Alternator maintenance

[PT20J]

There have been a couple of threads lately where alternator maintenance and especially periodic inspection of brushes and bearings has been recommended. This gave me pause because while I check the belt tension at every preflight, and check the wiring and mounting security every time I have the cowling off (these being the common causes of failure), I never check the brushes or bearings. This is based on my experience of hundreds of thousands of miles in automobiles where I have never once replaced brushes in an alternator -- or needed to. So, I thought perhaps I am missing something.

To get an opinion based on actual experience, I called the Director Maintenance at a large flight school nearby. The school has a fleet of 4 C-172s, 8 C-172s, a Warrior, a C-182 and a Seneca. That's 16 alternators and the airplanes fly a lot. The shop works on customer's airplanes as well as maintaining the fleet airplanes and I'm familiar with their work because they have worked on my airplane. It's a good shop. What the Director told me is that his experience is that an alternator will run well beyond TBO with no service. He recommends replacing or overhauling the alternator when the engine is replaced or overhauled and not messing with it if it is working to avoid "maintenance induced maintenance."

Now, that's just one guy's opinion. But, he does have some real experience to back it up.

For those that prefer to periodically replace brushes, it would be interesting to get reports of how many hours were on the brushes and how much they have worn compared to new. Then we would have some better data. 

 

[EricJ]

I've mostly encountered failure-related brush replacement, or at least servicing, rather than due to periodic maintenance.   With GA airplanes there's usually no telling how many hours or age or type of use on them if there's a related failure, so that's hard to track.   FWIW, though, that also not a bad reason to inspect them once in a great while if there are issues.

IIRC I replaced the brushes on my old, servicable, alternator after somebody misdiagnosed a failed alternator (which turned out to be another broken field wire).   I replaced the brushes just because the alternator was out and easily accessible.   I think the brushes were more than half worn, so I put a new set in and now the whole unit is a spare.

[GeeBee]

I think you have to differentiate under what conditions the alternator operates. An alternator on a C-172 is not going to take the beating that one on a Bravo does and not even close to an Ovation. Cooling is different, current demand is different. Just look at their ratings in amps. Equally so where they are mounted for cooling and how they are mounted makes a huge difference. The alternators on Continentals for instance take a huge beating and worse the consequences of failure of the drive coupling can be engine failure, so they deserve attention and care.

[philiplane]

On Cirrus aircraft, which have a 100 amp primary and a 70 amp secondary alternator, the primary alternator is inspected at 500 hour intervals. Most have engine-driven air conditioning, and in warm climates you'll find the brushes 75 percent worn at 500 hours. If you skip the inspection the brushes will fail at 600 hours and generally ruin the $1800 alternator. I have two clients with the all-electric Kelly AC added to their older Cirrus, and that uses a Plane Power 150 amp alternator. Those brushes barely make 400 hours and they're wiped out. 

Diamond DA40's have a single 70 amp alternator, which takes the same brush assembly as the 100 amp Cirrus version. DA40's will go 800 to 1000 hours before their brushes wear out. 

Both of these are all-electric aircraft. Put the same alternator on an older plane with lower draw, and you'll still only get 1500 to 1800 hours before the brushes are worn enough to cause problems. The key is the total average draw. If you're consistently using more than half the alternator's rated output, the brushes won't last as long. 

Brushes, and the time to inspect or change them, are cheap. I keep several sets in stock and I change at least 30 sets annually. 

[PT20J]

I assume that the Cirrus alternator inspections are called out in the Cirrus maintenance manual. My M20J maintenance manual, and the latest (Aug 2018) Mooney 50 hr/100 hr/Annual inspection checklist, do not call for any brush inspections or specify a replacement interval. I think my M20J is more similar to a C-172 than a Cirrus.

I have a 70 amp alternator. My airplane in normal cruise configuration draws 19A. An alternator (at constant rpm) is a current-controlled current source. Most alternators have a current gain of about 50, so at 19A the field current is only about 380 mA.

[201Mooniac]

I added a new plane power alternator when I replaced my engine 1500 hours ago.  No maintenance at all since.  Like Skip, my M20J draws 18A in cruise from the 70 amp alternator.  I've never really thought of checking the brushes.

[takair]

From PlanePower ICAW:  

Inspection Checks
50 Hours TIS & each 50 hours thereafter. 
(1) Perform a check of the alternator assembly. Inspect the alternator mounting bracket and 
electical connectors for condition and security. Inspect alternator belts for condition, proper 
alignment, and tension. 
100 Hours TIS & each 100 hours thereafter. (or each annual/event, the first to occur)
(1) Remove belt and turn alternator rotor by hand to check for grinding noises or rough running. 
Replace alternator if detected. 
1000 Hours TIS. (or each 5 years, the first to occur)
(1) Remove field brush assembly per drawing 17-9999-1 and inspect brushes for wear. Replace 
brushes if brushes extend less than .250 from edge of brush holder case.

[McMooney]

hmm, are there brushless alternators available ?  be nice to have less to maintain

[201Mooniac]

2 hours ago, takair said:

From PlanePower ICAW:  

Inspection Checks
50 Hours TIS & each 50 hours thereafter. 
(1) Perform a check of the alternator assembly. Inspect the alternator mounting bracket and 
electical connectors for condition and security. Inspect alternator belts for condition, proper 
alignment, and tension. 
100 Hours TIS & each 100 hours thereafter. (or each annual/event, the first to occur)
(1) Remove belt and turn alternator rotor by hand to check for grinding noises or rough running. 
Replace alternator if detected. 
1000 Hours TIS. (or each 5 years, the first to occur)
(1) Remove field brush assembly per drawing 17-9999-1 and inspect brushes for wear. Replace 
brushes if brushes extend less than .250 from edge of brush holder case.

Well we do the 50 and 100 hour checks at annual but I haven't done the 1000 hour inspection they list.  I'll have to ask my IA about that one.

 

[N201MKTurbo]

On the Prestolite alternator, removing the brushes is very easy. Hardly worth fussing about it. The hardest part of R&Ring the brush assembly is removing the field wires. You don’t need to remove the alternator, it is easy to do with the alternator mounted.
 

I have never had brushes wear out, but twice I have had brush assemblies fall apart from vibration. Specifically, where the brush wires are riveted to the wire studs. In both cases, a periodic inspection would have probably found the issue before it became a problem.

The other thing is the parts are cheap. The whole brush assembly with brushes is less than one fine wire plug. 

[Bartman]

In my 15 years of ownership of my J the field wire connection to the alternator required attention 2 or 3 times.  The field wire broke at the crimp in the connector, but that was due to vibration and cured with proper routing and securing of the wire bundle. The one time it was not the field wire the shop found it had some engine oil on the inside from a chronic engine oil leak prior to engine rebuild.  After cleaning they replaced the brushes in 2020 there have been no problems.

 

[Yetti]

I rebuilt the wires to the alternator about 6 years ago.   I had a voltage failure last year.   I replaced the brushes in the alternator as a first step.   Then replaced the voltage regulator and solved the voltage issue.  Best I can tell the brushes and alternator are from 1974.  Won't worry about that for a good while.