Alternator Check on Run-up

[xshuttlefa]

I was taught during my initial private pilot training that doing the "Alternator Check" on a piston airplane during run-up involved:

1.)Turning the alternator side of the master switch off.

2.) Look for the ammeter to indicate a discharge and...

3.) for the "Low Voltage Light" to illuminate

4.) Turn Alt side of master on.

Then, the school I was at ( the same one I am instructing at now) stopped that practice and now says to "turn on the landing light or lower the flaps" to test the alternator.

My feeling is that you do not get an appropriate "alternator check" just loading up the electrical system. So, my question is, am I harming anything in the airplane doing a "proper" alternator check? The only damage I can see possibly being done is to the voltage regulator.

Thanks for any help. I'd like to show the students my way but I don't want to damage anything either.

 

[erj-145mech]

What does your POH say? You aren't a test pilot.

 

[avbug]

You only have one master switch. That's the battery master switch. It controls your battery master relay, which applies power to the aircraft from the battery.

The other "half" of that switch is a separate switch; same assembly, different switch. It's your alternator switch, and it controls field current; it is what tels your alternator if it's okay to start making electricity.

When you turn it off, what have you tested? You have tested that you can turn it off. Have you discovered that the alternator works? No.

What does the alternator do? It produces electricity, right? To be precise, it produces alternating current electricity, which is recitified to DC...think of it as food being put through a strainer to make baby food...it goes through diodes and comes out direct current. The alternator's job is to produce electricity. Somewhere in your system you have a voltage regulator, which tells the alternator what the voltage is to be, and controls it by vaying the field current the alternator receives.

Producing electricity is one thing, but the important thing is that it produces enough. The pressure of the electricity is the voltage, and this you can read if you have a voltmeter. You don't need to turn the alternator on or off to check that; if it's more than battery voltage, you're seeing alternator voltage...that shouldn't change when you apply a load.

What you're talking about is the other part of the equation, current. Current is measured in amps, and you're looking at the ammeter. Some aircraft have ammeters, some voltmeters, and some loadmeter...which don't tell you much of anything. An ammeter tells you exactly what the electrical load is in amps. You will know from your study of the aircraft flight manual what the capacity is for the alternator on your aircraft...say, 60 amps. If you're approaching that point, or even half of that point, you've got it loaded up too much and need to start turning things off.

Getting back to testing it...you want to know if it's putting out the correct voltage, and you can only tell that if you have a voltmeter. It should be putting out two to four volts more than the battery voltage. If you don't have that, you need to know that it can produce the necessary volume of electricity...again, amps...to keep things running in the airplane. The only way you will know if it can do this is to turn something on, something big enough to produce a verifyable current draw, and see that it takes the load. If it takes the load and powers the high-current appliance (such as the landing light, typically 125-250 watts), then you have an indication that the alternator is prepared to do it's job.

If you turn on an applicance and see a discharge on a loadmeter, a drop in voltage, or an ammeter doesn't show a load or amp increase, then you have a problem.

 

[xshuttlefa]

Following the advice given above, I checked the POH for the Skyhawk. The POH says to use the landing light or place a load on the alternator to see if it is charging properly. So I will use that as the alternator/ammeter check. Although the useful suggestion to monitor the ammeter on start (it's near the oil pressure gauge anyway in most C172) is a good one. Thanks to all.

BTW, was anyone else taught to do the alternator check by turning the alternator off?

 

[erj-145mech]

No, I was not taught that procedure, I usually just cycled the landing light switch and observed the ammeter. I was already a licenced an practicing mechanic when I started flying, which was helpful in knowing how the systems functioned with different inputs.

Never cycle the alternator switch with the radios on. Voltage spikes can wreck havoc with newer (1975 vintage and up) avionics.

 

[icefr8dawg]

wanna see something interesting about shutting an alternator down. Somewhere in the piper repair manual for aztecs it says "dont shut the alternator down" cause it causese a spike or something to the regulators and may wipe them out. After causing many a line mechanic at the little freight company I worked for headache I stopped doing it...
In a twin you can check the sides on startup or in a single load the system up and observe draw. I do it that way because i've blown up my share of piper electrical systems and cause Mr. Piper Jr. saw to write it into the manual...