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I recently flew on an Airbus A319. When boarding the aircraft from the front entrance we passed in front of the left engine. The fan blades were spinning and there was a rattling noise (like when a chain is being rolled up) coming from the engine.

Is this normal? What causes this sound? Is it the fan blades that are somehow loose?

Farhan
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Fabrizio Mazzoni
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1 Answers1

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Yes indeed, the fan blades are loose and rattle in their sockets when the fan is freewheeling in the wind. Once the engine is running, centrifugal forces will make sure they do not rattle any longer.

The base of the fan blades have a shape like a fir tree, and they slide into sockets on the fan disk with a loose fit. When the fan is spinning slowly, they will sit on one side of the socket on the way up, and gravity will pull them into the opposite side on the way down. The rattling is caused by one blade after another falling into that opposite position once they rotate over the top.

enter image description here

Sorry for the bad picture (source) - I could not find a better one right now.

There are concepts to move to bladed disks ("blisks"), but while they are lighter and don't rattle, they are a headache once a single blade is damaged and needs to be replaced. Therefore, the single, easily removable fan blades are the solution of choice in all gas turbines.

Peter Kämpf
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    It should also be mentioned that that fan blades are made of such a material that their 'rattling' will not cause any degradation or wear. – user2479 Oct 01 '14 at 12:42
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    Interesting. See also A closer look at the front fan of the RB211-22B and an explanation of the rattling noise. – RedGrittyBrick Oct 01 '14 at 12:54
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    Does this happen on both of the engines available for the A319/320/321? What about the A318 Pratt & Whitney engine? I've never noticed this on anything but the A320 and derivatives thereof; does it happen on other aircraft? – gsnedders Oct 01 '14 at 12:55
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    @gsnedders: I am not aware of an engine where this ratting doesn't occur. The fan has to windmill, though. – Peter Kämpf Oct 01 '14 at 13:31
  • @gsnedders On parked 747-100 and -200 aircraft with P&W engines in a nice breeze, you can get a symphony so to speak with all four engines rattling away. Makes quite a racket. – Terry Oct 01 '14 at 16:53
  • Does expansion because of high temperatures play a role in this rattling as well? I.e. the fans expand and fit snuggly when they're operating and shrink and rattle when they cool down. – Saaru Lindestøkke Oct 01 '14 at 20:22
  • @BartArondson: The thermal expansion of the fan is negligible, but is substantial in the turbine section (the part of the engine that is exposed to the hot combustion gases). There, free play is larger to allow for thermal expansion. – Peter Kämpf Oct 02 '14 at 04:26
  • How serious is the rattle? Can it cause a fan blade to fall out and enter the engine's compressor? – shortstheory Oct 02 '14 at 11:06
  • @shortstheory: No way! It is just a gentle clicking when the blade assumes a new position. No lengthwise motion is involved, and those blades are tough, so they won't break off even after a million such clicks. – Peter Kämpf Oct 02 '14 at 11:11
  • I also heard it on an Airbus A320 and told the steward about it. He said it's designed that way to deal with metal expansion. This is the first time I heard that noise however. – ronenfe Jan 24 '20 at 14:59