What aircraft was involved and where did it happen?

The accident on 2006-10-28 involved a Reims Cessna F172N, registration G-BHDZ, at 900 ft above Snetterton, Norfolk, GB.

What was the probable cause of the accident?

The fire and subsequent engine failure were caused by a high-power electrical surge through the aircraft wiring, triggered when the pilot reset a tripped alternator circuit breaker, which led to the failure of the voltage regulator and subsequent damage to the magneto wiring.

Electrical failure leads to in-flight fire in Cessna 172

A private flight in a Reims Cessna F172N experienced an electrical fire and engine failure, resulting in a forced landing near Snetterton.

What happened

On 28 October 2006, a Reims Cessna F172N, registration G-BHDZ, was conducting a local private flight departing from Great Ashfield. During the flight, the pilot noticed the fuel quantity indicators were showing a significantly lower level than recorded during the pre-flight inspection. Upon inspecting the circuit breakers, the pilot discovered that the 60 amp alternator unit breaker had tripped and attempted to reset it.

Following the reset, a loud noise occurred, and smoke and flames began emanating from behind the left instrument panel. The pilot attempted to contact Old Bucken and, following advice from the airfield, switched off the battery master. Despite this, smoke continued to emerge, and the flames reappeared during the final approach to Runway 25 at Old Buckenham. During the approach, the engine stopped due to the grounding of the magnetos caused by wiring damage. The pilot was forced to perform a landing in a field adjacent to the runway. There were no fatalities and no injuries to the two occupants, though the aircraft sustained fire damage behind the instrument panel.

The investigation

Investigators examined the aircraft's electrical system and found extensive damage to the wiring behind the instrument panel, specifically around the magneto/start switch and the battery/alternator switch. The investigation revealed that the alternator circuit breaker casing had suffered an explosive event, with melted brass and copper terminal fittings.

Further examination of the voltage regulator, an Electrodelta VR515GA, showed evidence of heat damage and corrosion on several terminals, likely caused by moisture ingress. The investigation also noted that the aircraft was using a temporary battery that had been installed following previous electrical issues.

Findings

The primary cause of the fire was a high-power surge applied to the aircraft wiring following the resetting of the alternator circuit breaker.
The voltage regulator was not effectively controlling the voltage, leading to a high current being applied to the alternator field coil and a subsequent high power output.
The pilot's attempt to reset the circuit breaker likely triggered a cascade of short-circuit conditions within the wiring loom.
The engine failure was caused by the grounding of the magnetos due to the extensive damage to the wiring around the magneto switch.
It is suspected that the alternator circuit breaker may have tripped shortly after engine start, a fact that went unnoticed by the pilot during the initial checklist.