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 want to show my students my method, but I don't want to break anything either. ( I will be posting in the maintenence section as well)

 

[unreal]

What's there really to accomplish with a "proper" alternator check, anyway? That the alternator turns off when you turn off the switch? Good for it, but you're not really checking its proper operation, just that of the alternator relay.

I think the better method would be to ensure that the alternator will provide the system with the proper amount electricity when it's loaded with the normal amount of electrical equipment for your flight. When you're running up, you typically have most of the lights turned off, the pitot heat is off, etc. An alternator just barely functioning might be able to support the electrical system under those circumstances, so you're not really proving that the alternator is going to provide enough electricity when you're in-flight and lit up like a christmas tree.

Do I really suggest that you turn on all of your lights and electrical equipment when you're running up just to check your alternator? Not really. I fly at night a lot, and any pilot next to me in the run-up area doing that would find a severed horse head in their bed the next morning. An appropriate check would be to check your ammeter for proper indications just after you add power on takeoff. It's just as good as when you're doing a run-up, and your electrical system would be loaded as much as it would need to be for your flight.

 

[nosehair]

I have developed the habit of checking the alternator output immediately after engine start; that is when it is always showing a substantial charge because the battery has been depleted from the start-up. Within 10 seconds or so from engine start, the charging will be done, and the amp needle maybe indicating zero or so close to it that I have to do these 'other' tricks to see if it is charging. But why wait until run-up or power up on take-off?

I have changed my mode of thinking on the immediate after-start. Instead of looking at the oil pressure guage, which always takes a few seconds, I look at the alternator needle and voltage light while waiting for the oil pressure guage to come up.

 

[xshuttlefa]

My thought on what you are supposed to be looking for doing it the way I was taught is that the alternator turns off and also turns back on, that the ammeter gauge is functioning properly (don't know why it wouldn't but...) and that the low voltage light comes on (not burnt out).

 

[avbug]

This question has been posted in several places; don't do that. One is enough.

Any time you move any control or switch, you should be looking for the corresponding response. If you turn on a generator or alternator, the first place you go look is the voltmeter and ammeter to see what it's doing. Is it producing the rated voltage? Is it producing a load comparable to what's desired or expected?

See the reply under the maintenance forum, but in a nutshell, when you shut off that alternator switch and see the low voltage light, are you testing the alternator? No, you're testing the switch, and you're also testing the low voltage indication circuitry.

The alternator hasn't been investigated until you've given it work to do. If you check it just after start, it should show the load necessary to recharge the battery (if the battery was used for the start). Generally you don't want to turn on anything electrical until this load is low and stabilized. Once it's stable and below a given value, then you can start turning things on, such as radio masters, air conditioners, etc. Once it's stable and you start turning on applicances, again watch the alternator to ensure it's taking a load. Any time you turn on anything electrical, go right to the ammeter to ensure that it's taking a load and that the load is appropriate to what is expected. This also tells you if the applicance you've just turned on is operating normally.

Many light aircraft don't have ammeters. Pilots call them ammeters, but they're not, because you can't tell how many amps are being drawn on that alternator. Instead, you usually have a loadmeter, which really ought to be the same as an ammeter, but it's generally not. It's usually an idiot gauge that shows a zero in the middle and a plus or minus. You get to know if it's doing nothing, charging, or discharging. However, ask most pilots what's being measured, and most won't know.

If your loadmeter is showing a discharge, what does this tell you about your alternator or generator? Is it discharging? Is the battery discharging? What's going on?

That loadmeter tells you what charge the battery is receiving. Ultimately everything gets drawn on the battery, and the alternator services the battery. Put a big demand on the system, it comes from the battery, which then calls on the alternator (and also feeds the alternator it's field current, which is another story for another time). So you're not quite seeing what the alternator is doing, but what the battery is doing, and you can fiat or apply the logic that the alternator is doing it's job. Positive on that loadmeter, the battery well is overflowing and giving, and negative shows it's running down and not receiving a recharge from the alternator or generator.

Another important instrument that's often missing is the voltmeter, which tells you the value of what's being delivered by the alternator. The electricity flow through the aircraft is like water, flowing from A to B. It flows from negative to positive (also confusing at first). The water pressure, or the force of that electrical flow is called voltage. The amount of water flowing, or the amount of electricity moving through any given point, is the amperage. You need to know both amperage (how much electricity is being delivered) and voltage (the value or pressure, of the electricity being produced). Both are important, and most aircraft don't give you either one.

When you do get either one, it's usually on a dime gauge that's not worth a dime, and it's usually wrong. Remember that the information delivered to you by any instrument in the cockpit is usually wrong. Cockpit information is idiot information; real information must be obtained by quality instruments usually through other means than what's seen in the cockpit...it gives you ballpark reading. The voltage you see displayed probably isn't very accurate, neither is the amperage. But it gives you an idea. That's the impetus behind giving pilots the idiot gage called the loadmeter...it doesn't tell you anything, but makes the pilot feel better and allows you to see a general trend of the battery being charged, or discharged.

If that's all you have available, then all you can do is apply a load and see if it takes the load, or does not. It's not much, but it's all you've got.

Incidentally, that master switch is a master switch, and the alternator switch is an alternator switch. They're the same assembly in some cases, but two different switches. The alternator switch isn't half of the master, because it's not the master switch. A little piece of plastic connects them mechanically, but they're entirely different switches electrically.

 

[svcta]

Hey, Avbug, how do a nut and a bolt work?

Not that I don't appreciate your level of knowledge, I just want to see how many paragraphs this will generate.

 

[wt219200]

See the reply under the maintenance forum, but in a nutshell, when you shut off that alternator switch and see the low voltage light, are you testing the alternator? No, you're testing the switch, and you're also testing the low voltage indication circuitry.

Avbug's explanation was very in depth and is probably the best way to check your alternator. But in response to the above, if you turn the alternator switch back on and it extinguishes the low voltage light how can that not be interpreted as the alternator coming back on, working properly, and charging the system.

 

[avbug]

Extinguishing the low voltage light tells you nothing about amperage, the ability of the alternator to support a load, or it's function. It tells you a light went out.

What value illuminates the light, and what extinguishes it? Does the light have anything to do with the alternator (no)...it's a system voltage light, and more directly tied too the battery than the alternator. Regardless, it doesn't tell you about the function of the alternator, and only gives an indirect indication of system voltage. System voltage does not mean the alterantor will take a load. It does not mean the full rectifying capability of the alternator is available to refine and convert AC to DC, and it does not mean that regulation capability is available to handle an increased loading on the alternator.

Applying a load to the alternator verifies that.

 

[xshuttlefa]

Following the advice given in the maintenance forum, I checked the POH to 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 172) is a good one. Thanks to all.

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

(I know posting in more then one category is frowned upon and I usually don't. But I wanted to get the mechanic aspect too, not sure how much cross-over there is in the forums)

 

[midlifeflyer]

One of the schools where I fly, based on their maintenance guys' recommendation, adopted the practice that is in some aircraft POH of starting the engine on the battery only. After the engine starts, the alternator is brought online and the gauges checked for charging - showing a greater charge rate in response to the battery drain and then tapering off to "normal."