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2 Answers

Engine Driven Alternator

Asked by: 4657 views Aircraft Systems

we say Engine driven alternator.

until engine is shut down, alternator runs automatically.

but, there is a alternator switch in the cockpit.

why do we shut off and turn on that switch?

why that switch need for airplane?

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2 Answers

  1. Best Answer


    John D. Collins on Mar 17, 2013

    Good question. I leave my alternator switch on all the time. Cars don’t have an alternator switch, but seem to have reliable operation. When the engine starts, the voltage regulator is commanding the alternator to increase its voltage output and the field current is at a maximum. The thought is that when the starter is disengaged, there will be a short term voltage spike until the regulator is able to adjust it back to a normal voltage and the voltage spike will adversely affect electrical circuits that are on. However, the battery is in the circuit and acts as a very large capacitor which prevents spikes in the voltage. I have been starting my airplane for 34 years with the alternator switch left on without adverse effect.

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  2. Bill Trussell on Mar 19, 2013

    Having had more than one alternator failure in my flying career I can offer that the switch in the cockpit is provided in order to more totally isolate the alternator from the electrical system in the event of a failure. This maximizes battery endurance and affords you a chance to get on the ground before everything does dark.

    Think of it as being similar to the “off” position on the fuel selector. When the power fails and fuel is no longer needed, turn it off (in this case for safety reasons).

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