Mechanical whine which appears only when ALT field current is ON

I cannot reproduce it now, but then a ground run would not get the alternator hot enough.

I am changing it anyway.

The crackling sound, readily reproducible at 1100rpm, turned out to be loose plastic in the overhead trim

Someone suggested the whine may have been a passenger

Anyway, in case anyone is interested, or someone with a similar problem reads about this in years to come: the alternator was changed and no matter what I do, with a 1hr test flight, I cannot reproduce the whine now.

I need to again obtain a spare to keep on the shelf. This is not a cheap model – close to £1k. I am now trying to get a quote for an ALU-8521LS but as usual here in Europe nobody wants to take in the old one as a core, because we are so many millions of nm away from where people overhaul stuff (instead of chucking it in a bin and billing the customer 3x more) I can send it to the US but the shipping cost on something so heavy wipes out the savings.

I can now finally report that the alternator whine was real and was caused by a crack

There may be more than one crack.

Under the belt tension, this crack opened up and the rotor was touching the stator.

I know for a fact that the belt was not over-tensioned, ever, because this is something I check very diligently.

Unfortunately this will cost me $$$ because the warranty on spares kept on the shelf always runs out while they are, ahem, on the shelf…

That’s the problem. I bought these parts directly from the US. But even if I bought them from the UK (Adams for example) the warranty will still start running when it is shipped to me. The only way to avoid that is by me pretending to be a maintenance shop.

I am slowly getting the view that a lot of overhauled stuff is junk. It gets standard prescribed checks and tests and if it passes these, it is “good”. Actually we already knew this when it comes to engines: almost everybody overhauls their own engine, until they cannot do so any more, so most of the exchange engines are 6000hr+ jobs. It must be the same for overhauled-exchange accessories.

The alternator I am now flying with is a new one, bought in the UK, but of course no warranty on that either because I had it stored for a year or two.

I didn’t spot the crack.

In fact I was originally not going to send it back to the US (or anywhere), because the shipping costs a lot and the core value is peanuts – under $200 and the company is now offering me only $100 because the housing is cracked.

But we found a shipping service which is quite cheap – about £50 – and I sent back the alternator together with a D3000 magneto which I had to send back anyway. It had a suspect sticky bearing and a suspect impulse coupling mechanism. It was quite obviously much harder to turn that the spare I had. The company says there is nothing wrong with it, and the stiffer rotation is because the magnetic flux is stronger. Is that even possible?

The D3000 magneto stuff is a different topic (and the mag is worth some $3k) but again I had it before – here.

I have been offered an exchange overhaul alternator for $675 which is close to buying a new one in the UK so this can’t be worth doing, especially as I might be getting somebody else’s dodgy one.

The “is overhauled any good” was discussed briefly here. I think there is a real problem with the quality/reliability of overhauled accessories, but in some cases you have more or less no choice due to the cost of buying a new one.