Alternator Failure Q

[uwochris]

Hey guys,

Let's say you're flying a 152 (or something similar), and you experience an alternator failure.

According to most POHs, the emergency checklist will be similar to the following:
- Shut off ALT side of master switch;
- Turn off non-essential electrics;
- Land as soon as practical.

Now, is there any harm in not turning off the ALT side of the master-switch? ie) will you be using extra current in keeping it on and thereby reduce the battery's reservoir power even more? I just do not understand the reason behind turning off the switch.

Thanks in advance for any comments,

Chris.

 

[Slye]

I could be way off, but I'm going to go out on a limb here and say that the reason you should shut off the alternator is because it's obviously failed and you don't know what it's doing in there. It could be flapping around trying break other things or it may decide it wants to catch on fire. I'm not sure if it mentions in the checklist to try cycling the switch, but I'd definitely try that first. Then, if that doesn't work, shut it off so it doesn't get crazy on you.

 

[UnAnswerd]

Now, is there any harm in not turning off the ALT side of the master-switch? ie) will you be using extra current in keeping it on and thereby reduce the battery's reservoir power even more? I just do not understand the reason behind turning off the switch.

It's possible for the alternator to fail and actually overcharge. If this continues for a while it will ruin the battery and possibly other electrical equipment.

Power from the battery is also used to regulate voltage outputs from the alternator. Even if the alternator fails, the power from the battery will still be supplied, and will be an unnecessary parasitic draw on remaining battery capacity.

Lastly, despite fuses, breakers, and other means of circuit protection, you could be risking a fire or substantial damage to electrical components if you continue to leave a failed alternator "online".

 

[qmaster3]

The alt will still be connected. So if it's "flapping" around or decides it wants to catch on fire. I would get the plane on the ground ASAP..

 

[qmaster3]

UnAnswerd,

Tony C couldn't have said it better

 

[minitour]

You're drawing power off of the battery to run the alternator control unit, so turning that off helps conserve battery power for more essential hardware.

Let's say that you forget to turn on the ALT side of the Master switch and you run the battery all the way dead. So you're at your destination and you hand prop the airplane (starter is battery driven, but mags are self sustaining). Will the alternator charge the battery? No. You need a certain amount of juice in the battery to kick on the ALT control unit.

At least that's how it was explained to me. I checked out this systems book hoping to learn a little bit more, but haven't gotten to the electrical system yet, so that's all I got for now.

The only experience I've had with an alternator failure, we told ATC, they cleared us to land and taxi to park and let them know when we were in the parking spot. Asked if we could kill the transponder and they said whatever we needed to do was fine including nav lights, beacon...everything (it was at night). We hit the landing light at about 300 feet on final and were able to park just before the battery died. You don't have a whole lot of juice in a 152. We got maybe 15 minutes.

On another note. If you're just getting a low voltage light, but the ammeter is "normal" (Centered on zero), recycle the alternator switch and you're actually re-setting the low voltage indicator. Sometimes we have problems with that and it just "clicks" it back off.

Again, that's what I got out of the ground school...haven't read a whole lot on it yet, so if I'm wrong...I'm sure someone will correct me on it.

Otherwise, hope it helps!

-mini

 

[EatSleepFly]

I could be way off, but I'm going to go out on a limb here and say that the reason you should shut off the alternator is because it's obviously failed and you don't know what it's doing in there. It could be flapping around trying break other things or it may decide it wants to catch on fire.

Huh?!

Do you even know what an alternator is? How is turning off the Alternator switch going to stop anything from "flapping around?"

 

[EagleRJ]

The alternator side of the master switch controls the alternator field, which consumes some power. If the alternator is not working properly, keeping the field energized is just using up vital battery power, and as others said, if the alternator has a voltage regulation problem or something, removing it from the electrical system is the safest thing to do.

Some alternators have a "self-exciting" field coil, which means that they can begin producing electricity without any battery power being applied. Those that cannot self-excite will not be able to produce any power in an airplane with a completely dead battery that is hand-propped (but if the engine is already running and the battery is dead, the alternator will provide its own power to keep working). There's really no way to tell which kind of alternator a plane has, other than to ask your A&P. You could try turning off both the battery and alternator after start, and then turn just the alternator back on to see if it works. Don't have anything besides light bulbs running if you do this, because the raw 14V output from the alternator will be hard on avionics and other gear without the battery acting as a buffer.

 

[Slye]

"Flapping" was used in an exaggerated layman sense and I could see how it could be misconstrued. No, I have no idea what an alternator is. And yes, I'd love some chocolate a$$cream...

 

[EatSleepFly]

Gotcha.


What is chocolate asscream? Nevermind...don't want to know.