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"High Voltage" or is it "Low Voltage"?

Let me see if I’ve got this straight. The “High Voltage” light on a Cesna 172 N (and similar models) is actually a low voltage light. Cessna calls it a high voltage light because the most probable cause of low voltage is high voltage causing the alternator to go off line. In that case, the light labeled “High Voltage” would be telling you that you now have low voltage because you used to have high voltage. Perhaps it should have been labeled the “Formerly High Voltage” light.

If your high voltage protection is disabled due to equipment malfunction or someone putting a wire on the wrong terminal, and you get a high voltage condition, the high voltage light will not come on. It will remain dark until frying radios, smoke, or some other subtle indication alerts you to the fact that something is wrong. In fact, the light will never come on under a high voltage condition, only a low voltage one. It will come on if there is low voltage for any reason but, it’s a “High Voltage” light so you might not know that you actually have low voltage.

They could have called it a “Low Voltage” light. That would explain why it always comes on when there is low voltage such as when the alternator is off. The pilot would then have to figure out that the most probable cause of low voltage was not having an alternator any more. Cessna must have figured that was too much brain work for someone who meets the minimal standards for a PPL.

I always wondered where Microsoft got the engineers who designed a computer that you turn off by clicking “Start”. Now I know. They hired them from Cessna!

-Bob C.

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