How Long Do Alternators Last

NealRomeoGolf

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How long do alternators usually last? I've been documenting the major systems in my plane and that might be the next one to address. It has 20 years and almost 1400 hours on it.
 
I can't say for sure but mine is way over due for bearings and brushes. The manufacture may set a time for it to be rebuilt. Hoses for example should be changed every 10 years.
 
PM or tag Dan Thomas on this. He's had a number of posts on this very topic and would give you the best advice.
What say you @Dan Thomas ? I can't tell inspection history based on the logs. I'll be asking my A&P if he looked at it last month during annual.
 
In our company's experience with Cardinals they average closer to 5-800ish...
 
My memory may be fuzzy, but didn’t you have alternator problems on a prior plane? If that was you, you’d better leave well enough alone! Don’t touch it dude!! (I’m kinda kidding...)
 
My memory may be fuzzy, but didn’t you have alternator problems on a prior plane? If that was you, you’d better leave well enough alone! Don’t touch it dude!! (I’m kinda kidding...)
Nope. So far haven't had an issue with an alternator. This plane has already had prop issues and gear issues. The last owner was late on the mags and that's now covered. This might need to be my next investment.
 
Cessna, in their inspection programs, asks for a 500-hour internal inspection of the alternator. I did them at 500, usually found the field brushes about a third gone, and after another 500 hours I replaced them because they were 2/3 gone and wouldn't make it to 1500.

Now, that depends on the type of use. Night flying puts more load on the alternator, meaning that the brushes have to carry more field current, and they erode faster. In the 172SP with the G1000 glass panel, I found the brushes almost 2/3 gone at 500 hours, so I set the inspection interval at 300 hours. That glass stuff draws a lot of current, a lot more than a steam panel with an electric turn coordinator. Doing the standby battery duration test, with only the PFD alive along with the transponder and a Com, it drew over 9 amps.

If those brushes wear down far enough they pop out of their holder and the spring behind them starts arcing and chattering on the rotor's slip rings and chews them up. Now you're buying a new $600 alternator instead of a $20 set of brushes. (plus the labor to put them in, of course).

Now some will say that this is bogus, because the alternator in my car has 250K miles on it. Sure it does, alright. At a much lower RPM than the one in your airplane. For an alternator to be able to produce its max capacity it has to be running at its redline, around 10,000 RPM. The car's accessory driving pulley is sized to get the alternator to redline RPM at the engine's redline; how often do you run your car at redline for extended periods? Never. In the airplane, the driving pulley (or gearset, for gear-driven alternators) is designed to redline the alternator at the engine's redline, which is where it operates most of its life. A Lyc, for instance, that redlines at 2700 will be cruised at 2500, or 93% of redline RPM. Those brushes are going to wear out a lot faster in the airplane, and we see a constant stream of posters here telling us about yet another alternator failure.

The same goes for vacuum pumps and magnetos. They're being run to failure. It's not smart. And in the end the cost can be someone's life.

Alternator brushes are not "brushes". They're blocks of graphite, conductive stuff that transmit current from a stationary supply to a rotating component.
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Now some will say that this is bogus, because the alternator in my car has 250K miles on it.

Many modern cars also put a computer in charge of modifying what's sent to the field winding for various longevity and other reasons.

Whether there's now any airplanes that do it, I don't know of any, but the old stuff certainly doesn't.

Watched a guy use a really nifty and small battery/alternator tester in the automotive world the other day. No idea if airplane shops use em, but it was about a four inch square box, couple of short leads and it talked to his hand held device via Bluetooth. That device handled his truck dispatch, billing, payments, and also had a well done user interface for that battery test function.

It told him to hook up the gadget, then when to start the vehicle, then when to rev it a little... Then it spat out the raw data for voltage at rest and running, and things it assumed from the raw data including battery ability to take a charge, how fast, cold cranking amps, number of amps it had taken since start, condition of the alternator diodes... And whether the car was modifying the alternator field for whatever purposes.

You still had to know things like a badly resistive battery would trick the gadget in a couple of ways... But generally any monkey could have operated the thing.

It was the line item that said it was watching for the vehicle messing with the alternator field that got me to go look up some more info on it. If a major tester company had included it...

Guy had all sorts of fun toys on that truck. A little mini me version of the big rig service trucks.
 
Those brushes are going to wear out a lot faster in the airplane
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Good points all. I'd add that aircraft brushes operate at lower ambient atmospheric pressure, too, and lower humidity typically (aloft), both of which accelerate brush wear.

Paul
 
I've had them fail in under 500 hours. On the plus side, the 1976-1979 Honda Civic alternator my plane takes may be bought new for under $50. Could just about add it to the annual replacement list like spark plugs at that cost.
 
I've had them fail in under 500 hours. On the plus side, the 1976-1979 Honda Civic alternator my plane takes may be bought new for under $50. Could just about add it to the annual replacement list like spark plugs at that cost.

I hate you, and because I do, I want to join you. :D Stupid credit frenzy ruined my timeline, but I'm gonna get there. #EABFTW
 
How long is a rope? There are too many variables to answer the question.

Alternators offer a huge longevity improvement over generators because only excitation current must travel through the brushes as opposed to the full charging current. Many generators back in the day had a bushing on one end as opposed to a bearing. Add to that the variable of how many times it is hard to start in the alternators life. Hard starting takes a toll on the alternator because the battery is drained deeply requiring hard work from the alternator to recharge. That’s why keeping a fresh, strong battery pays off in improved starter and alternator life.
 
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