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Electrical Fire in flight

We normally associate electrical fire with problems in the instrument panel, since that's where most of the electrics are. That's not always the case. Electrical fires are usually identifiable by the smell of burning insulation. Most people are familiar with this smell. Smoke may or may not be visible in the cabin.

Putting out the fire

Most electrical fires will go out once the electricity is removed. How this is done is aircraft specific, but normally you would switch off the battery and the generator/alternator. If the fire goes out, the pilot may elect to localize the problem by pulling circuit breakers, restoring power, and then turning each circuit back on one by one. If the fire starts again, the last system restored is probably the culprit.

Depending on the circumstances, this debugging may be better off if done on the ground rather than in flight.

Ventilating the Cabin

Electrical fires often produce noxious fumes and smoke, so ventilating the cabin is usually a good idea. Opening windows, or even the doors, will usually help clear the interior of smoke and fumes.

A word about Rotor Tachometers

There are two common ways to sense main rotor RPM and display it on the Rotor Tachometer. One is to use a mechanical cable which runs from the main rotor transmission (or some other point in the drive system) to the tachometer. The other typical way is to use a "tach generator" which is a small generator attached to the main transmission. Voltage produced by this generator is proportional to RPM, and the voltage moves the tachometer hands. Since both of these systems are independent of the electrical system, neither will be affected by electrical fire or failure.

One exception to this is is the Robinson R22 and R44 helicopter. These helicopters use an electronic tachometer which requires external power to operate. A bypass circuit is installed so that turning off the battery and alternator will not cause the tachometer to fail, but if the fire damanges the wire harness sufficiently the bypass circuit could be compromised (this could also happen to the tach generator system if the wires for it run through the same bundles as the other electrical wires).

Paul Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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