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Generator problem on turbine twin

10 Posts

Hello colleagues,
I have a problem on the aircraft, which nobody of the certified (very) expensive professionals with lots of certificates and ratings can’t seem to solve. It’s not a car, so simply ordering and exchanging expensive (2k+) components is not a viable option, only replacing after verifying. I hope collective knowledge might push some ideas.

Aircraft in question: Piper Cheyenne II (PA-31T) YOM 1980

Problem: Right generator… does not work, or works strangely, you judge, here are the details:

(Ng >= 68%)
L GEN ON, R GEN ON: L amps: ~50, Voltmeter 25-26V, R amps: 0, NO lights on annun p.
L GEN ON, R GEN OFF: L amps: ~70, Voltmeter 28.5V (!), R amps: 0, “R GEN INOP” light
L GEN OFF, R GEN ON: L amps: 0, Voltmeter 0V, R amps: 0, “L GEN INOP” light

What was done so far:
- Replaced brushes on right generator
- Tried swapping voltage regulators, picture becomes a bit different:
L GEN ON, R GEN ON: L amps: ~70, Voltmeter 25-26V, R amps: 0, NO lights
L GEN ON, R GEN OFF: L amps: ~70, Voltmeter 25-26 (!), R amps: 0, “R GEN INOP” light
L GEN OFF, R GEN ON: L amps: 0, Voltmeter 0V, R amps: 0, “L GEN INOP” light

Likely, right ammeter is inop, but given the strange picture it’s not 100%. It worked a few flights before as clock (always 28.5V on all settings, as per manual), so it would be strange if regulator adjustment is needed – because mechanic who did runup told me that it worked (28.5V). Only when I did the runup, it didn’t and there was the picture above.

Shop I am currently in can’t fix the issue! ;-)

LYTV, Montenegro

I’m certainly no expert in aircraft electrical systems, but the first thing I would do is make sure the right ammeter works, then I would measure the alternator field current of the right alternator in all of your three scenarios. That should give you a better understanding of what’s happening. With a clamp ammeter it shouldn’t be too difficult. I guess you can access the alternator field circuits at the circuit breaker panel?

Exactly what conditions trigger the GEN INOP lights? Maybe you can post the relevant part of the electrics schematics?

Last Edited by Airborne_Again at 13 Sep 14:23
ESKC (Uppsala/Sundbro), Sweden

I would measure the alternator field current of the right alternator

It is probably a starter generator and not an alternator. I am no expert at all but does the thing start the engine properly? If not I would get it overhauled. The cost is small compared to a hot start. Overhaul in the US should be 2500 USD. If it starts fine you will need the electrical schematic to figure things out…

www.ing-golze.de
EDAZ

Just ideas, I’m not an expert in GA neither regulator, but in general these pieces gets sensitive to heat more than usual when it gets old. The it could may be work on the ground during run-up, but could show a strange behaviour once in the air, especially if it draws a lot of current. May be you can swap ammeter to test the second generator, with the risk of burning it as the right failed regulator may have already burnt it.

LFMD, France

So, it was main contactor (huge heavy-duty solenoid). It is situated right before the shunt, that’s why ammeter shows 0. Just to put some information into common experience… :)

LYTV, Montenegro

Interesting – thanks for the update.

Sounds like one of the Stancor relays…

These things are easy to troubleshoot if one has the schematic (the circuit diagram) a voltmeter and some basic electrical knowledge. Still, somebody I know spent thousands to fix an issue with the starter motor not running, while the maintenance company changed every item in the chain and billed him for it

Administrator
Shoreham EGKA, United Kingdom

It’s MS24171-D2. And yes, that’s what we did – took schematics, traced, tested, and then we tested the solenoid itself in-place with a spare battery (disconnected everything else) and vuala.

On the other matter you mentioned, see my other recent post… ;-)

LYTV, Montenegro

Those guys (who didn’t fix) changed brushes (ok, that’s not prohibitively expensive), but then wanted to change voltage regular, which is made out of solid gold probably (given the cost).

LYTV, Montenegro

Peter wrote:

while the maintenance company changed every item in the chain and billed him for it

Which only goes to show … if you dont fly a C172 or PA28 the maintenance company is often learning at the customers expense as we have all found out

United Kingdom

More than true. But in my case I was lucky to (eventually) find one with experience on the type. Although, in that particular case, problem could be solved by just following the schematics and using basic electrical knowledge. And that means, some shops are SO inexperienced/uneducated that even C172 would be a problem for them…

LYTV, Montenegro
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