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.

 

[qmaster3]

"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...

LMAO...

 

[Slye]

Austin Powers, anyone? Anyway...I didn't mean to tangentize the conversation, I tend to wander sometimes.

 

[TrafficInSight]

An alternator that overcharges is not an alternator failure, it's a voltage regulator failure. The alternator always has the capacity to overcharge but the voltage regulator throttles it.

If the alternator itself isn't charging then leaving the alternator master on isn't going to make much of a difference. If it's overcharging then leaving it on could make your battery explode or catch fire... this will obviously ruin your day.

Of course there's no real way to tell in flight if it's the alternator itself or the voltage regulator that's failed aside from a couple failure modes so turning off the alternator master for any charging system failure is good advice.

 

[UnAnswerd]

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.

Huh? I don't think this is correct. Assuming the alternator contains an internal voltage regulator, its output will be around 14V DC. Most 12V electrical systems are actually designed to draw from this slightly higher voltage, because such a voltage is necessary to keep the battery charging at a faster rate than the discharge rate associated with the operation of various electrical devises. Subsequently, you'll likely find that a fully charged battery, with the engine running, will actually measure well over 12V with a multimeter. In other words, the battery does not act as a "buffer" and it isn't even necessary because the electrical system is designed to handle 14 volts. Not sure what you mean by a "buffer". If certain avionics equipment requires a reduction in voltage, it will be accomplished through the use of integral, solid-states devises. The battery is simply a box utilizing a crude chemical reaction to store electrical energy. It is not a buffer, and is certainly not used to reduce voltages/amperages for the operation of avionics equipment.

Also what is the difference between "raw" voltage and "refined" voltage, assuming they're both DC?????????

Not nagging, just wondering where you came up with this....

 

[UnAnswerd]

An alternator that overcharges is not an alternator failure, it's a voltage regulator failure.

Yes, but we're assuming here that quite a few alternators utilize internal voltage regulators. If the device fails, you may as well conclude that the alternator has failed. Keeping the alternator offline during the failure of a voltage regulator WILL prevent further damage to other electrical components.

 

[TrafficInSight]

A voltage regulator is a seperate unit completely. I don't know of any aircraft alternators that have integral voltage regulators.

On a 152/172 it's a little box mounted at the top of the firewall near the center. It has a bundle of wires and a plug on it and some solid state ones have an LED on them.

Notice that I said turning off the alternator master is a good idea even if it wont change anything?

 

[EagleRJ]

Huh? I don't think this is correct. Assuming the alternator contains an internal voltage regulator, its output will be around 14V DC. Most 12V electrical systems are actually designed to draw from this slightly higher voltage, because such a voltage is necessary to keep the battery charging at a faster rate than the discharge rate associated with the operation of various electrical devises. Subsequently, you'll likely find that a fully charged battery, with the engine running, will actually measure well over 12V with a multimeter. In other words, the battery does not act as a "buffer" and it isn't even necessary because the electrical system is designed to handle 14 volts. Not sure what you mean by a "buffer". If certain avionics equipment requires a reduction in voltage, it will be accomplished through the use of integral, solid-states devises. The battery is simply a box utilizing a crude chemical reaction to store electrical energy. It is not a buffer, and is certainly not used to reduce voltages/amperages for the operation of avionics equipment.

Also what is the difference between "raw" voltage and "refined" voltage, assuming they're both DC?????????

Not nagging, just wondering where you came up with this....

You're right- a healthy alternator puts out 14V and normally, an aircraft's electrical system operates at 14V. I'm talking about the quality of the power.

If you were to look at the system voltage during normal operations on an oscilloscope, the line would be reletively stable and level. If you looked at the raw output from the alternator, it would be a rough, jagged line with a lot of fast variations in voltage. The internal power supplies in avionics aren't able to adjust that quickly to voltage variances, so they would experience static, flickering displays, and maybe even malfunctions.
The alternator doesn't put out a rock-stable 14V anyway. It will put out around 12V at a low idle (when the alternator warning light is flickering), and go up to around 14.5V at full throttle. That's normal, even with the voltage regulator working properly. Unregulated, the alternator would put out 20V or more at full throttle.

The power supplies in most avionics are able to handle input voltages from around 10V to 30V, to accomodate installations in both 14V and 28V aircraft. It's noisy, unstable power they don't like. One of the functions of the battery is to help absorb that fluctuation and stabilize the voltage for the aircraft's systems.

 

[mtrv]

One of the functions of the battery is to help absorb that fluctuation and stabilize the voltage for the aircraft's systems.

Which is why those red Cessna alt/battery switches interlock to keep the battery on line anytime the alternator field is on.

 

[UnAnswerd]

The power supplies in most avionics are able to handle input voltages from around 10V to 30V, to accomodate installations in both 14V and 28V aircraft. It's noisy, unstable power they don't like. One of the functions of the battery is to help absorb that fluctuation and stabilize the voltage for the aircraft's systems.

Okay that does make sense. Perhaps the alternator output is not as stable as I had believed. You're correct about the battery providing consistent voltage to the electrical system, but nevertheless I was confused about what you meant by "buffer", as I thought the avionics equipment was capable of dealing with minor voltage fluctuations.Time to hit the books.....

 

[UnAnswerd]

A voltage regulator is a seperate unit completely.

Frankly, I'm surprised by that. The trend for 30 years in the automotive industry has been internal regulators. Yeah, we're dealing with airplanes, bad comparission I guess....

Speaking of that, anyone know what aircraft fuel-injection systems are like? One of the great benifits of aircraft engines has long been the ability to operate without any need for outside electrical power. Is this still possible w/ fuel injection???