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A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

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Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network. Suggestions for future Aero-Tips are always welcome, as are additions or discussion of each day's tips. Remember... when it comes to being good pilots, we're all in this together.

Aero-Tips 02.17.06

For several days we’ve been demystifying an airplane’s electrical system by comparing it to water flow.  Check out the diagram, sharpen your #2 pencil, and see what you’ve learned. Answers are at the end of the article.

1. Before engine start you turn on the battery master switch. What indications should you see on the:
   a) ammeter
   b) buss voltmeter
   c) alternator loadmeter
2. While engaging the starter you look at your electrical gauges. What should you see on the:
   d) Ammeter
   e) Buss voltmeter
   f) Alternator loadmeter
3. Immediately after engine start with the alternator turned ON, what should each gauge read?
4. After a minute or two of alternator operation what indications should you expect on each gauge?

Answers

1. The battery is slowly discharging, with little load applied. Hence the ammeter shows a slight discharge, the buss voltmeter 12 volts or 24 volts, as applicable to the airplane, and the alternator loadmeter reads zero because the alternator’s not yet running.
2. The starter draws a lot of electricity, but the alternator is not spinning quickly enough to recharge the battery. Therefore the ammeter shows a big discharge, the buss voltmeter 12V or 24V as appropriate, and dropping, and the alternator loadmeter reads zero.
3. Right after turning the alternator ON the ammeter shows a charge as the battery replenishes; the buss voltmeter shows 14V or 28V as applicable, and the alternator loadmeter reads 25% or more…working hard to make up power lost during start. (Note: most alternators need at least about 700 rpm to generate electricity. Generators may need 2200 rpm or more. Indications from answer (2) above continue until the recharging system is fully active).
4. Once the battery is recharged, the ammeter should read neutral (neither charge nor discharge), the buss voltmeter 14V or 28V as applicable, and the alternator loadmeter usually under 50% of its rated output.

Please note: This “water visualization” is not technically correct—but it does a superb job of helping pilots comprehend what’s going on with the electrical system to accurately monitor it and troubleshoot problems…as you’ve probably seen by your ability to answer these questions. Engineers, mechanics and purists, please forgive this teaching tool.

Aero-tip of the day: You may not have all three types of electrical indicators in the airplane you fly, but knowing how the electrical system works helps you understand the gauges you have. We’ll end this discussion with a few more questions, tomorrow in Aero-Tips.

FMI: Aero-Tips