When to replace/overhaul alternator?

[422Phil]

Looking for advise on when to replace/overhaul the alternator in my 1977 172XP. I’ve owned the plane for 3 yrs, and have had absolutely no issues with the alternator. It’s a Ford DOFF10300F. According to the engine log book the last time it was overhauled was in 2006 (1300hrs ago). What’s the expected life of these alternators? Is it time to replace or overhaul? Thanks

 

[RyanB]

Cessna recommends a brush inspection every 500 hours (it’s good to do this at the same time you do the 500 hour mag inspection). If you’ve done that and the brushes are replaced when they need to be, you will reduce the odds of an alternator failure quite a bit. How’s the wiring? If it’s not frayed or showing wear, it might have more life to it, but if nothing has ever been done to it, you’re on borrowed time. This past annual, the Chrysler alternator on the Archer was replaced with a Plane Power. IMHO, it’s a much better unit than the original Ford’s and Chrysler’s.

 

[Gary Ward]

IMO It should be inspected every 500 hours and like said why not do it at the same time you do the mags.

 

[422Phil]

Thank you both. I hadn’t thought of combining with the mags, but great idea.

 

[Bell206]

Is it time to replace or overhaul?

FYI: You'll find that if you want to get the most service out of any item on an aircraft a simple route to take is via performing preventative maintenance as needed and following the OEM recommendations. In addition to the 500 check mentioned above, keeping your electrical system in good working condition can add life to your alternator... and your battery. The OEM (airframe, engine, prop, accessories) offers a number of recommendations and bulletins to assist you in getting the most out of your aircraft and one I would recommend you at least take a look at for possible compliance.

 

[Dan Thomas]

Looking for advise on when to replace/overhaul the alternator in my 1977 172XP. I’ve owned the plane for 3 yrs, and have had absolutely no issues with the alternator. It’s a Ford DOFF10300F. According to the engine log book the last time it was overhauled was in 2006 (1300hrs ago). What’s the expected life of these alternators? Is it time to replace or overhaul? Thanks

At 1300 hours it's about to quit. I did the 500-hour inspections on the flight-school airplanes' alternators, and most were ready for new brushes at the 1000-hour mark, with enough left for maybe another 100 or 200 hours. In airplanes that have more power consumption, like the G1000 172SP, the brushes wore faster and I had to do the checks at 300-hour intervals, brushes pretty much gone at 600 hours.

Beware of the Plane Power stuff. They have brushes, too, and they will wear out. Advertising hype about them being better only goes so far. Until someone comes up with a small brushless alternator, we'll have to put up with this stuff. Heavy trucks have had brushless alternators for a long time already. Over 30 years.

 

[Whitney]

At 1300 hours it's about to quit. I did the 500-hour inspections on the flight-school airplanes' alternators, and most were ready for new brushes at the 1000-hour mark, with enough left for maybe another 100 or 200 hours. In airplanes that have more power consumption, like the G1000 172SP, the brushes wore faster and I had to do the checks at 300-hour intervals, brushes pretty much gone at 600 hours.

Beware of the Plane Power stuff. They have brushes, too, and they will wear out. Advertising hype about them being better only goes so far. Until someone comes up with a small brushless alternator, we'll have to put up with this stuff. Heavy trucks have had brushless alternators for a long time already. Over 30 years.

And they still work fine at 25,000 hours most of the time.

 

[Dan Thomas]

And they still work fine at 25,000 hours most of the time.

Sure they do, if they've had about 20 sets of brushes and 5 or 10 sets of bearings. And the slip rings wouldn't last through more than 10k hours anyway.

 

[flyingbrit]

At 1300 hours it's about to quit. I did the 500-hour inspections on the flight-school airplanes' alternators, and most were ready for new brushes at the 1000-hour mark, with enough left for maybe another 100 or 200 hours. In airplanes that have more power consumption, like the G1000 172SP, the brushes wore faster and I had to do the checks at 300-hour intervals, brushes pretty much gone at 600 hours.

That's a whole lot shorter life than what I've seen in automobiles. Worst case for me was 60,000 miles in my '85 Astro. Back then you could still buy brushes from the dealer so I did. Found one brush severely worn. The other one was missing, so it likely wore enough that it fell out of the holder. 60,000 miles at 25mph (a reasonable average for suburban driving) is 2,400hrs. Best case for me is my current 2007 HHR with 190,000 miles and still the original alternator. That works out to 7,600hrs. Any theory why airplanes should be so much worse?

 

[Bell206]

Any theory why airplanes should be so much worse?

Operating parameters and electrical loads. Even the auto alternators used in E/AB aircraft tend to have issues. High electrical loads at low RPMs while on the taxi way or ramp start to add up over time.

 

[Daleandee]

Until someone comes up with a small brushless alternator, we'll have to put up with this stuff. Heavy trucks have had brushless alternators for a long time already. Over 30 years.

And they still work fine at 25,000 hours most of the time.

FWIW ... my experimental aircraft uses a Dynamo (permanent magnet alternator) made by Yanmar that is brushless. It's only 20 amps but that's plenty for a VFR sport plane. These things run a long time and when they fail it's usually because one of the two bearings went out. Not much else to go wrong.

 

[flyingbrit]

Yeah, many motorcycles also use a PM alternator. Problem is you have to regulate the output directly with zener diodes or SCRs etc. instead of regulating the lower current field winding so it's only practical for 20A or less. What Dan is referring to is a much fancier system which essentially uses a small alternator to regulate the big alternator. Here is a description:
https://www.marineengineersknowledge.com/2021/06/explained-working-and-use-of-brushless.html

 

[Daleandee]

Yeah, many motorcycles also use a PM alternator. Problem is you have to regulate the output directly with zener diodes or SCRs etc. instead of regulating the lower current field winding so it's only practical for 20A or less. What Dan is referring to is a much fancier system which essentially uses a small alternator to regulate the big alternator. Here is a description:
https://www.marineengineersknowledge.com/2021/06/explained-working-and-use-of-brushless.html

Cool link! Thanks. I knew what Dan mentioned was very different as my PM puts out all the time and the regulator turns a lot of that into heat. So far so good.

 

[Bell206]

my experimental aircraft uses a Dynamo (permanent magnet alternator)

FYI: Just about all FADEC systems use a PMA to power the ECU/EECU once the engine is running. Even the Lycoming FADEC has a PMA on the accessory case.

 

[Dan Thomas]

That's a whole lot shorter life than what I've seen in automobiles. Worst case for me was 60,000 miles in my '85 Astro. Back then you could still buy brushes from the dealer so I did. Found one brush severely worn. The other one was missing, so it likely wore enough that it fell out of the holder. 60,000 miles at 25mph (a reasonable average for suburban driving) is 2,400hrs. Best case for me is my current 2007 HHR with 190,000 miles and still the original alternator. That works out to 7,600hrs. Any theory why airplanes should be so much worse?

There are good reasons why alternators in cars last much longer than in airplanes. Here's the major one:

The alternator has a redline. Around 10K RPM or so. In both the car and the airplane, its pulleys or gearing are sized so that the alternator reaches redline when the engine is at redline.

The car redlines at ~6000 0r 6500 RPM or whatever, but almost never reaches that. It will cruise at a third of that, maybe a bit more. The airplane redlines at~2700, and cruises at 90% or more of that.

So in the car the alternator is running, while cruising, at around 3000 or 4000 RPM. In the airplane, it's doing 9000. That means that the brushes, running on the slip rings, are experiencing far greater surface travel at the higher RPM, and friction simply wears them out a lot faster. More speed means more heat, which adds to the wear. Bearings also wear out faster.

Comparing cars to airplanes is a big mistake. The operating environments have little in common.

 

[Jdm]

I had an old A&P teacher once who had an interesting view about short alternator life. He emphasized the importance of not making practice of operating with a drained battery. Said it requires a lot more of the alternator and can result in shortened service life. He suggested keeping a fresh battery in the airplane and not allow it to become fully dead before replacing.

 

[Gary Ward]

Don't know if it is true or not but I leave the alt switch off while starting so there is not a lot of amps pulling through the alt while starting which can shorten it's life .
I use these under wing courtesy lights often at night after flying. They are LED and can run for several hours without killing the battery. I have a GPU that I plug the plane into in the hangar that recharges the battery before the next flight so the alternator does not have recharge the battery.
I have pull breakers on my electric turn coordinator and one of my G-5s that I pull out while using the power with the motor off and the main power switch on. I also turn the alternator switch off, it saves some amp draw from the battery.

Aerotech of Louisville Ky overhauled my ford alternator, they were great and fast to deal with and my alternator has been flawless.

Got back right before the snow started this night.

 

[Dan Thomas]

A 12-volt alternator draws about 4 amps when the master and ALT switches are on and the engine isn't running. That's a minimal draw, but if one happens to forget to turn stuff off after shutdown, that four amps slowly heats up the rotor's field winding and can roast it. I have seen that only once.

The alternator will take a bit of energy to turn during start, increasing the load on the starter a little bit. When starting might be a marginal proposition, such as in cold weather, better to leave it off, then turn it on once started. The risk here is forgetting to to turn it on. Keeping the battery charged up while parked is fine, but starting the engine eats a whole pile of power and the alternator has to replace all that anyway.

It does take a bit more for the alternator to recharge a low battery, but a low battery also won't start the airplane too well and most owners will replace it.

The alternator's job is to recharge the battery after start, and to supply all the airplane's needs during that recharge and after the recharge. In the typical lightplane, a 60-amp alternator has no trouble whatever doing this. It is designed to handle a whole 60 amps continuously. Where you might see some problems is at night, with all the lights on, and taxiing endlessly. Low RPM means high field current (that four amps), high output, and poor cooling, as the fan isn't pulling much air through the alternator. Still, alternators are a hundred times better than the old generators were. We should be grateful for them.

Most alternator failures are due to worn-out brushes. It's false economy to let them get that way, since a worn brush falls out of its holder and the spring behind it then arcs on the slip ring and chews it up and burns it out. Now you're not buying new brushes ($20), you're buying a new alternator. $500?

And a failed alternator can kill you if it's at night or in IMC and you have some distance to go. Battery dies, dark cockpit, no radios, no nothing. Does that sound like fun? Shouldn't that alternator get some preventive maintenance?

Some rebuilders are in the habit of putting far too much grease in the rear bearing. That grease squeezes out when the alternator is assembled and the rotor shaft goes into the bearing, and then it ends up on the slip rings, sludging them and the brushes up and causing electrical resistance that limits the field current and therefore the alternator's output. Kelly/Hartzell probably still haven't learned that lesson even after 20 years. I filed way too many SDRs on it for them to have any excuse. Some independent rebuilders are just as bad.

 

[Gary Ward]

A 12-volt alternator draws about 4 amps when the master and ALT switches are on and the engine isn't running. That's a minimal draw, but if one happens to forget to turn stuff off after shutdown, that four amps slowly heats up the rotor's field winding and can roast it. I have seen that only once.

The alternator will take a bit of energy to turn during start, increasing the load on the starter a little bit. When starting might be a marginal proposition, such as in cold weather, better to leave it off, then turn it on once started. The risk here is forgetting to to turn it on. .

I read that for split second while the starter is engaged the alternator is trying to provide way more than 60 amps that it is designed for since the starter pulls more than 60 amps. So that is why I leave it off. Yes the risk is forgetting to turn the alternator switch on after the engine starts. That is where my 20 buck USB charger comes in that plugs into my cig lighter. It makes a beeping noise I can hear even with my headset on to remind me to turn it on indicating low voltage. Pretty handy for 20 bucks from Sportys.

My LED lights draw less than 4 amps when on, and when I switch on the alternator switch it draws at least 4 amps on my 24 volt system. So leaving it off helps a lot when the engine is not running and I am using the under wing lights.

 

[Whitney]

I was taught to leave the alt switch off for engine start, since day one. Well 26 years later of flying, I still leave it off. If you did forget to turn it on within seconds of engine start, which is what I do, you would notice that you have no charging on your pre-take off gauge scan anyway.
As a part of my immediately following engine start checks are for sufficient oil pressure, and alt on. At run up, I double check its on, and showing me a positive charge rate, before I do my mag checks, while the rpm are up, as I do my mag checks at 1500 rpm.

 

[Dan Thomas]

I read that for split second while the starter is engaged the alternator is trying to provide way more than 60 amps that it is designed for since the starter pulls

It can't produce 60 amps or anything like it during starting. Its RPM is far too low to produce much at all. You can't get max amperage at engine RPMs much under runup RPM unless the battery has sulfated and shorted some cells and you'll see a pegged ammeter right after start, at 1000 RPM or so. Shutdown is necessary.

In the flight school, over 20 years and many airplanes and many thousands of hours, we never lost an alternator to starting loads. Never lost any significant numbers of alternators at all. Looked after bearings and brushes and they just kept going. Ultimately, it was worn-out slip rings that forced replacement.

 

[flyingbrit]

I read that for split second while the starter is engaged the alternator is trying to provide way more than 60 amps that it is designed for since the starter pulls more than 60 amps.

Agree with Dan that the alternator doesn't produce any current during cranking. However what you say is relevant when giving someone a jump from your automobile. If the good one's engine is running while the bad one is cranked it will stress the good alternator. In addition to the high current draw during cranking, there is a phenomenon known as "load dump". This is a voltage spike caused by the good car's field current going from max during the bad car's cranking to almost zero when it finishes. It takes a finite time for the current to decay and during this time (10s of milliseconds) the alternator is putting out a high voltage. The alternator's diodes are designed to limit this to about 28V by acting like zeners but it still represents a stress. I had a long ride with a tow truck driver once and asked him if tow trucks ever broke down. He replied of course they are like any other vehicle, but they also go through a lot of alternators. I figure this is due to them giving a lot of jumps. I still give jumps, but will NOT have my engine running. Not quite as good a jump, but less likely to cause any damage.

 

[Jdm]

Don't know if it is true or not but I leave the alt switch off while starting so there is not a lot of amps pulling through the alt while starting which can shorten it's life .

I’ve always tried to operate by the 172 POH. Don’t believe it says to crank with the switch off (not that it would hurt anything.)
My alt switch has always remained in the on position other than momentarily during the run-up check. Had it 21 years. Replaced one alternator. That’s was about 15 years ago.
Seems like I remember the A36 Bonanza POH calling for the the alternator switch to remain off until the after start check. Not sure why the difference.

 

[Dan Thomas]

Seems like I remember the A36 Bonanza POH calling for the the alternator switch to remain off until the after start check. Not sure why the difference.

Probably an engineer's opinion. There may have been a legitimate electrical reason in that model, but I can't imagine what it would be.

I did some troubleshooting on an Aerostar. The POH and checklist called for the alternators to remain off until after start, but sometimes they would not come online when the pilot turned them on once the engines were running. I traced the circuitry and saw some odd ideas there. I asked the mechanic who had consulted me to check the SBs for the airplane to see if there was anything on the issue, since it surely wasn't limited to one airplane. Not with that circuitry. He did some digging and found the SB; it called for a diode to be added to the circuit and to start the engines with the alternators on. The diodes were already there, so we started it with the alternators on and they worked every time. The guy who had done the SB diode installation had failed to amend the checklist.

The voltage regulators were triggered on by a signal from the start circuit through that diode. Once on, the alternator output locked them on. Dumb setup in the first place.

 

[Whitney]

A relatively small electric motor has to start our engines, especially on cool days, it is struggling. I figure that every tiny speck of extra effort I can reduce from that start up, the better.
I also hand swing the prop as part of every walk around.

 

[Dan Thomas]

A relatively small electric motor has to start our engines, especially on cool days, it is struggling. I figure that every tiny speck of extra effort I can reduce from that start up, the better.
I also hand swing the prop as part of every walk around.

That small electric motor produces several horsepower. It's small because its operation is intermittent and so heating and the need for cooling is minor. But leaving the alternator off does help a bit, by removing its load from the starter and by taking the field amperage load off the battery.

Don't swing that prop. People have been killed or maimed when the engine fired. One cannot trust that the mags are both dead. Ever. As mechanics, we never got careless around it. If we needed to work on it, we disconnected all the sparkplugs. If it HAS to be rotated, turn it backward but keep out of its way. Some will say that the vacuum pump can be damaged that way, but if that pump gets damaged, its vanes were shot and the thing was about to fail in flight anyway. Never did I ever have a pump fail when I was working on the engine and rotating the prop backwards. Never.

https://www.avweb.com/flight-safety/technique/dont-touch-that-propeller/