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3rd of January 1994

A Piper Aircraft Corporation PA-31 (Piper Navajo) was operating a non-commercial business flight on this day in aviation history.

Piper Navajo VH-MNT in March 2007 (©Andrei Bezmylov)

While in the cruise phase of the flight at 10.000 feet ~3000 meters) the left alternator light came on and all the electrical load was taken up by the right alternator. At the same time, the pilot observed that the left engine manifold pressure was down to 23 in hg from the cruise setting of 31 in hg. This led the pilot to believe he had a failed turbocharger on the lefthand engine. Unable to maintain altitude he requested ATC for a descent to 7000 feet (2100 meters) and a clearance to return to his departure airport.

Typical PA-31 Dual Alternator System (from public domain)

In an attempt to restore engine power he switched the fuel supply to the left-hand engine from the main tank to the auxiliary tank and also switched on the left fuel boost pump. This restored engine power for a short period of time before the engine suffered a complete failure, shortly afterwards followed by a complete electrical failure and smoke entering the cabin.

Typical PA-31 fuel system, Left Fuel Selector Valve highlighted in yellow (From public domain)

The pilot completed the necessary checklist, and (amongst others) feathered the left-hand propellor, turned the left fuel selector and all electrical switched off. With radio communication lost the pilot managed to get in touch with ATC via a mobile telephone to inform them about the situation. An uneventfully single-engine landing was made after the gear was manually extended.

An investigation into the multiple failure incident was launched and the following findings were recorded;

  • The Left Alternator field wire had chaffed on the left engine fuel supply line, This caused the insulation to wear through. The bare wire then caused arcing to the fuel line, eventually burning a hole in the fuel line.

  • The loss of fuel through the hole caused the drop in manifold pressure

  • Subsequent activation of the fuel boost pump temporarily restored engine power

  • Fuel spraying from the leaking fuel line was ignited by the arcing alternator field wire.

  • This fire caused extensive damage to the engine and wing and related components, as well as hoses, and oil and hydraulic lines.

  • The electrical loom between the engine and the cabin was also affected by the fire and this caused, amongst others, the failure in the electrical system of the left engine magnetos, causing the left engine to fail.

The full incident investigation report is available by clicking here.

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