May have heard a disgruntled wizard operating the engine. With that being said, I have no idea what you heard
My hypothesis is that, maybe something was stuck to one or more of the impellers, causing an imbalance, and therefore vibration. What would get stuck to an impeller without damaging it, I don't know, but that's my somewhat kind of educated guess.

Omag 2.0 wrote on May 20^th, 2010 at 4:26pm:



Hi Kris

Firstly.....bugger, been a long time since I saw you post. Life must be keeping you busy.

Ok, now down to business. The engine was just wind milling, being turned by the breeze. It does not take much of a breeze to start one turning. The whine you heard was more then likely caused by the engine wind milling backwards. The engine was most probably being turned by a tail wind. If the wind is strong enough, not only will the fan turn at the front, but it may also cause the HP compressor to turn too. It is not connected in any way to the fan at the front, so you would not know. If the HP compressor is also wind milling (and in this case I suspect backwards too) it will be turning the gearbox, also backwards. No biggy, but when they do this, they whine as the gears are cut to travel in the opposite direction. As I said, no biggy. Your second point, the A320 has a CFM56 engine (not to be confused with the CFM56 on a 737NG, totally different engine)It is a Hi bypass engine, that is to say, 80% of the thrust is produced by that single fan you see at the front. This fan is made up of blades that are fixed to a rota. In the case of the CFM56 in the Airbus, they are a long blade and if you look at the blade closely you will note at the top end are small horizontal wings. These are called snubbers. The blades in the Airbus CFM56 are able to "flop" from side to side. As the engine rotates, they are mealy hitting the snubber of the blade next to them. Once the engine is started, centrifugal force prevents them from making contact with each other making the clattering sound you heard. What you heard is a perfectly normal noise for a the Airbus engines to be making. If you fly in a 737NG, you will note that the blades are almost three times as wide and do not have snubbers. That is because theses blades are fixed in place and do not have any sideways motion, so a wind milling 737NG will be silent. However if you fly in a 737 classic, then they will sound just like the Airbus. As for you oscillating vibration, hard to say. May have been a high vibration from the engine. 4 units is the limit and the point of starting to investigate (usually adjustment of the balance screws in the fan spinner) but even to the experienced crew, 2 units can feel like 6 but it well within limits. Also depending where you are sitting, the hydraulic system of an Airbus can make some weird noises too.
Hope that helps.

Matt

Thanks all.

Davysims and Expat, you guys are the reason why I posted this question here. I knew there was a logical explanation for what I heared. I just needed to know from a reliable source.

I hardly can imagine that so much personel around the plane wouldn't have noticed it anything was really wrong.


And yes, it's been a long time. Maybe I should visit some more again.

Cheers!

Kris

DaveSims wrote on May 20^th, 2010 at 6:06pm:


That's the exact answer for the vibration; mystery solved.