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Strange noise from up-front - magneto

10 Posts

On last 2 flights, 35 mins from EGKA to LFAT and back, I noticed a strange screeching noise after landing. It was audible only with headset lifted off, only after touchdown, only when the rpm was below about 1300, and it lasted only a minute or two, it was rpm related, and not related to aircraft motion. It sounded like a motor bearing failing…

The causes could have been

  • the engine (not terribly likely to get a high pitch noise out through the crankcases, I am told, and very few mechanisms for such an intermittent one)
  • the magneto
  • the vacuum pump
  • the high pressure fuel pump (cam operated; no obvious way to make such a noise)
  • the vacuum controller (curiously, Rapco say they should be binned after 10 years because the rubber diaphragm wears out and it can make a screeching/whistling noise which is obviously suction- i.e. rpm-related, and this item is on the cockpit side of the firewall so any sound would be very audible)
  • the alternator (almost everybody said it must be that)

So, with 15hrs on the oil I did oil analysis and checked the two filters and it showed nothing out of line, except 1.5% fuel in the oil (normally I see 0.5 to 1%). We checked there was no fuel coming out of the fuel pump drain tube, but I bought a new pump anyway as it’s cheap (£300). Most likely, with a new engine, this is fuel getting past the rings.

We took the belt off the alternator and had a look at the bearings etc and it looked good. I do have an induction motor on a bench drill at home which when it heats up expands the shaft and jams the bearings, so this was a real possibility.

The one item which will stop the engine dead is the magneto (single shaft dual mag D3000) and since I have a spare on the shelf we fitted that. Everybody said it cannot be the magneto an I didn’t think so either, but who wants to risk it?

I always carry a spare vac pump anyway but I ordered a new vacuum controller (£500 PMA from Rapco) since mine is 15 years old, plus however long Socata had it in stock (knowing how they work, perhaps another 15 years ).

No amount of ground running could replicate the noise. Only a landing after a flight would show it.

I have been away on a ski holiday, the wx has been crap, etc, but finally today we did a test flight. Very carefully, 4700ft around the airport and always within glide range, with FR24 showing a spider on LSD:

I even ran a sound recorder in the cockpit.

Then we landed and… no more noise!

So unless something really weird is going on, it was the magneto making that noise. Along with all the engine accessories, it’s done only ~120hrs, since being installed in April.

Interesting…

If changing the accessories had not fixed it, it would have been a strip of the engine, costing about 10k. Basically a shock load job but without the NDT.

Administrator
Shoreham EGKA, United Kingdom

Wouldn’t have guessed it would be the magneto. Everyday is a school day. Those single drive magnetos are very expensive, I’d be disappointed if it only ran 120hrs. Will be interesting to see what the repair station who inspect it have to say.

Buying, Selling, Flying
EISG, Ireland

Peter, shouldn’t it need few more flights to be sure ? Intermittent noise is not the easiest to troubleshoot ! I guess, internal inspection of the magneto will also confirm it was actually the faulty equipment, just in an other way.

You may well be right, but it was very consistent after the two I mentioned. And because it cannot be heard through the headsets, it could have been there for longer. This time neither of us two could hear anything.

I would be surprised if it can be reproduced on the standard quick and dirty mag bench test. I don’t think a mag even warms up on that test. The best I can hope for is an overhaul and lots of fresh parts.

Yes they are pricey – I paid $2.5k for that spare magneto. I bought it when Bendix stopped making them and nobody else could make any of the parts which weren’t PMAd. Now that’s been sorted but a spare is really good to have.

At the Annual, next month, I will throw away the vac pump, the fuel pump and the vac regulator anyway, so if the noise comes back, the alternator will be next.

However I plan to do a few more flights before the Annual, just to make sure.

Administrator
Shoreham EGKA, United Kingdom

Let’s hope you were right !
It wouldn’t be the first time the least suspected part proves to be the faulty one :-)

Scientifically, you should now put in the old magneto and see if the noise reoccurs. Wisdom, however, dictates otherwise…

So – wouldn’t running the magneto mounted on a bench at 1300 RPM reproduce the issue? Maybe after putting it in the oven to get it to operating temperature? If you still have some of the temperature measurement strips you used on the engine, maybe you could use those to establish the operating temperature, and heat it to that before putting it on the bench.

Biggin Hill

Alternator drive belts sometimes make a screeching noise. I squirt of WD40 usually cures it.

Indeed; it could be some other factor which “got fixed” by accident at the same time as the magneto was changed.

The alternator belt did get loosened and re-tightened, for example.

I will do a flight tomorrow – a short gap in the British wx is forecast – and that will be another data point.

The crankcase temp is about +90C during a flight which is slightly above the oil temperature, which is what one would expect. I reckon the magneto casing reaches around +80C.

I think it is temperature dependent, not RPM dependent, because it could not be reproduced at all on the ground run.

The good news is that it looks less likely to be the engine…

No, I am not putting the old magneto back in. I am not doing a PhD thesis

Administrator
Shoreham EGKA, United Kingdom

Another thing that produces screeching is a worn electric T&S gyro. You only hear it when power is reduced and headset off, sometimes it needs to get hot to make the noise so is difficult to reproduce on the ground. Sometimes electrical items sound RPM related because they spool up with increased output from the alternator from idle to low power.
I once had a continuous screeching on an overwater flight which I eventually tracked down to the stall warning horn (the stall vane had frozen up with earlier icing over the mountains). After it had been running for some time it sounded quite different from the usual sound – the horn was obviously getting tired. You would have thought that I would have recognized what it was immediately, but I didn’t – it sounded very metallic – just like a bearing failing. That horn also sounds when the throttle is closed with the gear retracted, an intermittent microswitch fault could also have the horn sounding off unexpectedly, especially just after landing. French light aircraft usually use automotive horns which are pretty feeble (probably off a 2CV).
Another good screecher is a kinked vacuum line, but that should be easy to reproduce on the ground.

One more flight done. 2 approaches, etc. Damn windy too – saw 216kt GS. No more funny noises. So it does look like it was the magneto.

I did wonder earlier whether I should have turned off the electrics when I heard the noise originally. It would have eliminated a lot of stuff. I did however remember to keep the alternator fully loaded up just like in flight, in case it was that.

Administrator
Shoreham EGKA, United Kingdom
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