039 – You Can’t Always Believe an “Idiot-Light”

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Warning and indicator lights on an airplane instrument panel… they give a lot of useful information when they’re interpreted correctly.

When everything is working properly, an “Idiot Light” can tell you when a system has failed.

According to Google, an “Idiot Light” is, “A warning light that goes on when a fault occurs in a device, especially a light on the instrument panel of a motor vehicle.”

This is a pretty good definition, as long as everything else is functioning normally.

But be careful, because sometimes these warning lights can “lie” to you!

Recently, I did an engine runup in a Piper Apache with a Plane Power alternator system installed.  It had these “alternator inop” lights at the top of the instrument panel.

During the runup, these lights turned off, and it appeared that both alternators were functioning normally.

However, I found something during the inspection, on the right alternator, that showed otherwise.

The alternator output wire was completely disconnected from the ring terminal on the alternator.

So, if I had turned the left alternator off during the runup, I would have discovered that the right alternator was not showing any positive amperage indication on the amp gauge.

So… you can’t always believe the alternator inop light… it must be verified with other available information.

Listen to today’s episode for more on this, and some recommendations for how you can better interpret the lights and information available to you on your airplane’s instrument panel.

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