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PMN
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Rattling CFM56's?

Sat Dec 03, 2005 8:53 pm

Hi All,

Last night as I disembarked my B733 flight home, I noticed what can only described as a loud rattle coming from the engine No.1 as the fan was turning in the wind. I've never noticed this before, and walking to the terminal building I heard it from another CFM56 on another B733.

Could someone please explain what creates this sound? Is it something that happens just on the CFM56 or is it a characteristic common with turbofan designs?

Thanks in advance

Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song
 
VuelingAirbus
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RE: Rattling CFM56's?

Sat Dec 03, 2005 9:09 pm

It happens on the A320 as well. The fanblades of the CFM56 are not attached to the engine. At least not with screws. They are kinda free in some sort of rail. So if the whole fan move the individual blades lean slightly to one side and when they pass the highest poit they fall to the other side and make that klicking, rattling noise. Hope that made sense but I didnt know how to explain it any better.

rgds
 
PMN
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RE: Rattling CFM56's?

Sat Dec 03, 2005 9:13 pm

Quoting VuelingAirbus (Reply 1):

That makes perfect sense. It sounded exactly like it was the fan blades moving slightly, but I couldn't see them not being solidly fixed. I guess I was wrong!

Thanks for the info.

Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song
 
charliecossie
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RE: Rattling CFM56's?

Sat Dec 03, 2005 9:48 pm

All jets "rattle" as they spin in the wind.
 
VuelingAirbus
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RE: Rattling CFM56's?

Sat Dec 03, 2005 9:53 pm

Quoting PMN (Reply 2):
It sounded exactly like it was the fan blades moving slightly, but I couldn't see them not being solidly fixed. I guess I was wrong!

I was as irritated as you when I first flew CFM56. I flew A320's with IAE engines and now with CFM and on one of my first flights I heard that noise and thought something is in the engine so I went to the downwind engine and rotated the fan slowly by hand and than you can actually hear and even see the blades tipping over. I was very surprised and thought thats very odd.
 
Matt72033
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RE: Rattling CFM56's?

Sat Dec 03, 2005 10:40 pm

most engines do it, but the cfm56's are louder than most, i notice it a lot on the 343

obviously they rely on the centrifugal force to hold them solid when they are rotating

Quoting VuelingAirbus (Reply 4):
I was as irritated as you when I first flew CFM56. I flew A320's with IAE engines and now with CFM and on one of my first flights I heard that noise and thought something is in the engine so I went to the downwind engine and rotated the fan slowly by hand and than you can actually hear and even see the blades tipping over. I was very surprised and thought thats very odd.

you fly these planes an you didnt know what it was? thought there would have been something about it in your training! especially for when you do a walk around!
 
Whiskeyflyer
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RE: Rattling CFM56's?

Sat Dec 03, 2005 10:40 pm

The blades are slotted onto the disc, so there is some play. I am most familar with the RR engines, so my comments on the CFM would be here say. On RR Blades are slotted using whats called a "fir tree" slot, basically looks like a christmas tree unside down, with the disc being the "female" with the "male" fir tree pattern being below the base of the blade. The pattern is most obvious when you see a disc removed from an engine during overhaul. The limits of play for the discs vary according to the disc (e.eg. practically no play allowed on turbine disc baldes)
 
wing
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RE: Rattling CFM56's?

Sat Dec 03, 2005 11:24 pm

Quoting Matt72033 (Reply 5):
you fly these planes an you didnt know what it was? thought there would have been something about it in your training! especially for when you do a walk around!

Thats exactly the way of talking which irritates the pilots to contribute to the forums and especially very unfair not only to the contributer but many other enthusiast who enjoy and learn from the experiences from the professionals.

The Airbus philosopy indicates to the pilots to know only "what they need to know" and exterior walkaround doesn't consist to know how the fan blades sounds.Lets keep A.net more civil as it was,thats for good of all of us.Best Wishes.WING
follow me on my facebook page" captain wing's journey log"
 
Matt72033
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RE: Rattling CFM56's?

Sat Dec 03, 2005 11:35 pm

Quoting Wing (Reply 7):
Thats exactly the way of talking which irritates the pilots to contribute to the forums and especially very unfair not only to the contributer but many other enthusiast who enjoy and learn from the experiences from the professionals.

i'm sorry, but i think you've taken this exactly how i didnt mean it! i was merely expressing my surprise! thats was all!

Quoting Wing (Reply 7):
The Airbus philosopy indicates to the pilots to know only "what they need to know" and exterior walkaround doesn't consist to know how the fan blades sounds.Lets keep A.net more civil as it was,thats for good of all of us.Best Wishes.WING

well i would have thought it was a fairly important thing, when you do a walk aorund, you need to know what is normal and what isnt normal!
 
Tristarsteve
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RE: Rattling CFM56's?

Sun Dec 04, 2005 4:53 am

Quoting Charliecossie (Reply 3):
All jets "rattle" as they spin in the wind.

Look at the fan blades. Older engines with lots of blades have clappers about 2/3 of the way out. It is these clappers hitting that you hear on the ground. Newer engines like RR Trent and IAE V2500 have less blades that are larger and do not need clappers because the blades are further apart.
The noisiest I know are the older RB211 (B757 Tristar B747-200)which really clack in a breeze. These clappers are lubricated with a dry film lubricant to allow them to slide against each other. If you have a fan vibration problem the first thing you do is rotate the fan and listen that each blade claps as it falls over TDC. If it doesn't then the clappers have locked, and need to be separated.
So clanking clapper blades is good to an engineer.
(But not as good as a Trent, or B747-400 RB211, without them!)
 
PMN
Topic Author
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RE: Rattling CFM56's?

Sun Dec 04, 2005 6:48 am

Quoting VuelingAirbus (Reply 4):
I was as irritated as you when I first flew CFM56.

It didn't irritate me, it was more curiosity! I've done plenty of flights on CFM56 powered aircraft, and I'm just surprised I've never noticed it before. It isn't like wind is a rare thing at LBA!

Thanks for the info guys. It's great to have a place to find accurate answers to these little questions!

Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song
 
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HAWK21M
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RE: Rattling CFM56's?

Sun Dec 04, 2005 6:42 pm

As mentioned above.Its the Play between the Blades & its mountings in the Spool.
As the Engine speeds up & Temperature rises.The Sound is Eliminated.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Airgypsy
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RE: Rattling CFM56's?

Mon Dec 05, 2005 2:46 am

I must have missed something in the past few years away from the airlines. Last I knew, GE did a mod that eliminated (or reduced) mid-span shroud contact (clacking) because of wear to the hard coat where they meet. Something like some rubber blocks at the blade roots. It was such a problem with the GE TF-39 (Grandfather of the CF-6) that the military sticks a rubber coated 2X4 in the fan and through the guide vanes to keep the fan from turning on long ground times (done this many times).
CFM blades are retained by an upside down "T" (very rounded) broached in the fan disk. Turbine blades have a "christmas tree" broached into the turbine disk. Turbines do make noise when they are stone cold but is is a very soft ticking of their tip shrouds. The heat while operating eliminates all the play.
On RR Spey, if the fan blades don't move freely on their retainer pins, you can get engine vibration. Lubed a few fans to cure that one.
It's all the background noise of a day on the line. Add the smell of burn JET A and life is good.
 
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HAWK21M
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RE: Rattling CFM56's?

Mon Dec 05, 2005 4:04 pm

Quoting Airgypsy (Reply 12):
It's all the background noise of a day on the line. Add the smell of burn JET A and life is good.

Don't forget the "Make my day SKYDROL"  biggrin 
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Molykote
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RE: Rattling CFM56's?

Mon Dec 05, 2005 5:13 pm

Quoting HAWK21M (Reply 11):
As mentioned above.Its the Play between the Blades & its mountings in the Spool.
As the Engine speeds up & Temperature rises.The Sound is Eliminated.
regds
MEL

My understanding was that the centrifugal effects on the spinning hub/blades cause the blades to fly outward and seat themselves snugly in the allocated blade root slots (I had never heard a thermal explanation as it pertained to the fan blades). If I am wrong or if thermal considerations also provide a contributory effect I'd be interested to hear about it.

This is partly the rationale behind doing a fan blade lube when high N1 vibrations are recorded. As the old lubricant gets dirty or gummed up in the blade root slots it slightly throws off the seating positions (and therefore balance) of the fan blade rotor set. By cleaning and relubricating these slots the original balance condition of the clean and well lubricated LP rotor system is restored and the N1 vibrations are reduced.

Quoting Whiskeyflyer (Reply 6):
The blades are slotted onto the disc, so there is some play. I am most familar with the RR engines, so my comments on the CFM would be here say. On RR Blades are slotted using whats called a "fir tree" slot, basically looks like a christmas tree unside down, with the disc being the "female" with the "male" fir tree pattern being below the base of the blade. The pattern is most obvious when you see a disc removed from an engine during overhaul. The limits of play for the discs vary according to the disc (e.eg. practically no play allowed on turbine disc baldes)

I believe that the fir tree slots are more common in turbine blades as you describe. For example I am looking at an RB211 (535E4) fan blade now with a triangular (non-fir tree) root. Most RR blades are probably as you describe.
Speedtape - The aspirin of aviation!
 
abbs380
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RE: Rattling CFM56's?

Mon Dec 05, 2005 6:13 pm

The JT9 is especially noisy on a windy day, and yes its because the mid span shrouds bang together as the blades go over the top and turn the entire fan into a huge cymbal. Several times I have had to do jobs on JT9/s that take a long time, to eliminate the noise I would climb up into the nose cowl and very carefully rub my foot into the fan blades to stop the rotation. Once the rotation stops I would jam a small rubber chock (meant for ground equipment) between the blades and the exit guide vanes, oh heavenly peace.
I don't think its in the book but it sure makes life easier.
 
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HAWK21M
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RE: Rattling CFM56's?

Tue Dec 06, 2005 12:29 am

Quoting Molykote (Reply 14):

Your Username Indicates you specialise in Engines.Is that true  Smile

Quoting Abbs380 (Reply 15):
I would climb up into the nose cowl and very carefully rub my foot into the fan blades to stop the rotation.



Quoting Abbs380 (Reply 15):
Once the rotation stops I would jam a small rubber chock (meant for ground equipment) between the blades and the exit guide vanes, oh heavenly peace

Your methods are Risky & damaging to the Equipment.Are you from Mx.  Smile
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Tristarsteve
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RE: Rattling CFM56's?

Tue Dec 06, 2005 4:46 am

Fan blades do not have fir tree roots. They have triangular or bulbous roots which fit in slots in the fan hub. When the fan starts to spin the blades centifuge out and the mid span shrouds have gaps between them. When the engine is powered up, the fan blades move fwd in their slots and start to "pull" the aircraft forward. If one blade does not move, then the mid span shrouds can touch and the fan set can lock up. On the Centre engine on a L1011 or DC10 this can cause bad airframe vibration. On a pod installation, because the strut is flexible it is not as bad. This is why the fan blade roots and mid span shrouds are lubricated with a dry film lubricant. When the aircraft is flying and the the engine is pulled back to idle the fan is pushed back into the engine. This for and aft movement is tiny, but you be amazed the vibration it can cause if one blade sticks. The clue to a locked up fan set is that the blades do not clack when the engine is windmilling on the ground. If this happens we have to take out all the blades and clean and lubricate the roots and reinstall them IN THE SAME POSN.
A JT8D also has mid span shrouds and these can lock as well, but they usually overlap rather than locking because the blades are so much smaller and less flexible.
 
Molykote
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RE: Rattling CFM56's?

Tue Dec 06, 2005 1:51 pm

Quoting HAWK21M (Reply 16):
Quoting Molykote (Reply 14):


Your Username Indicates you specialise in Engines.Is that true

Thankfully it is true.  bigthumbsup 
Speedtape - The aspirin of aviation!
 
abbs380
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RE: Rattling CFM56's?

Tue Dec 06, 2005 6:50 pm

Hawk (reply 16). Yes I am in maintenance, and I agree the procedure I described is risky. A Jt9 low press compressor/turbine & shaft must weigh- I don't know- hundreds of pounds, that's why its risky. You don't try to stop it quickly, you just add a small amount of drag by allowing the bottom of your shoe to rub on the fan blades, it takes many revolutions to stop the fan. You would only do this if you have a tail wind (which gives backwards rotation to the fan) so the flat surface of the blade rubs against your foot. You certainly would be a fool to try this if the fan was turning in the normal direction.
Does it damage the engine? I doubt it. Those fan blades are mighty tough. Why would anyone do this? Because these rotating fan blades make a horribly loud clacking noise and ear protectors are almost no help, I guess because they don't protect against the right frequency range. As I said, I (and others) would only do this if we had a job which called for a long time working near the fan, say an engine component change.
By the way, speaking of damaging the engine, did you know that P&W allow you to run the engine with the fan strapped so it can not turn?
 
Matt72033
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RE: Rattling CFM56's?

Tue Dec 06, 2005 8:06 pm

we slow down the fans on our cf6's in a similar way. i normally lean against the spinner, facing the wingtips and take a step or so up the inlet cowling to apply some pressure to the spinner with my back! but this is to inspect the fan blades, not to stop them rattling.
 
A/c train
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RE: Rattling CFM56's?

Tue Dec 06, 2005 9:04 pm

If the winds in the right direction relative to the airplane just deploy the thrust reversers, or if your not working in the nose cowl, and you can get one, put the intake blank on!, last resort, jam a piece of wood bewtween the fan and the guide vanes, or the last stage L.P turbine rotor/stator, as long as you take it out after it doesnt hurt it !
regds a/c
 
Matt72033
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RE: Rattling CFM56's?

Tue Dec 06, 2005 9:26 pm

Quoting A/c train (Reply 21):
as long as you take it out after it doesnt hurt it !
regds a/c

lol that leaves a huuuge gap for human error! lol
 
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HAWK21M
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RE: Rattling CFM56's?

Tue Dec 06, 2005 9:37 pm

Quoting TristarSteve (Reply 17):
The clue to a locked up fan set is that the blades do not clack when the engine is windmilling on the ground. If this happens we have to take out all the blades and clean and lubricate the roots and reinstall them IN THE SAME POSN.

What type Lubricatant....Molykote.

Quoting Abbs380 (Reply 19):
By the way, speaking of damaging the engine, did you know that P&W allow you to run the engine with the fan strapped so it can not turn

No.Tell me about this.Any link.

Quoting Abbs380 (Reply 19):
Why would anyone do this? Because these rotating fan blades make a horribly loud clacking noise and ear protectors are almost no help, I guess because they don't protect against the right frequency range

Why not use Intake & Exhaust Blanks.Wouldn't that help.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Matt72033
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RE: Rattling CFM56's?

Tue Dec 06, 2005 11:23 pm

Quoting HAWK21M (Reply 23):
Quoting Abbs380 (Reply 19):
By the way, speaking of damaging the engine, did you know that P&W allow you to run the engine with the fan strapped so it can not turn

No.Tell me about this.Any link.

ive heard stories of this, engineers using seatbelts and such to hold the N1 spool! they said it was for a particular check on the engine, but i cant remmber what!
 
A/c train
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RE: Rattling CFM56's?

Tue Dec 06, 2005 11:50 pm

On all the blades ive lubed, there is 2 lubricants, a baked on Molydag coating and a molykote dry film lubricant on top, most MM's say apply 3 coats of molydag 15 mins apart and place in oven too bake. Then put 3 coats of dry film lubricant on top and allow to air dry.
Most blade lubes just involve cleaning the blades up and the blade slots on the fan hub, then applying the 3 coats of dry film lube too the blade and the pressure face of the fan hub.
When the schedule calls for a blade lube, sometimes when you take the blades out you find the baked on coating is damaged this will mean sending it too the bay for baked coating, less frequently you have too send a fan blade set for complete blade lube, this involves meda blasting the blades down too the bronze protective coating and starting from scratch again.
To rectify some N1 vib/ BB vib problems, you may have too take the blades out and do the dry film lube, that is if the seals on the annulus fillers are not flapping in the wind or not pushed down with a credit card !!
regds a/c
 
dl757md
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RE: Rattling CFM56's?

Wed Dec 07, 2005 12:30 am

Quoting PMN (Thread starter):
Is it something that happens just on the CFM56 or is it a characteristic common with turbofan designs?

Not all CFM56s rattle in the wind. The CFM56-7 on the 737NGs have a part called a platform installed between the blade near the root that has rubber seals preventing the blades from clanking. Other enignes that I'm familiar with that have this feature are the IAE V2500 and the RR Tent on the 777. I'm pretty sure there are others.

Dl757Md
757 Most beautiful airliner in the sky!
 
abbs380
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RE: Rattling CFM56's?

Wed Dec 07, 2005 6:11 pm

Yes I agree plugs (or blanks) are a good idea but the only blanks we had were for the inlet. They are large plastic tubs which are just slightly larger than the diameter of the nose cowl and are held in place by friction only. I have tried to install these by myself, if you have say a 10 knot tail wind they just blow back out. Most guys thought they weren't worth the effort.
Shutting down the engine while in reverse is also a good idea, I think some DC8/CFM operators do this routinely, however on a 741/JT9 if the engine is in reverse you can't get the cowling open.

As to strapping the fan, I looked this up in the A310 mm, though Im pretty sure it applies to all JT9's, sorry cant link it.
The mm says you CAN use this procedure while performing test 03. Test 03 is basically a leak test which is required after the replacement of many engine components. P&W calls it a "locked fan rotor ground run". The mm is very specific about how you strap the fan, they call for installing seven straps which must be woven through the fan blades at specific clock locations. The straps must be positioned "outboard of and against the mid span shroud as far outboard on fan exit vanes as possible".
The advantage of using this procedure is you can run the engine with ALL of the cowling open. The disadvantage is it obviously takes lots of time to set up and you are supposed to have fire fighting equipment standing by because the engine fire bottles will be ineffective with the cowling open. You can not go above ground idle. As an alternative you can do test 03 with the cowling closed (no straps) then open it up and look for leaks.
 
Matt72033
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RE: Rattling CFM56's?

Wed Dec 07, 2005 8:30 pm

why cant you run it at idle with the cowlings open?
 
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HAWK21M
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RE: Rattling CFM56's?

Wed Dec 07, 2005 9:16 pm

Quoting Abbs380 (Reply 27):
I have tried to install these by myself, if you have say a 10 knot tail wind they just blow back out.

Isn't there a mechanism to lock it ion.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
kaddyuk
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RE: Rattling CFM56's?

Wed Dec 07, 2005 9:38 pm

The CF6-80C2 that is on the B747-400, MD11 and B767 is reasonably quiet... You can hear a rattle, but its minimal and you have to be stood in the intake to hear it.
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