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Alternator / Generator failure

A failure of the electrical generator is not usually a serious emergency. Most aircraft will fly perfectly well without electrical power. An exception would be a helicopter being flown on instruments in IMC, but these aircraft typically have redundant sources of electrical power.

Checking for overvoltage

Most aircraft have overvoltage protection, to protect the electrical system from damage if the voltage regulator fails. Typically this is a relay which trips if output voltage exceeds some threshold. These circuits are normally reset by turning off the alternator/generator for a few seconds, and then turning it back on. Failure to reset may mean the overvoltage situation is still present, or that the problem is other than an overvoltage condition.

Shedding electrical load

If the power source can not be brought back on line, the next step is usually to shed electrical load in order to preserve battery power. Some aircraft have an essentials bus. Essential equipment is connected to this bus, and the other electrical busses are turned off. With one switch, the pilot can shed all non-essential equipment. If the aircraft does not have an essentials bus, the pilot can turn off equipment by hand which he does not feel is required for the safety of flight.

The aircraft I normally fly have about 10-15 minutes of reserve power with a normal daylight load. The more equipment that gets turned off, the longer the battery will last. At night, running the position lights is going to use up electrical power much more rapidly. I would not normally extinguish my position lights, but would plan on a landing almost immediately if the failure occurs at night.

Equipment which I would immediately consider turning off would be transponders and communications radios (after telling ATC that I am doing so), landing lights, cabin fans, non-essential nav radios, interior lights, strobes, etc.


During daylight, I can probably shed enough load to make it to an airport for a landing. This has the advantage that there may be a mechanic there who can fix my problem. At night, however, I want to be more conservative and shed load, but also pick a spot to land almost right away. I want to have plenty of power to run the landing light to help spot wires and poles on my approach to land. I would be more concerned with getting the aircraft on the ground, than with getting to an airport, and let the mechanic sort it out in the morning.
Paul Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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