A319 weird high pitch whine...

[Gallinazo]

During my walk around at Rio Negro the other day I heard a loud, high pitch whining sound emitting continually from the left horizontal stabilizer. The same sound the yellow hydraulic system electric pump makes when it is switched on, in fact. I was baffled, and began wondering if I had the whole thing about the servo's wrong, that perhaps they did in fact have their own little electric pumps and that it had stayed on! It made no sense. The base maintenance, myself and the Commander were all baffled, and we tried all sorts of things with the flight controls and the different hydraulic systems, SEC's and ELAC's. No joy. Then we decided to switch off the nav / logo light and the sound disappeared. The breaker was pulled and the item deferred, and we flew back to Quito.

I have heard no more of the event from maintenance at home base, so I want to ask here.

What on earth can it be that a logo light has that can make a sound like a electric hydraulic pump? :confused:

[nitpicker330]

The A330 with the APU running can offload excess bleed air, it makes a screaming whining noise when it does and it goes up and down in volume. It's worse with the APU bleed selected off ( naturally )

[Uplinker]

Hi Gallinazo,

I can easily believe that the logo light was causing the whine - good spot by the way for working out what it was. I can't recall if logo lights are fed from 28VDC or 115AC - I would have thought 28VDC. But there could easily have been a break in the light or the wiring insulation allowing a short circuit arc to earth. If the physical dimensions of the short were just right, it could have caused a constant discharge - like a mini arc welder - and the hollow construction of the horizontal stabiliser could have amplified this sound. If the lights are fed from AC, then even AC fittings themselves can sometimes vibrate (at 400Hz in this case), or there could have been a bad connection, either of which could make a pump-like sound.

I was snorkelling once in a harbour and could hear what sounded just like a pump running under water. I investigated and eventually traced the sound to a broken wiring box under water which had exposed wires and bubbles fizzing off the connections! Mains wiring would you believe going to lights on the harbour wall!!! (No, I didn't touch it and Yes, I did tell someone about it and got them to switch it off!).



U

[nitpicker330]

I've suggested what the noise was and its a lot better than your Nav light idea!!

:D

[Uplinker]

Nit; You're entitled to your opinion, and I'm entitled to mine. The OP said A319, not A330. If it was as you suggest, why did turning off the LOGO light cure the problem???

Don't start another PPRUNE argument for goodness sake.

You may not understand electrics, but I do.


U

[Gallinazo]

Thanks for the suggestions!

nitpicker330; It is worth bearing in mind for another occasion, but there was one catch this time around. The APU was off.

:eek:

Uplinker; I managed to talk to someone in maintenance more in the know of the A319's electrics today, and he informed me that the power that arrives in the horizontal stabilizer for the logo light is, indeed, 115 AC (this checks out in the FCOM), but it is stepped down locally (in each horizontal stabilizer) with a small transformer to 28 V for the light itself. Whether it is 28 VAC or 28 VDC he was not sure. I asked him that particular question as I know from years ago that "bulbs" are indifferent to AC or DC and will work equally well on either. I asked him to please do me a favor and check it with a multimeter the next time he was doing anything around that area to determine the type of current, or look a bit closer in his texts for the answer, as I am curious now. Anyway, the noise was coming from a faulty transformer, he said.

I would not have imagined it could make such a racket; during the walk around I initially thought it was a constant siren from some hangars in the distance, until I was under the horizontal stabilizer and located the sound, after which it seemed to be the exact same sound as the yellow system pump.

Totally new to me...

[McGoonagall]

Maybe a long shot and I am talking as a lowish hours NPPL but a very long time train driver. We encountered a problem with a new model of locomotive. The engine at full power had a lower cab noise than the next notch down which caused a howling noise in the cab which was unbearable for long periods. After extensive tests it was found that the actual dB level of the lesser power setting was lower than the full power setting, the excess noise was something called Harmonic Resonance. It would not show up on a decibel test of total sound and was of a frequency that the human ear was rather apt to pick up.

It was tracked down to an electrical relay that emitted a sound wave which at that power level caused the resonance within it's location and the resultant bloody noise. Relocation of the relay and having it somewhat insulated cured the problem.

Apologies for the cack handed explanation. Driver not fitter.

[fantom]

Y pump? Or is that too silly?

[Uplinker]

Ah, well there you go - an AC transformer. As I said, AC devices can vibrate. From what you say, 115VAC is fed to a small transformer next to the light which steps it down to 28VAC to feed the light. I am guessing now, but perhaps the light, (which is external and subject to the weather), is fed with AC rather than DC to avoid electrolytic corrosion problems?

A transformer works by magnetism - the magnetism generated by one coil induces a current flow in the other coil. If the windings in the transformer are loose, the AC magnetic field will make the windings vibrate more than normal, giving rise to an audible sound, (same mechanism as a loud speaker, but obviously unintentional and undesirable in a transformer).



U

[ironbutt57]

As well have encountered the high pitched whine from the stab area, engineer described it as some strange internal bulb fault and turned off logo lights...noise disappeared..

[nitpicker330]

wow only on an Airbus would the Logo lights make a funny noise!!

[Mr Optimistic]

if it's magnetostriction wouldn't it only be pitched at twice the a/c frequency ?

[Uplinker]

I just had to look up the term 'magnetostriction'. It could be that, but I doubt it. I reckon that the transformer windings, which should be securely mounted on the transformer core and fixed in a resin have just loosened for some reason, possibly bad manufacturing, or old age or tail plane vibrations?

Aircraft AC is 400Hz of course, but the panel the faulty transformer was attached to could in theory surely resonate at any frequency mathematically related to that - simple harmonics?


U

[TURIN]

Oh nooooo!

I've just got my head around the idea of electrical circuits at resonance and now this thread throws all my thinking in to turmoil.

Good job the exam was yesterday. :)