Got a Meter? Check Your Alternator

It’s cold, it’s dark, it’s winter. The alternator is the electrical lifeline for your aircraft. While the battery on board most planes, if you lose the alternator, the life of the battery is typically less than an hour at full load, and even shorter with all the lights on! But many pilots pay little attention to the alternator in their airplane, since it is such a dependable piece of equipment. The problem with alternator problems is that they usually come when the alternator is under heavy load. Translation: The alternator usually fails when you need it most.

A WORST-CASE EXAMPLE: You are flying between the mainland and Martha’s Vinyard, along roughly the same path that JFK Jr. managed to make so well-known with his accident. The skies are clear but as usual, when you are flying over water, there is a bit of a haze in the air, and the horizon is hard to see.

As you reach the halfway point, you notice your battery charge meter is pointing toward the discharge end of the scale — instead of registering the slow charge you are expecting. After working through the checklist and cycling the field breaker for the alternator, the charge meter shows no change. You correctly declare an emergency, and start getting rid of all your non-essential loads by turning off everything you don’t need. At this point, with a little luck and some help from ATC, you might just make it to your destination with some of the lights (and instruments) on … if you are lucky!

While the inner workings of an alternator are unknown to many pilots, any pilot or mechanic can check the parts that turn the AC of the alternator in DC — and it’s surprisingly easy. Parts, known as diodes, are used to make this change, and apart from brushes and bearings, they are the most failure-prone component in your average alternator.


TOOLS“: A decent digital voltmeter, the airplane, and a minute of your time.


    • With the engine running at 1500 rpm and the aircraft secured by either a pilot or parking brake plus tie-down, turn on the landing light and taxi light (if the aircraft is so equipped). This puts a pretty good load on the alternator, and helps to uncover any weaknesses.
    • With the digital volt- or multi-meter set to read AC volts, measure the AC voltage on the aircraft’s DC electrical system.

The Voltmeter To Aircraft Connection: For the best results, connect to the aircraft’s electrical system inside the cockpit, through an accessory (previously known as the cigarette lighter) plug. With your own “pigtail” connector you can hook in quickly and easily. These connectors are available for a couple of bucks from Radio Shack or any reputable electronics shop.

WHAT TO LOOK FOR: An indication of 0.75 volts or greater generally indicates that one or more of the diodes has gone bad, or that the alternator filter has gone bad.

WHY ONLY 0.75 VOLTS AC? Well, that is a small value of AC, but it indicates that one of the diodes may not be working correctly. Generally, this means the other diodes have to “pick up” the load, which can cause them to fail prematurely. Either way, it’s a predictor of incipient failure of the aircraft’s electrical system — one you likely don’t want to experience!

THE BOTTOM LINE: Keeping an eye on your alternator makes good sense. When you consider how exciting your flight can get without electrically operated avionics and instruments, keeping this under your own (or your shop’s) watchful eye, is both good and simple. This simple test, plus the required preventive maintenance could easily spare you from in-flight alternator failure.