GE90 Engine Noise During Start

[Fuzzy Math]

Any ideas as to why GE90s are so noisy during start?

[awblain]

I second the question.

I sat between a pair of them for the first time a couple of weeks ago on a new New Zealand 773ER, and noticed they were remarkably noisy when starting compared with the PW4000s or Trents that usually push me places, or what I recall from earlier model GE90s.

As I remember, it was an almost-constant-tone whine-grind noise, rather than a rising tone from something whose speed was increasing. I assumed it had to be from an air turbine, which didn't turn again for the rest of the flight after the engine started. I also see something perhaps similar has been discussed before at
http://www.pprune.org/fragrant-harbour/430389-ge90-engine-start.html#post6008093

[STOLskunkworks]

I asked the same question to someone a while ago and if I recall the answer I was given was:

the airflow through the engine is no yet stable and the awful noise is various compressor blades at a stall relative to the airflow in the engine.

or something along those lines this may not be correct however.

[3holelover]

My own experience with GE90's (-110's and -115's) is that they only make ugly noises during start if they weren't cooled properly prior to their last shut-down. They call it "bowed rotor effect" and if one doesn't wait the recommended 20 minutes since the throttle was last off the idle stop prior to shut down, the next start will be noisy.

So, obviously, if it takes less than 20 minutes from landing roll-out to gate, (and surely, it often does!?), it's going to happen.

[777AV8R]

Natural harmonics......

[grounded27]

The "cool down period" is detrimental to this motor, shaft warping can cause major problems. The 115/derated110 have huge fan blades, suspect this is the source.

[3holelover]

The "cool down period" is detrimental to this motor:ooh: Could you expand on that statement? Is this a language thing, or did you really mean to say one shouldn't cool the GE90 before shut down?

In any case, I'm not just spittin' wind here... I'm tellin' ya, if you cool them down nicely, the next start will be no more noisy than a cf6-80 or PW4000 or even one of those funny backwards turning, three spooled, weird things the Brits call engines. ;) .....and if they're not cooled really well, I can guarantee your next start will sound awful!

[lomapaseo]

I never heard a GE90 that I didn't like

and I have no idea what this subjective noise sounds like or what causes it

When talking about cool down in jet engines (large or small) some issues that come to mind are:

Depending on mass and shape different parts cool at different rates. and even the same part will cool faster at some of it geomentry areas than others.

Exampes are:

Case structures surrounding the rotor blades cool much faster than the rotor disks themselves burried deep in the center of the engine. Beacuse of this rate difference there could be a critical time when the blade tip clearenace is not optimum (too loose or too tight). The too tight part of it is at one time and the too loose part of it at yet another time.

too loose creates air distortions at the blade tips leading to rumbles and sometimes even a surge.

Too tight leads to binding at very low RPM and inability for a starter to overcome and/or excessive rub out which will show up as too loose when the rotors get to speed and equalize.

Then there is the effect of depth between the top of the engine and the bottom of the engine in the gas cavities where heat rises creating a bow in the rotor. This acts like a tire that has been allowed to sit too long leaving a flat spot and sets up harmonic vibration (noise) of some parts createing a cacophony of sounds until the spinning itself evens out the temperatures around the rotor.

All engines are subject to all the above, but to different degrees and rarely does once change a design just to change a subjective noise that only acts for a brief time.

Engine surge is of course to be avoided

[3holelover]

The problem I'm having is an inability to cut and paste from employer provided info... They'd get upset about that. But I've seen text from GE that says what I'm saying... I'm pretty certain they called it "Bowed Rotor effect" (the flippin' things shake and shiver while they find their happy place) and it's cause is said to be inadequate cool down.
I've heard/felt it many a time during many starts... And, I've also seen, many a time, when I give the poor dears their twenty minutes at idle after a hard run... the next start is as smooth as a baby's bum.

[Hedge36]

We ran into the same thing with naval steam turbines. It always made me wonder why the OEMs don't fit small electric jacking motors to get them turning at a relatively slow RPM to cool off properly.

[Papa2Charlie]

The most likely cause are HP bleed valve(s) without silencers being open during the start. As the engine reaches idle, the valve will close and the noise levels will drop significantly.

[3holelover]

The most likely cause are HP bleed valve(s) without silencers being open during the start. As the engine reaches idle, the valve will close and the noise levels will drop significantly.:hmm: The HP valve is definitely not closed at idle. ....Given that you're evidently not too familiar with the beasts, I'm a little curious as to how it is you feel you can say what "the most likely cause" is? HP bleed valve "silencers"?? Never run across one on a GE or a Pratt? ....could that be one of the odd features of those RR (or should that be VW?) makes?

In any case, the noise I've been talking about, and which I took the question to be about, is not an air related noise at all.... it's an ugly sounding vibration. Maybe I'm in left field though.... wouldn't be the first time.

[A320skoda]

Its to do with fan blade material. GE90 fan blades composite so make diffrent noise to metal ones.

[TopTup]

Its to do with fan blade material. GE90 fan blades composite so make diffrent noise to metal ones.

I agree.... These GE90 (115 & 110) are damn big engines & their design needs to be respected just as any engine does. As a side note, it is a personal pet dislike seeing pilots disregard the need to confirm stabilized engines prior to pressing the TO/GA switch. Laziness and complacency.

[Landroger]

....could that be one of the odd features of those RR (or should that be VW?) makes?

Trolling under the wrong bridge there, I think, bro'. Rolls-Royce, the makers of cars - automobiles? - are owned by Volkswagen, but Rolls-Royce (1971) Ltd., - or whatever they are called nowadays - make some of the world's finest aeroplane engines. And you can see from the name, how far back it was that they were the same company. Keep with the programme. :)

Roger.

[3holelover]

;) Thanks Mate... I thought so, but can't seem to resist poking some fun at me Limey Cousins for those funny three spooled backwards turners. :E

[Old Smokey]

Skip all the rest, go straight to 3holelover's first post - "Bowed Rotor effect" for all of the reasons given.

Case closed your honour:D

[STBYRUD]

Hmm, how about a sub-idle debow run to increase service life? Or would that be too Concorde-esque? ;)

[lomapaseo]

Hmm, how about a sub-idle debow run to increase service life? Or would that be too Concorde-esque?

Its to do with fan blade material. GE90 fan blades composite so make diffrent noise to metal ones.



http://fromtheflightdeck.com/MEL/PPRune/Whoa.jpg

[STBYRUD]

Hahahaha, had to look for a while, but here it is:
Yet another Concorde question [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-424156.html)
Apparently one had to run the Olympus for a while at sub-idle to debow the shaft prior engine start if the engine had been run recently...

[Turbine D]

I would tend to go along with what 3holeover and lomapaseo have posted. The vibration sound and feel may also be influenced by harmonics as 777AV8R pointed out, depending on where you are seated.

There is no influence, one way or the other, caused by the composite fan blades.

The 20 minute stabilization after landing before shutting the engines down certainly helps, but it probably doesn't occur on a regular basis. With a bowed rotor on startup, the LPT area is most affected until things settle in. Additionally, as the rotors start to turn and pick up rpm's, there are a lot of frequencies being crossed which are designed to be outside normal engine operating range (idle through TO thrust). These sounds and vibrations are often transmitted from the engine, through the pylon to the wing and fuselage giving some strange sounds when seated near the engines or wing area. These all go away as the engine approaches idle speed and everything is in sync.

[easyduzzit]

I have to support Papa2Charlie's contribution here, however he had the HP bleed valve confused with engine handling bleeds/start bleeds!
It is very obvious, that the deep 'awesome' vibration sounding resonance, experienced during engine start/spool-up, distinctly shuts off, disappears, as the engine reaches steady state idle, which can only result from some sudden change, which is the appropriate engine handling bleeds closing, hence silencing the aweful rumble, eminating from the mass compressor air mismatch of N1 to N2 airflow, during sub idle/operating RPM's.
An engine of these proportions, will probably never reach max motoring spec's without all the handling bleeds wide open, dumping the excessive air and unloading the starter turbine, let alone accelerate, once fuel burn is induced.
Once the GE90 reaches steadystate idle, it rapidly becomes incredibly quiet. Smooth, matched, controlled and sealed compressor airflow.

I agree with 3holelover about his 'bowed rotor effect' on GE engines, however this pertained specifically to the CF6-50/80's, which had very skinny & long LP shafts.
I doubt this phenomenon is creating the majority of the loud audible & physical rumble that every GE90 delivers on almost every engine start.
That kind of vibration/resonance being the result of a long shaft whipping within its bearings will no doubt have caused GE to issue an immediate AD to force 20 min cool-downs, as it would seriously shorten engine reliability and life.

[gas path]

Right just to clear up a few details.! The vibration and noise is caused by compressor airflow mismatch through the gas path. :p In normal operation the engine would never been operated at above say 70% N1 immediately prior to being shut down. However heavy use of the reversers should invoke the requirement of the 20 min cool down cycle. The bowed rotor effect is a GE thing, very noticeable on any subsequent start, you definately don't need loose fillings.. or kidney stones:ooh: The STB (Start Transient Bleed [valve]) was removed very early on in the engines service life, as it was found to be unnessessary! The Bypass Valves (10 off) are fully open during engine start and dump excess air from the back of the LP compressor (booster) into the fan stream they only start to modulate closed with advancing throttle, the VSV (Variable Stators) are fully closed at idle and only start to modulate open with increasing throttle. :8

[3holelover]

Gaspath is absolutely correct. The VBV's modulate and are not closed at idle. (The VSV's when "closed" are obviously not closed shut like the VBV's can be, but they're as "closed" as they can get.)
I've probably run as many CF6-50's and -80's as many who fly with them have, and I never once experienced -nor even heard of - the bowed rotor effect with them... many other troubles, but not that! ;)

As for your doubts Easyduzzit. What can I say.... GE didn't issue an AD, but they did issue the recommendation ( probably had a fancier name -SB? EI maybe? I forget) to cool the GE90 for 20 minutes after the throttles were off the stop. (never saw any such recommendation for any other GE motor)... and as you've noted, the noise/vibration on a GE90 start is not an every time occurrence, it ONLY occurs when there's been insufficient cooling time prior to shut down... and as it takes 20 full minutes, it is very often the case.

If you're ever running at idle for that long and you have the opportunity -waiting for deice, or great lines of congestion or whatever, try shutting one down and restarting.... you'll see. Smooth as a babies bum! Guaranteed! (but don't cheat! 20 minutes at idle! Not 18!, and not 20 minutes which included just a little breakaway throttle use ...it IS that fussy.) --and again, that's the GE90-115 (&-110) I'm referring to.... Don't know about the lesser GE90's.

CF6-50's were always smooth starters, especially when we adopted our PP engineers' recommended method of putting the fuel on at 10%.... Seemed odd at first, but made for a very smooth start, as the thing would light off at about 15% and, they claimed, the thermal shock was was much less that way compared to motoring to max motor before lifting the fuel cock. Haven't run a -50 for about 10 years now... I miss those beasts! Great engines to work on!

CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course. :)

[grounded27]

CF6-50's were always smooth starters, especially when we adopted our PP engineers' recommended method of putting the fuel on at 10%.... Seemed odd at first, but made for a very smooth start, as the thing would light off at about 15% and, they claimed, the thermal shock was was much less that way compared to motoring to max motor before lifting the fuel cock. Haven't run a -50 for about 10 years now... I miss those beasts! Great engines to work on!

CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

Slightly off topic, it seems a natural inclination for pilots to throw in the fuel at max motor. I have experienced with CF6-10 through 80's that unless high and hot throwing in the fuel above minimum motor just as N1 starts to slow in acceleration saves starter life and is just as healthy for the motor.

[3holelover]

I have experienced with CF6-10 through [...]
Yikes! CF6-10's!? I remember them... bare pipe... no vsv's... looked like a jt8 with a fan strapped to the front?.... lol... I only messed with one once... was on a United DC10-10, and #1 had blown it's last breath out the tailpipe.... nothing but melted stubs where that last LP turbine should've been... But that was the United way... (:rolleyes:) I'll bet they were good engines for their time.

[grounded27]

We operate the largest fleet of them as converted to MD-10's on the airframe. They still run well. Operated domestically, the company is looking for a more efficient aircraft to fit the bulk of cargo this aircraft provides on domestic routes.

[3holelover]

They still run well.Of course they do, early CF6's were built right. ....but fuel was cheap. ;) That's cool to hear the -10's are still running! Say hello to a Dear old Diesel Dixie for me, will you? ....sigh....:)

[grounded27]

Love them every day brother. We take good care of these old dawg's.

[Shanewhite]

....could that be one of the odd features of those RR (or should that be VW?) makes?

Trolling under the wrong bridge there, I think, bro'. Rolls-Royce, the makers of cars - automobiles? - are owned by Volkswagen, but Rolls-Royce (1971) Ltd., - or whatever they are called nowadays - make some of the world's finest aeroplane engines. And you can see from the name, how far back it was that they were the same company. Keep with the programme.

Roger.



Not quite, but almost. When VW purchased the carmaker then known as Rolls Royce Motors from Vickers, who owned it at the time, they were unaware that the trade mark still belonged to Rolls Royce (1971) Ltd, and was not included in the deal. BMW were also in the chase as they had been supplying engines to RRM for some time, so knew the situation. When the sale went through VW outbid BMW, so BMW nipped in quick and bought the rights to the name from RR (1971) for a bargain £40 million. That's why VW now build up-market Audis with Bentley badges in the old Rolls Royce Motors factory in Crewe, and BMW build posh but extraordinarily ugly Beemers with a Spirit Of Ectasy on the front in a purpose-built factory in Goodwood.

[Papa2Charlie]

Let me just clarify something here....

When I said a HP Bleed Valve, I wasn't speaking about cabin bleed. I mean handling bleed valve(s) in the HPC. At low N2, the HPC BV's will be wide open to help stabilise the compressor airflow. As the engine nears idle then the offending BV will close. The remaining valves on the HPC will open / close as usual during accels / decels. Without knowing the details of the booster arrangement on the GE90, it's possible the noise could come from an LPC BV also but my intuition would say the HPC is more marginal on stability during engine start.

I don't know the in's and out's of the GE90-115B but from my own experience of hearing a couple of them start, the noise is consistent with a handling BV being open at start then closing once the engine nears ground idle.

[gas path]

GE90-base or the -115 No 'handling' BV.