What causes that noise?
 

Boarding my A320 this week I heard a rattling noise coming from the slowly turning fan. It was not unlike a washing machine drum slowly turning with a load of loose metal chunks inside. I’ve heard this many times before and I’m still here to ask the question so it’s obviously nothing serious. I am intrigued though as to what causes the noise. The power pack was a CFM56-5B4/P if that makes any difference. Any ideas guys?

The fan blades are connected to the hub in such a way that there is a small amount of movement (probably for thermal expansion and so the whole assembly is not as brittle as it would be if completely fixed and rigid).

When the fan is spinning at high speed the fan blades are locked in place by centrifugal force and the fact that they are pushing the air through the engine. When not running, any head or tail wind will slowly turn the fans and you can hear the blades rattling their fir tree shaped joints in the hub sockets.

As Pelikanpete says it's the way the blades are fitted to the hub. The roots of the blades are loose to allow for expansion. The picture isn't great but without climbing into the loft and digging out some old books it's the best I can find.

Look at the picture on the left, the 'christmas tree' is the bottom section. Hope it makes sense.

http://www.msm.cam.ac.uk/phasetrans/...oating_HPT.jpg

Almost right.

The clacking noise is made by the mid span snubbers hitting each other as the fan blade goes over top dead centre.

If you look about half way along the span of a CFM56 fan blade you will see them. They are there to ensure the blades remain rigid especially in the event of a bird strike.
More modern engines such as the IAE V2500 or RR Trent have more advanced wide chord fan designs that do not need them. They are virtually silent when windmilling. :ok:

Agree with the above.

It's also interesting to note that the CFM56 engines fitted to the 737NG don't do this!

SW

Fan blades are held in by dove-tail roots not fir-tree (normally that for turbine blades) anyway been discussed before here

Thanks for the replies guys and for the link to the previous thread. I must improve my searching skills as I missed that one. :ok: