History of the Flight
On 28 October 2006 at 1112 hrs, a Reims Cessna F172N (registration G-BHDZ) was conducting a private local flight from Great Ashfield. The pilot noticed the fuel contents indicating significantly less than pre-flight readings. He attempted to contact Old Buckenham Airfield but received no reply. Checking the circuit breakers, he found the 60 amp alternator unit had tripped and reset it, causing a noise described as a 'phut'. He then retransmitted his call to Old Buckenham, stating his problem, when smoke and flames immediately issued from behind the left instrument panel. The fuel contents indication briefly returned to its original reading.
The pilot received a reply from Old Buckenham advising him to turn off the battery master switch. After making a 'PAN' call, he complied, which diminished the flames but smoke continued. He positioned the aircraft downwind for Runway 25. On final approach, flames reappeared from above and below the left instrument panel, and smoke hindered forward visibility. On short final, the engine stopped. With insufficient height to clear obstacles, the pilot performed a forced landing in a field to the right of the runway. The landing was successful, and the two occupants evacuated immediately. The airfield fire crew, having just completed a practice session, arrived promptly and likely prevented more serious fire damage.
Examination of the Aircraft
The aircraft sustained no damage from the forced landing, but fire damage occurred behind the instrument panel. Damage was extensive around the magneto/start switch and the combined battery master and alternator switch. The battery had an 'unserviceable' tag and 'Jump Battery' written on it, along with the registration of another aircraft that had been de-registered in July 2006. The owner stated it was a temporary replacement.
The maintenance organization that examined the aircraft suspected a faulty voltage regulator. The regulator (Electrodelta VR515GA) was removed and tested, confirming it was not controlling the voltage. Internal inspection revealed heat damage to a transistor and at least two resistors, and corrosion on terminals of the amplifier block. The regulator had been installed on 31 July 2002 as a replacement due to the alternator not charging the battery. The aircraft has a 28V DC electrical system, and the regulator was the correct type.
Similar Occurrences
The CAA database lists six occurrences of cockpit smoke in UK-registered Cessna 172 aircraft since January 2002. In March 2005, an incident involved high and low voltage lights illuminating; the pilot recycled the battery master switch, followed by a burning smell and black smoke from the left side of the instrument panel. A Mayday was declared, and a forced landing was carried out. The voltage regulator (Electrodelta VR600 for a 14V system) had failed. This incident was not investigated by the AAIB.
In July 2004, the FAA issued Special Airworthiness Information Bulletin CE-04-72 for Cessna models with Electrodelta VR600A regulators in 14V systems, prompted by over-voltage conditions after a regulator failure that did not trip the alternator circuit breaker. Recommendations included replacing the regulator with a VR600 unit and reinstalling the over-voltage sensor.
Discussion
Evidence suggests the alternator circuit breaker tripped shortly after engine start, unnoticed by the pilot. The poor condition of the temporary battery likely caused rapid voltage deterioration, leading to low fuel gauge and radio indications. The probable failure mode of the voltage regulator resulted in high current to the alternator field coil and high alternator output. Resetting the circuit breaker applied this output to the aircraft wiring, melting insulation and causing smoke and flames. A cascade of short circuits likely rendered the battery master switch ineffective. The engine failure probably resulted from grounding of the magnetos due to wiring damage around the magneto switch.
The pilot's decision to reset the tripped circuit breaker, though following a once-only attempt guideline, endangered the aircraft and occupants. The aircraft was at low altitude, and the pilot had already decided to land due to the spurious low fuel indication. Two safety recommendations were made following this event.