FastFlyer
Topic Author
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Engine Fans Spinning At The Gate

Sun Sep 28, 2003 7:53 am

The other day I saw an engine on a 737 spinning slowly at the gate. I've seen this before, and Im curious as to why it does this.
 
cancidas
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RE: Engine Fans Spinning At The Gate

Sun Sep 28, 2003 8:14 am

it could be due to wind entering the engine from behind and spinning the fan. do you remember what direction is was spinning?
"...cannot the kingdom of salvation take me home."
 
manzoori
Posts: 1459
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RE: Engine Fans Spinning At The Gate

Sun Sep 28, 2003 8:27 am

It's almost certainly the engine quietly windmilling.

The shame of it is if you are outside and near the engine then you will hear it makes a hell of a racket as it windmills, caused by the fan blades clonking around in their retention slots.

You won't get this when the engine is running as the CF loads will prevent the blades moving.

HTH

Rez  Big thumbs up
Flightlineimages DOT Com Photographer & Web Editor. RR Turbines Specialist
 
N243NW
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RE: Engine Fans Spinning At The Gate

Sun Sep 28, 2003 9:30 am

Hello all:
As I was reading this thread, a question came to my mind. Once at the gate, approximately how long does it take for a commercial jet's engines to completely stop turning (provided there is no wind)? Anyone know? Thanks!
-N243NW Big grin
B-52s don't take off. They scare the ground away.
 
ha763
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RE: Engine Fans Spinning At The Gate

Sun Sep 28, 2003 5:26 pm

Can't tell you how long it takes for the blades to stop turning since I never spent the time to actually watch it stop, but I do know that it can stop in about 15 min. I've been on 767 ETOPS audits and seen the blades stopped when the mechanics check the engine oil roughly 15 minutes after shutdown. Here at HNL, you can hear the blades rattling as it windmills in the open air areas of the terminal, provided it isn't drowned out by all the APU noise. However, the 717 BR715's are not supposed to rattle while windmilling. If it's making any noise, it should be inspected.
 
Goldenshield
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RE: Engine Fans Spinning At The Gate

Wed Oct 01, 2003 6:07 pm

General rundown times for a 737 is about 2-3 minutes. Its safe to walk in front of one after 30 seconds.

An A320 is about 45, and spools down in about 3-4

757 60, and 3-4 as well.

It would also depend on how well the oil is lubricating the parts...
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 
manzoori
Posts: 1459
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RE: Engine Fans Spinning At The Gate

Wed Oct 01, 2003 11:17 pm

However, the 717 BR715's are not supposed to rattle while windmilling. If it's making any noise, it should be inspected.

Thanks for that snippet, I didn't know that!

On a related note, I was at the Airport Hotel beer garden at MAN recently and an Embraer regional jet (possibly Crossair) taxied past and I swear there was something horribly wrong with the port engine! It was making this horrible sound very similar to someone shaking a large tin can filled with one or two bolts. I should point out that I've heard BA aircraft do the same so this isn't a dig at Crossair maintenance... More a comment on the noises coming from the Ae3007 engine!

Cheers!

Rez
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Mr Spaceman
Posts: 2723
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 12:10 am

Hi guys.

Regarding the amount of time a typical jet engine takes to run down (2-3 & 3-4 minutes), is this based on internal mechanical drag of gears, shafts, etc and the eventual loss of centrifical force after complete shut down, or is the compressor & turbine stage ONLY (no combustion stage), purposely still running at very low RPM's for these few minutes to help disipate residual heat from the burnner cans in the combustion chamber & heat in the exhaust pipe?

I belive I learned this before, but , of course I could be wrong. Big grin


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Chris  Smile

"Just a minute while I re-invent myself"
 
Shenzhen
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 12:22 am

On Boeing Airplanes, once you close the spar valve (by moving the switch from run to cutout/or fire handle) that supplies the fuel, the engine is shut down.
 
Mr Spaceman
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 1:52 am

Hi guys.

> Shenzhen, Thank You for clearing that up for me. Big grin

Perhaps what I actually learned (in this forum), was that as a jet engine is spooling down after fuel cut-off (no more hot combustion), the intake compressors & exhaust turbines help to force heat out off the engine, both internally & externally, which helps prevent heat damage.

If this is the case for Boeing airliners, then I guess it's most likely the same for Airbus, McDonnell Douglas, Lockheed, Fokker, Fairchild, BAe, Canadair, Embraer, etc.

Question:

When you see an airliner sitting on a ramp all alone and it's intake fan blades are slowly turning in the breaze, are the internal N1 & N2 compressors plus the turbine blades in the exhaust pipe also turning? Or are they isolated from being able to windmill freely via all the different shafts within the main shaft?

I'm asking about this because I'm wonder how the high & low pressure compressors would be lubricated if they're spinning while the engine is off, plus I thought it was only the jet blast of hot gases from the combustion chamber that made the rear turbines spin.

I've seen many jet engine fan blades turning in the wind at Toronto Intl, but never noticed if the exhaust turbines were spinning too.

Chris  Smile
"Just a minute while I re-invent myself"
 
Staffan
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 2:38 am

"When you see an airliner sitting on a ramp all alone and it's intake fan blades are slowly turning in the breaze, are the internal N1 & N2 compressors plus the turbine blades in the exhaust pipe also turning? Or are they isolated from being able to windmill freely via all the different shafts within the main shaft?

I'm asking about this because I'm wonder how the high & low pressure compressors would be lubricated if they're spinning while the engine is off, plus I thought it was only the jet blast of hot gases from the combustion chamber that made the rear turbines spin.

I've seen many jet engine fan blades turning in the wind at Toronto Intl, but never noticed if the exhaust turbines were spinning too."


High pressure compressor and high pressure turbine are directly connected, so whenever one spins, so does the other. Same goes for lpc and lpt.

When the engine is windmilling, it's mostly the fan, lpc and lpt turning, since these will be driven by the fan which is most easily driven by the wind.
With regards to the noise during windmilling, it's only engines with spacers (they have another word, not shure what though) on the fan blades, that make the sound. The sound occurs when the blade passes the 6 and 12 o'clock positions and the blade swings over and hits the blade next to it.

Staffan
 
Staffan
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 2:41 am

Here you can see the "spacers" on the fan blades about 2/3 of the length from the centre.


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Staffan
 
manzoori
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 7:02 am

Hi Staffan,

Not sure what the competition call them but at RR we call the 'Clappers'. Their job is to provide a support between all the fan blades on these older engines and prevent the blades from twisting open under high power conditions.

Modern engines with their Wide chord fan blades no longer need these.

HTH

Rez  Big thumbs up
Flightlineimages DOT Com Photographer & Web Editor. RR Turbines Specialist
 
JohnM
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 7:08 am

Fan blade "clank" is very irratating on an otherwise quiet flightline. It is a rare time when no APU's or power units are running, and the damn fans are spinning from the wind. We do have fan blockers, which are high speed aerospace 2X4s with a rubber surface to stick in the inlets, if inlet work is to be done.
 
Staffan
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 7:13 am

Thanks! Didn't know about the twisting issue with older blades! Anyway, here are some better shots:


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Staffan
 
air2gxs
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 8:39 am

We call the "spacers" mid-span shrouds. Or at least that's what we call them on the JT9.
 
ha763
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 1:09 pm

I actually have some close ups of a DC-10 engine that was being worked on due to FOD damage to the blades. You can see the spacers quite well. They didn't use any rubberized stoppers, but just a broom to keep the blades from windmilling. I've actually seen this done many times here in Hawaii.
 
challengerdan
Posts: 145
Joined: Sat Sep 27, 2003 3:17 pm

RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 4:42 pm

Even some engines without midspan shrouds will rattle when windmilling. The GE CF-34 is like that. It doesn't have to do with the shrouds, but with the way the blades are secured on the fan disk.
if your flight goes MX in YUL, I might be called to fix it!
 
FredT
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RE: Engine Fans Spinning At The Gate

Thu Oct 02, 2003 6:12 pm

Regarding the amount of time a typical jet engine takes to run down (2-3 & 3-4 minutes), is this based on internal mechanical drag of gears, shafts, etc and the eventual loss of centrifical force after complete shut down, or is the compressor & turbine stage ONLY (no combustion stage), purposely still running at very low RPM's for these few minutes to help disipate residual heat from the burnner cans in the combustion chamber & heat in the exhaust pipe?

The only way to stop the combustion is to stop providing the fuel. Once you do, the engine is shut down and the rundown time will only depend on friction, drag, wind etc. No running it past that point at any RPM.

In some aircraft, you motor the engine for a while using the starter (but with the fuel cut off) in some situations. Typically, you’d do this after a hot start.

When an engine windmills, it does a few RPM per minute as opposed to thousands of RPMs when running. They’ll do fine without oil pressure. Besides, if there was enough friction to ruin things it’d hardly windmill anyway.  Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
FDXmech
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RE: Engine Fans Spinning At The Gate

Fri Oct 03, 2003 12:04 pm

The clanking of the fan blades during windmilling (so very loud on the P&W engines) is due to "tip shake". Tip shake allows the fan blade to better absorb an impact such as a bird strike rather than having the blade mounted rigid, unable to deflect.
You're only as good as your last departure.
 
Staffan
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RE: Engine Fans Spinning At The Gate

Fri Oct 03, 2003 4:50 pm

It also helps to balance the the fan when the blades are allowed to move slightly in their mounts.

Staffan
 
avioniker
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RE: Engine Fans Spinning At The Gate

Fri Oct 03, 2003 9:53 pm

It also shouldn't be heard on a CFM-7 except very quietly.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
Mr Spaceman
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RE: Engine Fans Spinning At The Gate

Sat Oct 04, 2003 6:14 am

Hi guys.

Thanks for all the additional info.

> Staffan, Thanks for explaining that the Fan, LP Compressors & LP Turbine are directly connected. So, whenever one spins, so does the other. I understand that the same goes for the HP Compressors & Turbine.

> FredT, you stated ......

"In some aircraft, you motor the engine for a while using the starter (but with the fuel cut off) in some situations. Typically, you'd do this after a hot start."

I believe it's this procedure of motoring the engine for a while by using the starter (no fuel), that I was thinking of as a method for removing heat from the engine. I'm sure I read about this technique in this forum.

Personally, I've always liked the clink clink clink clink clink clink clink clink clink sound of a jet engine windmilling in a breaze. Big grin

Chris  Smile
"Just a minute while I re-invent myself"

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