Alternator kaput?

[ArrowFlyer86]

Have a trip I was going to fly out for tomorrow morning. I do my oil change and ground-run to check for leaks. When I start it up to taxi it over to fuel up, I realize my alternator isn't generating any juice. I troubleshoot it during taxi to try and see if I can fix it.

Status:
- Alternator belt tight, nothing obviously out of order from looking at that.
- Battery works like normal
- Note: Alternator belt replaced ~1 year ago
- If I'm not missing any log entries, the alternator was installed 33 years and well over a thousand tach hours ago. Digging through the logs I don't see any other mention of alternator OH/repair/installation.

Symptoms:
- Breakers: nothing blown, all good
- Ammeter = 0
- Add load to the system (landing light, pitot heat) = no change on the ammeter
- Radio #1: coming in a little muffled. Constant low volume crackling sound.... Radio #2: reception is fine. Constant low volume crackling sound.
- As soon as I turn off the battery switch but leave alternator running, my Garmin G5 detects a power input loss (so alternator not generating anything)
- Turning off the alternator switch while the engine is running eliminates the constant low volume crackling sound in both the radios.

Cycle the alternator switch (ON -> OFF.. wait... -> ON) a couple times to see if that helps. No dice. No impact on the ammeter at all, just impacts the low crackling sound in the radio.

I refueled at the self-serve pumps, turned the plane back on and on the taxi back to my hangar it's the same issue. I got it back as soon as I could so I wouldn't be burning too much battery power.

Question:
Any other troubleshooting steps I can reasonably do? Or is it off to an A&P? In either case my trip for tomorrow is almost certainly cancelled .

The only positive about this is that it happened at my hangar and not on my flight to Arizona tomorrow so I'm stranded somewhere.

 

[RyanB]

Yeah it’s probably toast. 33yrs is a good run and long overdue for replacement. Get a PlanePower - you’ll thank me later.

 

[ArrowFlyer86]

Yeah it’s probably toast. 33yrs is a good run and long overdue for replacement. Get a PlanePower - you’ll thank me later.

Appreciate the recommendation. I was initially ****ed off until I looked at the logs and couldn't find any mention since 1990
Any thoughts on what an alternator replacement costs? I assume they have to pull the prop off and stuff (I know they have to for replacing the belt, of course).
Wondering if this is a 1.5 AMU thing or a 3 AMU thing lol

 

[Eric Pauley]

Did you check bus voltage with the alt switch on? Wondering if it’s an indication error.

I’d have to double check the wiring diagram (assuming this is an Arrow) but I don’t think turning the master off (assuming that’s what battery switch means) allows running off the alternator even if it is working. I’ve never tried this in mine.

Edit: looking at the 28R-201 diagram I guess in theory it could be possible to run off just alternator with the master off, but I could imagine some voltage stability issue that causes a reduction/loss of field current quickly enough that it doesn’t work in practice.

 

[Jim K]

Appreciate the recommendation. I was initially ****ed off until I looked at the logs and couldn't find any mention since 1990
Any thoughts on what an alternator replacement costs? I assume they have to pull the prop off and stuff (I know they have to for replacing the belt, of course).
Wondering if this is a 1.5 AMU thing or a 3 AMU thing lol

Prop won't have to come off. Replacement is $800ish (or was last time I checked) or $1500 for a plane power. Swapping it takes a couple hours.

In the future, have the brushes replaced every 500 hours or so. I also learned this the hard way. Bad news is by the time they stop producing power, the commutator will have been eaten up, so replacing the alternator is your only real option.

Spruce almost certainly has one you could get tomorrow, so if you have a friendly mechanic it might be doable.

 

[Magman]

Why not do some quick checks first?

Assure no terminals are broken off on Alternator.

With ALT ON you should get battery voltage to the Field Terminal.

I will generally disconnect it first and see if you have it.

Defects in the Circuit Breaker, ALT switch or wiring can make the unit inop also.

 

[Capt. Geoffrey Thorpe]

Trace through the circuits with a volt / ohm meter.
Check the output from the regulator.
Prolly a good time to get the alternator overhauled, but I wouldn't assume that this will fix the problem.

Warning: I ain't no A&P.

 

[Bell206]

Any other troubleshooting steps I can reasonably do?

First, check the wires and connections between the alternator, regulator, switches, and battery. Be sure to put a hand on them other than just look at them. If you're good with a volt meter, check the reading per a troubleshooting chart like the ones below and go from there. If it has been 30+ years on the alternator you definitely got your money out of it x2.

Chart starting on page 484
http://www.mikeg.net/library/files/pa28r-service.pdf

https://planepower.aero/aircraft-alternator-support/troubleshooting/

 

[Eric Pauley]

Is the alternator light illuminated? This wasn’t mentioned in the post but I guess it’d be notable if it weren’t.

 

[4RNB]

If it happened associated with an oil change, I'd be suspicious some wiring with near short status converted to full short.
Happened to me my flight home after a new engine...

 

[Domenick]

Why not do some quick checks first?
Assure no terminals are broken off on Alternator.
With ALT ON you should get battery voltage to the Field Terminal.
I will generally disconnect it first and see if you have it.
Defects in the Circuit Breaker, ALT switch or wiring can make the unit inop also.

Listen to Magman. Check the field cable/wire carefully. Sometimes vibration breaks the conductors INSIDE the sheath at the connector. You can't see it from the outside.

The issue could be as simple as worn brushes.

The Arrow MX manual has an extensive section on troubleshooting/repairing the alternator.

 

[Dan Thomas]

The alternator will not work with the master off. It needs field current in order to generate, and that comes from the bus in a Piper. In a Cessna, it comes from the alternator's hot terminal and is bootstrapped by current feedback from the alternator output line to the bus. The regulator in both cases controls it.

The alternator light won't illuminate in most of these old airplanes. The regulator is not wired into the circuit the same as it was in a car. It's S terminal, the terminal that turns it on, is controlled by the ALT switch or master switch. In the car it was fired by the S (stator) terminal on the alternator, which would put out a little current when the car was started. In the airplane we need to shut the alternator off if it's in a runaway situation, or if it's not generating but still consuming field current. In some of these airplanes that light will only come on if the master is on and either you shut the alt switch off, or the overvolt sensor does it. The whole alternator could fall out of the airplane and that light would not illuminate. The introduction of the ACU (alternator control unit) around 1980 fixed all that. Plane Power has an ACU for their alternator, but that alternator will still run off an old regulator. The Plane Power alternator is still a brush-fed alternator like the dead one on this Piper, and its brushes will wear out just the same. Pay attention to the ICA that comes with it (Instructions for continued airworthiness).

Cessna's inspection sheets recommend a brush check every 500 hours. I usually found the brushes a bit more than 1/3 gone at that point, and at 1000 hours they would not make another 500 hours, so they got replaced. They're not expensive and the job isn't difficult but a lot of mechanics have never done it. That alternator gets run until the brushes fail, then the brush springs chew up the slip rings and now you're replacing a $500+ alternator instead of $20 brushes.

Piper's inspection sheets don't mention the brush check, and FAR43 Appendix D doesn't either, so they get ignored and the thing usually quits at a bad time, not outside your own hangar. It'll fail at night or in IMC or at some remote airport.

Airplanes with glass panels use a lot more electricity, meaning that the alternator works harder, meaning that the brushes are carrying more field current, so they wear out sooner. I started inspecting the 172SP's brushes at 300-hour intervals instead of 500, and they were shot at 600.

The regulator could also be at fault. Anyone with a multimeter and a bit of knowledge can figure it out, but one can pull off the F (field) wire at the regulator and check the resistance between that and ground. It should read around 3 to 5 ohms. If it reads nothing, there's a problem with the brushes. If it's high, like 30 or 50 ohms, the slip rings and brushes have been contaminated with oil, or with grease off the alternator's rear bearing, and a LOT of rebuilders put way too much grease in there and it extrudes out and flings around and gets on stuff it shouldn't and the thing fails. Turning the prop should not change the reading much if everything is OK and the resistance is good.

With the master and ALT on, the field terminal should have 10 or so volts on it. If there's nothing, the regulator is dead or there's a fault with the supply from the bus-breaker-alt switch.

1000 hours and 33 years means it needs overhaul or replacement anyway.

 

[dbahn]

Even with all of these great suggestions wouldn't you go ahead and just replace an alternator that's 33 years old and has had over 1000 hours of violent spinning?

 

[Country Flier]

I just had one replaced (same symptoms as yours, btw), but mine was only a few years / 500 hours old. A new plane power was less than 1 amu.

Edit: Make that 2 amu...I got it confused with my vacuum pump replacement...or maybe my starter replacement...its been a rough year.

 

[ArrowFlyer86]

If it happened associated with an oil change, I'd be suspicious some wiring with near short status converted to full short.
Happened to me my flight home after a new engine...

This is exactly what I was worried about. How did it happen *right* at oil change. An owner oil change no less. I took the cowl off to inspect and make sure I didn't bump anything else.

@Bell206 @Dan Thomas
I've got an A&P taking a look at it this morning and they said they'll check the wiring first to make sure it's not a bad connection and do some other troubleshooting. They said based on the time on the unit they won't be surprised if it needs to be replaced but they want to confirm it's not something simpler first. They mentioned most of the cases they encounter are due to a bad connection.

 

[ArrowFlyer86]

Also, just want to mention the reason I forewent any additional troubleshooting on my behalf is because I'm right at the critical time of still, maybe being able to make my trip down to AZ work. Getting a pro involved quickly seemed like a good plan. I'm about to taxi the plane over to them and I'm hoping they don't mind if I watch the work get done so I can learn a bit about it.

 

[RyanB]

How did it happen *right* at oil change. An owner oil change no less. I took the cowl off to inspect and make sure I didn't bump anything else.

I’m sure it’s just a coincidence. I’m not sure how you would bump the alternator changing oil. They’re on opposite ends.

 

[Magman]

It may be that Piper had a bad batch of CB’s but I’ve had to move the Field Wire to another Breaker

for the ALT to function.

The quick checks others have mentioned are worthwhile even if you intend to replace the unit.

 

[ArrowFlyer86]

Sorry quick separate but related question...

I've written about this on another thread but I'm wondering if it's related. Since buying the plane I've always had an issue where my battery sounds "weak" when I start the plane (many people have commented on that, A&Ps included). This is the case even after having replaced the battery at annual last year. After a couple flights it just sounds like it struggles a little at start-up. It remained undiagnosed because it didn't impede my flying, it was just annoying and a little bothersome.

And I fly all the time, like 4x+ per week so it's not like the battery was losing charge from sitting idle.

I'm wondering if the alternator was perhaps failing for a while now and not charging the battery all the way? Or is that not the way alternators work? Are they more of a binary situation where they're either working or they're not? Because if replacing this alternator kills 2 birds with one stone, particularly one that I've been lugging around in my shoe for a while, I'll be happy.

Also, just dropped the plane off at the shop and got the game plan. They're fairly confident they can have me out the door today. Thanks @Jim K for mentioning the job can be a quick one, otherwise I wouldn't have even bothered checking until I got back from AZ.

 

[Magman]

Generally the ALT should put out about 14V; measured at the battery.

Any less will result in a semi- charged battery.

Low cranking can also be indicative of aluminum wiring or voltage drops at connections.

Whether it is any of these can be determined with a Harbor Freight Aircraft Supply Voltmeter .

Or course there are better though.

 

[Country Flier]

Sorry quick separate but related question...

I've written about this on another thread but I'm wondering if it's related. Since buying the plane I've always had an issue where my battery sounds "weak" when I start the plane (many people have commented on that, A&Ps included). This is the case even after having replaced the battery at annual last year. After a couple flights it just sounds like it struggles a little at start-up. It remained undiagnosed because it didn't impede my flying, it was just annoying and a little bothersome.

And I fly all the time, like 4x+ per week so it's not like the battery was losing charge from sitting idle.

I'm wondering if the alternator was perhaps failing for a while now and not charging the battery all the way? Or is that not the way alternators work? Are they more of a binary situation where they're either working or they're not? Because if replacing this alternator kills 2 birds with one stone, particularly one that I've been lugging around in my shoe for a while, I'll be happy.

Also, just dropped the plane off at the shop. They're fairly confident they can have me out the door today. Thanks @Jim K for mentioning the job can be a quick one, otherwise I wouldn't have even bothered checking until I got back from AZ.

Generally the ALT should put out about 14V; measured at the battery.

Any less will result in a semi- charged battery.

Low cranking can also be indicative of aluminum wiring or voltage drops at connections.

Whether it is any of these can be determined with a Harbor Freight Aircraft Supply Voltmeter .

Or course there are better though.

Can also be a "going" bad solenoid.

 

[mandm]

You’d be surprised, some wires are so old and they rub or get frayed and loose then cause issues. For the mechanic it’s just easier (and faster) to swap out the alternator. Get a Plane Power alternator. It’s around 1k and add a couple hundred to your mechanic. Probably 1200-1500 out the door (I forgot the exact amount). Yes your mechanic can try to open it up and fix it, but it’s old and you want your airplane to work now without any issues so sometimes just easier to get a new part.

For the battery, I would get a battery minder installed and plug her in between flights (just a few hundred bucks for the parts and installation). This was kind of a must by my mechanic too.

 

[ArrowFlyer86]

Low cranking can also be indicative of aluminum wiring or voltage drops at connections.

Whether it is any of these can be determined with a Harbor Freight Aircraft Supply Voltmeter .

Thanks! I upgraded to the Bogert copper cables when I bought the plane, so aluminum wiring shouldn't be a thing anymore for this plane, I think.
I'll have to read back up on testing some of these things using multimeters/volt meters. It's been about 5 years since I really was in the weeds with them on them microelectronics stuff. Good to know harbor freight sells one, there's one right down the street.

EDIT: Probably a stupid logistical question, but how can you measure how much voltage the alternator is putting out, measured at the battery. Assume I'm one pilot trying to do this. I have to have the aircraft running in order to see the power output of the alternator (meaning I'm at the controls), how do I simultaneously measure the voltage of the battery? Or is this just a 2 person job?

 

[Magman]

Safest with 2 people.

or

Or you can purchase HF Aircraft Diagnostic Kit consisting of some wire and jumpers.

Might change reading a tad.

or

You can check at the bus or cigar lighter

or

Get creative with your phone and video,

ALT output must be above batt voltage in order to charge.

Have you tried cranking with a fully charged battery? Perhaps with charger ON?

A poor or missing ground cable will cause cranking issues.

Throttle, Mixture Cable and Fuel Lines make a poor substitute but it could

get exciting!

 

[Bell206]

It remained undiagnosed because it didn't impede my flying, it was just annoying and a little bothersome.

FYI: one of the most neglected systems on an aircraft, especially a private Part 91 aircraft, is the electrical system. Few meet original spec. My 1st guess is your problem is a worn start relay or a bad connection. A voltage drop check during the start sequence will isolate the point if that is the issue. My 2nd guess would be to have the amp draw checked on the starter as it can draw down the entire system.

how do I simultaneously measure the voltage of the battery?

You can fabricate some test leads (or buy them) and hook them up at the points you want to check voltage and run the other end into the cockpit. Make them as long as you need but use at least 16 gauge or larger wire. Tape the leads to the fuselage so the prop wash doesn't disconnect them. Be sure to label the leads if using more than 1. I usually used a "break-out" box I made so I can troubleshoot issues by myself but that is way overkill for your requirement.

 

[Daleandee]

Even with all of these great suggestions wouldn't you go ahead and just replace an alternator that's 33 years old and has had over 1000 hours of violent spinning?

If it ain't the problem, it will be soon enough ...

EDIT: I see below that the OP found the alternator was newer than previously stated. Good news!

 

[ArrowFlyer86]

FYI: one of the most neglected systems on an aircraft, especially a private Part 91 aircraft, is the electrical system. Few meet original spec. My 1st guess is your problem is a worn start relay or a bad connection. A voltage drop check during the start sequence will isolate the point if that is the issue. My 2nd guess would be to have the amp draw checked on the starter as it can draw down the entire system.

You can fabricate some test leads (or buy them) and hook them up at the points you want to check voltage and run the other end into the cockpit. Make them as long as you need but use at least 16 gauge or larger wire. Tape the leads to the fuselage so the prop wash doesn't disconnect them. Be sure to label the leads if using more than 1. I usually used a "break-out" box I made so I can troubleshoot issues by myself but that is way overkill for your requirement.

Thanks! When you say "or larger" wire, do you mean higher gauge or physically larger (lower gauge)?
I have a several identical alligator-clip leads lying around that I could daisy chain together to make them long enough, as long as that's not a problem.

Regarding the relay, that's something that was mentioned as a potential thing to look at by the last shop I used. Though they never got around to actually looking at it while the plane was in there.

 

[ArrowFlyer86]

Update:
Just got a call back and it turns out they found a broken wire. The PoA crowd was right to have me ask them to check that first! . Should be good to pick up the plane in an hour.
They also found that the alternator is actually much newer. I was looking in the airframe logs last night when trying to find the alternator history, but there was an entry in the engine logs instead marking a new alternator installation. I didn't expect to see that in engine logs. Thank you all for your help and recommendations. And now I'm definitely just going to make a spreadsheet for all of the major components where I can track their age and maintenance rather than having to go back to the well each time and dig it out of the logs like I've been doing. Inefficient and leads to errors like this.

The issue of the low crank is something I'll have to dig into with the recommendations above.

 

[Gary Ward]

Glad they got it fixed for you!!

My AP mechanic pointed out that my original starter, voltage regulator and alternator were due to be replaced during an annual one year.
Months maybe weeks later the voltage reg went out, I replaced it and all was good for more hours. Then the starter went out, sob I guess he was right. This was all within a years time of him mentioning it. Then months later it quit charging, it appears the alt was 30 years old with less than 800 hrs on it. He suggested I call Aerotech of Louisville KY.

This was Fri night when it quit working. Which I did call on a Sat morning. They answered and said they had a core on the shelf that he could rebuild for me Mon morning. I said great please do it. I called again early Mon morning and she said he was in the back working on it for me. The lady called me back at 11am and said it was ready for pickup. My wife drove down there with our old one to pick it up. They took a quick look at the core and sent here on her way. I put it on the plane that afternoon and was flying it that night. They were very nice to my wife and did what he said he would do.
This was just a couple years ago and I am nearing 500hrs since. It is going to come off and the brushes will be inspected soon. It cost 395 for overhauled 60 ford style alternator with all paperwork.

 

[Dan Thomas]

Can also be a "going" bad solenoid.

Regarding the relay, that's something that was mentioned as a potential thing to look at by the last shop I used. Though they never got around to actually looking at it while the plane was in there.

The master and starter contactors (also sometimes called relays or more often solenoids) Get old. Their contacts oxidize with age. Those contacts burn every time they open and arc a bit. They end up introducing a tiny resistance that you will never spot with your ohmmeter, but the massive starter current sure will see it. Ohm's Law says that voltage is equal to current times resistance. An old contactor that has a resistance of 1/50th of an ohm, with 200 amps surging through it, will cause a voltage drop of 4 volts, something that reduces the starter's current a whole bunch and makes it lazy.

Some guys change the starter contactor and nothing improves. They forget (or don't know) that the master contactor is in series with the starter contactor, and they both have to be in good condition.

Bad ground are also a big deal. Loose or dirty or oily or corroded ground straps between the engine and airframe will do it. Voltage drop measurements can be taken across the contactors, engine and airframe, and so on to find the culprit.

 

[Dan Thomas]

The OP sounds like he's going to chase down the component times and ages. I wish all owners would do this. It's not just alternators that typically get run until they quit; it's the magnetos (another 500-hour item) and the vacuum pump, which Cessna tells us to replace every 600 hours unless it has a provision for vane wear inspection. I don't know why anyone would buy a vac pump that doesn't have that provision. Last I checked, Airborne still didn't. Tempest and Rapco have had it for a long time. First check at 500 or 600 hours, then every 100 hours thereafter until the vanes are worn to limits, then replace the pump. This gets you the biggest bang for the buck, and prevents the most common cause of pump failure: badly worn vanes that cock in the rotor slot, jamming and breaking it. And it's not just the pump that suffers; the vacuum in the system will suck back carbon debris from the pump, sometimes all the way into the instruments. That can damage the instruments, and even it doesn't, and all the vacuum lines don't get cleaned out, that debris will get sucked into the new pump and chew it up and shorten its life.

It's false economy to run stuff to failure.

 

[Morgan3820]

The OP sounds like he's going to chase down the component times and ages. I wish all owners would do this. It's not just alternators that typically get run until they quit; it's the magnetos (another 500-hour item) and the vacuum pump, which Cessna tells us to replace every 600 hours unless it has a provision for vane wear inspection. I don't know why anyone would buy a vac pump that doesn't have that provision. Last I checked, Airborne still didn't. Tempest and Rapco have had it for a long time. First check at 500 or 600 hours, then every 100 hours thereafter until the vanes are worn to limits, then replace the pump. This gets you the biggest bang for the buck, and prevents the most common cause of pump failure: badly worn vanes that cock in the rotor slot, jamming and breaking it. And it's not just the pump that suffers; the vacuum in the system will suck back carbon debris from the pump, sometimes all the way into the instruments. That can damage the instruments, and even it doesn't, and all the vacuum lines don't get cleaned out, that debris will get sucked into the new pump and chew it up and shorten its life.

It's false economy to run stuff to failure.

I keep pointing this out to my mechanic. It’s a lot cheaper for him to replace it before it breaks here and now that it is some other place when I’m broke down on the ramp having to get a hotel for three or five days but he still thinks I’m wasting money. Which is I guess nice of him to care, but….

 

[Dan Thomas]

I keep pointing this out to my mechanic. It’s a lot cheaper for him to replace it before it breaks here and now that it is some other place when I’m broke down on the ramp having to get a hotel for three or five days but he still thinks I’m wasting money. Which is I guess nice of him to care, but….

You need a new mechanic. These guys with the "if it ain't broke, don't fix it " mentality are dangerous. Besides, he's not the one stuck far from home when something quits.

 

[mandm]

Safest with 2 people.

or

Or you can purchase HF Aircraft Diagnostic Kit consisting of some wire and jumpers.

Might change reading a tad.

or

You can check at the bus or cigar lighter

or

Get creative with your phone and video,

ALT output must be above batt voltage in order to charge.

Have you tried cranking with a fully charged battery? Perhaps with charger ON?

A poor or missing ground cable will cause cranking issues.

Throttle, Mixture Cable and Fuel Lines make a poor substitute but it could

get exciting!

Is it more exciting to smoke a cigar while flying an airplane? Does the smoke trap inside? Maybe I’ll try it one day haha and wait a year for the smoke to disappear.

 

[mandm]

Update:
Just got a call back and it turns out they found a broken wire. The PoA crowd was right to have me ask them to check that first! . Should be good to pick up the plane in an hour.
They also found that the alternator is actually much newer. I was looking in the airframe logs last night when trying to find the alternator history, but there was an entry in the engine logs instead marking a new alternator installation. I didn't expect to see that in engine logs. Thank you all for your help and recommendations. And now I'm definitely just going to make a spreadsheet for all of the major components where I can track their age and maintenance rather than having to go back to the well each time and dig it out of the logs like I've been doing. Inefficient and leads to errors like this.

The issue of the low crank is something I'll have to dig into with the recommendations above.

Awesome, you can write down all of your major components and the last time they were overhauled/hours and any repairs. Makes it easier.

Fuel Servo
Magnetos
Alternator
Starter
Fuel pump
Seals & Belts
Landing gear springs
Battery
Spark plugs
Vacuum pump
I’m sure there’s more

 

[Bell206]

now I'm definitely just going to make a spreadsheet for all of the major components where I can track their age and maintenance rather than having to go back to the well each time and dig it out of the logs like I've been doing.

Another item I recommend would be to review the factory inspection program and the SB/SL listings for your specific aircraft, engine, and prop. While I'm not a fan of using all factory references for various reasons, they do contain specific items that should be inspected or monitored, as mentioned above, that are not addressed in generic inspection checklists or Part 43 Appx D. You could easily add these specific additions to your spreadsheet and use it as a discussion reference at your next annual or other interaction with your mechanic or shop. When I provided owner-assist mx we would develop similar lists which allowed the owner to be more "engaged" with the aircraft maintenance requirements.

When you say "or larger" wire, do you mean higher gauge or physically larger (lower gauge)?

Lower gauge. Most of my test leads were/are 12awg, but 16awg is the smallest I found that did not have the potential to induce any errors over longer distances.

 

[Dan Thomas]

Awesome, you can write down all of your major components and the last time they were overhauled/hours and any repairs. Makes it easier.

Fuel Servo
Magnetos
Alternator
Starter
Fuel pump <-----
Seals & Belts
Landing gear springs
Battery
Spark plugs
Vacuum pump
I’m sure there’s more

Good catch there. The aux fuel pump often has a ten-year life assigned to it by the manufacturer. Most get run until they won't run anymore, and that could be deadly if the engine-driven pump failed in flight and the aux pump was dead.

Stuff like the battery and sparkplugs are on-condition items. They should be getting checked and tested regularly, at least at every 100 hour or annual, whichever comes first. If that stuff is being ignored, what else in the airplane is being ignored?

Airplanes aren't cars. If the airplane's engine dies, you could die too.

 

[Morgan3820]

You need a new mechanic. These guys with the "if it ain't broke, don't fix it " mentality are dangerous. Besides, he's not the one stuck far from home when something quits.

Oh, he is coming around, besides he is a partner in the airplane. It would be awkward to go elsewhere. Plus, the third partner agrees with my approach, the partner/mechanic is typically out voted.

 

[Eric Pauley]

You need a new mechanic. These guys with the "if it ain't broke, don't fix it " mentality are dangerous. Besides, he's not the one stuck far from home when something quits.

Doesn't this depend on what the failure curve looks like for a given component, though? If failure is a memoryless (Poisson) process, for instance, then early replacement offers no benefit (of course everything wears out in practice so this is only true to a point). Worse, if the part is subject to infant mortaility/defects then replacement increases odds of failure!

I don't know the actual failure curves but to speculate:

The vacuum pump is definitely an example where timed replacement makes sense, since clearly the vanes wear out over time. I'm glad I won't have to think about this one anymore.

Aux fuel pump feels like it might side more towards memoryless (though I have limited evidence to back this up), for instance because debris jamming would be memoryless. Anecdata: I had a fuel pump crap out something like 5-10h after replacement! Both fuel pump failures for me were on the ground and were confirmed (before replacement) with percussive maintenance. Given the tendancy of vibration to free siezed fuel pumps I kind of suspect that fuel pumps would be far more likely to fail on the ground than in the air.

Alternator feels like a case where infant mortality could be a concern.

 

[Dan Thomas]

I don't know the actual failure curves but to speculate:

The vacuum pump is definitely an example where timed replacement makes sense, since clearly the vanes wear out over time. I'm glad I won't have to think about this one anymore.

The vanes wear, but with the inspection provisions of Rapco and Tempest you can monitor it. Continentals spin their vacuum pumps faster than Lycomings do, and they wear out faster. You can't use Continental pump life to judge the life of a pump on a Lyc.

Aux fuel pump feels like it might side more towards memoryless (though I have limited evidence to back this up), for instance because debris jamming would be memoryless. Anecdata: I had a fuel pump crap out something like 5-10h after replacement! Both fuel pump failures for me were on the ground and were confirmed (before replacement) with percussive maintenance. Given the tendancy of vibration to free siezed fuel pumps I kind of suspect that fuel pumps would be far more likely to fail on the ground than in the air.

It isn't debris. The pump is downstream of the fuel strainer, which has a very fine screen. It's the life, in years and also affected by engine heat and the chemistry of the fuel on the polymer vanes in the pump. They get old and brittle and break. The bearings of the pump also suffer.

Alternator feels like a case where infant mortality could be a concern.

Looking after many airplanes, and doing many, many brush inspections and replacements, I can tell you that the alternator is a robust device whose weakness is the brushes. 95% of alternator failures will be those worn-out brushes. I never once had a diode fail. I did have one fail when the field coil in the rotor burned out because someone left the master on overnight, which keeps that field alive with maximum current when there is no cooling air flowing through, and the windings burn their varnish off and they short.

I came across many alternators, especially low-time Kelly Aerospace (now Hartzell) alternators in which the assemblers had put far too much grease in the rear bearing. When the rotor shaft is inserted, that excess grease squeezes out and gets on the slip rings and causes dissolution of the brush carbon and a resistive sludge forms on the slip rings, limiting the field current and therefore the alternator's output. No matter how many SDRs I filed, the problem continued. Poor training at the factory, and no one is reading the Electrosystems manual that covers this stuff.

Alternator feels like a case where...

Technicians don't go by feelings. They use wisdom and knowledge gained through training and experience. And the training should never stop. Ever. If one isn't learning, he's stagnating and falling behind.