated from an internal failure of the alternator regulator.
History of the Flight
While cruising at 2,500 ft during a local flight from Leicester, the occupants noticed smoke entering the cabin from around the base of the windscreen. The pilot handed control to the front seat passenger, who carried out Emergency Checklist items from memory, including turning off the alternator field switch and closing cabin air vents. The battery master was turned off but periodically turned back on for radio communication. The smoke intensified, and the initial pilot opened a storm panel and briefed passengers on brace position and evacuation. On instructions, he opened the left-hand door and used a dry powder fire extinguisher. The aircraft made a rapid descent and landed safely at 90 kt. After stopping on the apron, doors were opened, engine shut down, and occupants vacated.
Investigation
Examination revealed an internal failure of the alternator regulator, with visible heat damage to the underside of its casing. The regulator was located behind the engine firewall and in front of the instrument panel. Smoke had originated from a layer of cabin insulation material near the regulator, which showed extensive charring. The 60 amp alternator circuit breaker had tripped and the battery had boiled, though it was unknown if the breaker tripped during the incident or was pulled after landing. The wiring loom was unaffected. The regulator's internal circuitry was filled with potting resin, rendering it inaccessible, so the failure mode could not be identified.
Similar Occurrences
A search of the UK CAA Occurrence Database found eleven events involving electrical problems in Socata TB series aircraft over 25 years. One similar incident occurred to another TB10 on 15 November 1996, involving a burning smell and apparent loss of communications. Investigation found a voltage regulator intermittent and relay unserviceable, resulting in a cooked battery and failure of both VHF radios. Another similar incident occurred to a Cessna 172, G-BHDZ, on 28 October 2006, where a voltage regulator failure caused the alternator circuit breaker to trip; resetting it led to melting of terminal fittings and insulation, with smoke and flames.
Certification Requirements
The TB10 was certificated under FAR Part 23, Amendments 1 to 16 dated 14 February 1975. The current EASA CS23 and FAA CFR Part 23 include requirements that failure conditions preventing continued safe flight must be extremely improbable, and other significant failures improbable. The nature of the occurrence to G-RIAM suggests the regulator installation would not meet current requirements.
Safety Recommendation
The AAIB made Safety Recommendation 2012-022 to the European Aviation Safety Agency, recommending a review of the alternator regulator installation of the SOCATA TB series of single-engine aircraft.