Back up alternator question

[Morgan3820]

Having had some issues with my alternator, I have been thinking about installing a back up alternator. How useful are these things? Are they a primary replacement or is it just an emergency back up to get you to the nearest airport?

 

[donjohnston]

I had a B&C SD-20 on my plane. It was capable of powering everything should the primary 60a alternator fail. I never had a primary alternator failure but I tested it a few times a year.

Two caveats:
1) When raising the gear, the current draw was in the mid-20's. But that was soon after starting the engine. Running the pump to raise the gear took about 15a. Lowering it only took about 8a.
2) I had an all glass panel. In cruise after the battery was charged with everything except the pitot heat on, the draw on the alternator was about 10a.

 

[Ryan Klems]

Define "primary replacement" ? Do you mean can it provide all the amps you need should the primary go offline? It's going to greatly depend on your plane & panel. Without knowing your plane and what backup system you were thinking about, the most common is the B&C which is rated for 20A. It can provide more for short times, but will flash a warning light telling you to reduce load. In some cases a full glass panel actually increases your baseline load, while they are pretty efficient, a 10.6" display vs an analog 6 pack, the display is gonna be using more power. In my T210 with a 10" TXi, GTN 750/650Xi, EDM930 and other things, I don't get the reduce load flash from the B&C if I turn off the main alternator. But if I start adding other things like landing lights, pitot heat, or transmitting on the radio, it does exceed the 20A. With pullable breakers I can load shed as needed. But another thing to consider... in my T210 (Vitatoe converted TN-550 engine), I have a gear driven alternator... if I lose the main alternator, I'm not really thinking about continuing my flight to a planned destination, I'm thinking about gear failure, drive coupling failure, etc that means I should be getting on the ground ASAP. Different story with a belt driven alternator.

 

[Morgan3820]

In cruise, I am consuming about 27 amps. So the back up should keep up. I am sure there are things that I could turn off to lower that number. I am just looking at the utility/value of such an investment. Is it appropriate to take off with just the back up functioning? Can I get home? Fortunately, both alternator failures where at my home drone but that may not always be the case. I am always looking for ways to avoid being stuck AOG away from home.

 

[aftCG]

27 amps in cruise? Wow. Running a toaster oven or what?

 

[Bell206]

I am always looking for ways to avoid being stuck AOG away from home.

FWIW: in my experience, the majority of electrical issues causing an AOG situation, to include the alternator, were directly related to not following the maintenance schedule recommended by the OEM. Perhaps spend the money on getting your entire electrical system up to spec before installing a 2nd alternator? Just my 2 cents.

 

[Morgan3820]

27 amps in cruise? Wow. Running a toaster oven or what?

That is what I recall.

 

[Morgan3820]

FWIW: in my experience, the majority of electrical issues causing an AOG situation, to include the alternator, were directly related to not following the maintenance schedule recommended by the OEM. Perhaps spend the money on getting your entire electrical system up to spec before installing a 2nd alternator? Just my 2 cents.

Well, my most recent alternator experience failed 38 hours after rebuild. I am not sure how I failed to follow any maintenace schedule with that.

 

[kyleb]

27 amps in cruise? Wow. Running a toaster oven or what?

Seems like a lot, but a heated pitot and incandescent lights might get you there. In my RV-10 with a single screen G3X, the burden is about 8 amps with just the panel (including AP) running. LED strobes and position lights add 5ish amps, the fuel pump adds 5ish amps. So I'm pushing 20 amps with all that stuff on. Add the heated pitot and a sustained load of 30A isn't out of reach.

 

[Dan Thomas]

The Garmin G1000, running on the backup battery alone, consumes a bit more than 9 amps. That's just the PFD, no MFD, one com, the transponder, and the engine instruments. I never measured the load with the whole thing on, but it would be around 20 amps, I think.

 

[Bell206]

my most recent alternator experience failed 38 hours after rebuild.

Who did the rebuild? Were both your failures out of the same shop? What is the actual electrical load on your system vs the alternator amp rating?
EDIT: maybe I should back up a step. What part failed in the alternator at 38 hours?

 

[Dan Thomas]

Well, my most recent alternator experience failed 38 hours after rebuild. I am not sure how I failed to follow any maintenace schedule with that.

Did you take it back and tell them to do it right?

There is a Service Difficulty Reporting System for stuff like that. Something that fails, not due to old age or way too many hours, should be reported by the mechanic. A few such reports, on the same things rebuilt by the same shop, will often trigger an investigation into the certification of that shop. I often filled out SDRs on troublesome or poor-quality work, and I also referred to the database to find the weak spots in the airplanes we had in the fleet so we could track and perform extra checks to catch that stuff.

 

[Morgan3820]

Who did the rebuild? Were both your failures out of the same shop? What is the actual electrical load on your system vs the alternator amp rating?
EDIT: maybe I should back up a step. What part failed in the alternator at 38 hours?

Same shop. A housing bolt came loose and must have done something on the way out. I don't know a lot about alternators. Thinking that the 'keeper washers' were not done well.

My question is more about the usefulness of backup alternators, than afixing blame.

 

[Dan Thomas]

My question is more about the usefulness of backup alternators, than afixing blame.

They're a good idea, considering that electrical problems, whether in a car or an airplane, make up about 90% of performance hassles. Alternators do fail, though it's most commonly because they're being run to failure. Lots of stories of that on POA.

But they're not the only problem. Electrical circuit components like breakers and switches get old and their contacts oxidize and fail. Cable terminals get corroded and dirty and oily and loose. Master and starter contactors oxidize and burn. Ground straps aren't looked after. Much of this stuff isn't even checked at annuals, based on what I used to find.

So a standby alternator is fine, but if the rest of the system is ready to quit, what good will it do?

 

[Morgan3820]

So a standby alternator is fine, but if the rest of the system is ready to quit, what good will it do?

Agreed, That is why I have recently replaced the original gear and starter soleniods. I am thinking about replacing the master solenoid as well. My AP thinks that I over think things, but he is coming around.
I guess the question for me is, assuming the demand does not exceed supply, is it ok to take off with just the back up alternator functional? My scenario is, flying to a destination, the primary alternator fails enroute. Can I continue to my destination and then return home where I can make repairs to the primary alternator?

 

[Bell206]

My question is more about the usefulness of backup alternators, than afixing blame.

My point was not about fixing blame but fixing the core problem. If the same shop is giving you sub-standard service then fix that problem. There is no reason you can't get 10+ years out of a properly serviced and maintained alternator. Batteries should give 7+ years with the right care and a serviceable electrical system. As mentioned above, what good is a standby alternator if the main electrical ground not up to spec and fails? So the usefulness is only related to the health of the entire system. Perhaps spend your money there first? Hate to see people throw money at an issue that doesn't fix anything.

 

[MauleSkinner]

Is it appropriate to take off with just the back up functioning?

A housing bolt came loose and must have done something on the way out.

I think the second one answers the first one from a practical aspect…especially in light of your comment that you don’t know much about alternators.

 

[John Collins]

If you have a POH, it is likely to provide in the limitations section, a list of Kinds of Equipment required for flight. In my Bonanza, the primary alternator is required for day/night VFR/IFR, so my aircraft is not airworthy to depart without a functioning primary alternator. I have a B&C backup alternator and it pretty much powers all necessary avionics.