jet engine warm-up/cooldown time

[flybubba]

Ok, looking for some expert direction here. I have heard every variation of how much, how long , and why one needs to warm-up (before applying TO thrust) or cooldown a jet engine after landing. Anyone care to weigh in on this?

[Dream Land]

The aircraft I fly has listed a minimum oil temperature prior to take-off (Limitations section), as far as cool down, we use three minutes after reaching idle. Think you will find that this is similar for all Turbine engine applications.

D.L.

[Diabolo]

Hello,

Normaly 3min Cooldown after landing if Thrust Reverser was used. Standart thermal stabilisisation. Valid for all cold stream reverser by-pass engines.
3 min thermal prior applaying TOGA thrust as well.

Happy trails,

[barit1]

But no one has answered WHY.

Startup: The casing is lightweight, but the rotor(s) very massive by comparison. The casing thus heats up and grows in diameter in a minute or so while the rotor takes much longer. During this stabilization interval the seal clearances, blade tip clearances, and axial clearances are out of whack, causing a temporary inefficiency in the engine operating cycle.

If takeoff throttle is applied in this condition, temperatures will be higher than normal because of this inefficiency. An extra few minutes at low power saves wear and tear on the engine.

Shutdown: If the rotor is still hot while the casing is rapidly cooled, there is risk of a rub. A couple of RJ pilots experienced this at FL410 a year ago with sad results (search for "core lock")

Hope this helps.

[westhawk]

Barit1:

I agree with your explanation of thermal expansion/contraction in turbine engines and the consequent advantages of thermal stabilization intervals. Well put.

I would however like to comment on the following statement.

A couple of RJ pilots experienced this at FL410 a year ago with sad results (search for "core lock")

Following the public hearing held by the NTSB regarding the Pinnacle Airlines RJ crash investigation, a rather large body of data gathered during the investigation to date was made available through the NTSB website. Along with the CVR transcript and FDR traces, numerous other investigative group reports were included. One of these was the GE engines paper on "core-lock" phenomenon. Testing on the CF-34 revealed this to occur following flameout during subsequent air re-start attempts. This may or may not explain the Pinnacle crew's lack of success in accomplishing a re-start for the following reasons:

1) GEs tests did not follow a severe over-temp. The Pinnacle FDR traces indicate an ITT indication well beyond maximum limits on one of the engines during the aerodynamic stall event. (IIRC, the r/h eng.) While a severe over-temp may result in causing a rotating group to "lock up", this is a separate issue to the phenomenon addressed in GEs paper on "core-lock." "Core-locked" engines tested could be turned by the starter once internal temperatures stabilized. Often not the case with engines which have been severely over-temped to a degree that causes permanent deformation or dimensional changes of internal components.

2) From the data presented, it is not clear exactly what the configuration of the pneumatic system was at the time that re-start was attempted. We can only hope that the final report will establish the cause for the lack of success in attaining a re-start on at least one engine. I get the impression that this is one of the focal points of the investigation.

I can draw no solid conclusions regarding whether "core-lock" is a contributing factor to this accident on the basis of of my reading of the data presented. It is my hope that the final report will present the FDR data in a clearer, more readable format than the information currently available to the public on the website. While I may have reached a few of my own conclusions as to the root causes of the event, I consider many aspects of the causal chain of events to be unresolved questions. The role "core-lock may have had is one of them.

Apologies for sidetracking the thread a bit.

Best regards,

Westhawk

[lomapaseo]

Westhawk

Core lock being affected by a turbine overheat depends on where in the rotor geomentry the lock is occuring (compressor vs turbine) and what torque conditions are on the rotor at the time of the thermal pinch.

If the pinch point is in the compressor it probably wouldn't be affected by thermal conditions in the turbine. If the pinch point only freezes to a stop when the engine RPM is depressed far below idle speeds then it should be avoidable by keeping the windmill rotor speeds up. Just how much contribution of all this in the referenced accident is as you say still under review.

[barit1]

As I read it, they attempted a VS mode climb to 410, stalled the aircraft, which induced stalls/overtemp/flameout in both CF34s, followed by core lock, botched APU light, botched airstart attempts, flew past several fields where deadstick landings might have been successful -

The litany of missed opportunities to salvage the flight reads like a Stephen King novel. There are times when the best crew in the cosmos runs out of luck - but sometimes a crew is in over their heads 15 min. after rotation.

[Old Smokey]

flybubba, whilst there's a few 'Engine knowlegeable' people close by, may I borrow your thread for a moment? (Actually, it's the reverse of your question).

I'm familiar with the need to allow cool-down before shut down, an engine that I operated some years ago, if shut down early, had to be motored within 20 minutes to expel residual hot gases and reduce uneven thermal distribution. So far so good.....

For the engine that I presently operate (RR Trent), start (Auto and Manual) is not initiated until motoring has reduced the EGT to 93°C. I was recently unable to answer a trainee's question of - "Wouldn't it be less thermal shock to initiate combustion in an already hot engine, rather than for a cool engine?". I had to admit that I didn't know why, but opined that the motoring / cool-down cycle was possibly required to ensure more even thermal distribution before the 'thermal shock' of combustion.

Any opinions appreciated, with apologies to flybubba for the thread-jack.:O

Regards,

Old Smokey

[barit1]

Smokey, on the face of it your trainee is correct about the "thermal shock" aspect of his question. However (in my experience) that is probably not the reason for the cooldown procedure. Various engines have required a similar procedure, and it probably is related to compressor efficiency during startup. (If you induce a starting stall/overtemp, that's quite a "thermal shock", ain't it?)

Some engines also have tailwind limits for start (early JT9D-3's were notorious for this), while others have no such limits. Some require waiting till 15% N2 or greater before fuel on, others are happy at 10%. DC-10's had derated APU's for many years, but the CF6's seemed to start just fine at low pneumatic pressures.

In short - the 93°C limit was likely determined empirically or statistically to give the least probability of a hot start.

[flybubba]

Old Smokey and barit1,
No offense taken about HJ'ing the thread. I had always assumed motoring the engine to get the EGT down was to reduce the chances of a hot start in an already warm engine. It makes sense that by motoring, the airflow would tend to equilibrate any temp. variations, thus reducing the chances of a hot start.

[Old Smokey]

Thank you, flybubba, for allowing me to 'borrow' the thread for a short time.

Thanks to flybubba and barit1 for the good answers, sounds reasonable to me, so much so that I'm going to quote you the next time the topic comes up!:ok:

I agree, barit1 , the CF6 is a 'good starting' engine. Works every time.

Regards,

Old Smokey

[Ignition Override]

Barit1: I have no idea whether the CRJ has typical VNAV, FLCH and VS climb/descent modes.

On the 757, we were taught to never use VS during climb, because of the very wide range of climb rates at different weights, altitudes. It was years ago when I flew it, but we only used VNAV.

VS was only used for non-precison approaches and can be risky during an approach; when you begin a descent, i.e. under 10,000' and make a mistake, VS did not capture your altitude, compared to FLCH. The syllabus required numerous non-precision approaches in Initial Training as a safeguard.

Pinnacle has fired numerous new-hires during Initial Training for the CRJ, following the tragic accident. The FAA must have put them under severe pressure (Don't know about training budgets, or whether they were reduced). Pinnacle allegedly have less tolerance for men who have training problems, compared to women. The wife of an Instructor Pilot there told me this. This is incidental, but a Pinnacle Check Airman (flew on us to DFW) told me that one poor guy went through a well-known (infamous?) regional airline program in Florida on the Beech 1900 and had too little actual instrument approach experience, despite his massive financial investment. Florida would not require as many as Michigan or Minnesota.

[Centaurus]

Old Smokey. You said that an engine you once operated needed 20 minutes of motoring time to cool down components. Must have been a mighty tough starter motor. Do you mean 2 minutes maybe?

[Diabolo]

If no reverser had been selected during Rollout or even at Idle power you do not need cooldown.
Reason, on Bypass Engine cold stream reverser you simply cut the cold stream of air that cool the core and the casing of the engine. After reverser check your EGT and you will notice that Ground Idle EGT values are greater that normal.
737-200 did not required cooldown after landing. Hot stream thrust reverser installations. Correct me if I am wrong :)



Sincerely

[Old Smokey]

Hi Centaurus,

On skipping back, I think that I said "had to be motored within 20 minutes".

You had me worried for a moment, I thought that my typing skills had gone for a Burton.

It only had to be motored until the EGT was < 200°, usually 30 to 45 seconds of motoring.

Regards,

Old Smokey

[barit1]

If no reverser had been selected during Rollout or even at Idle power you do not need cooldown.
Reason, on Bypass Engine cold stream reverser you simply cut the cold stream of air that cool the core and the casing of the engine. After reverser check your EGT and you will notice that Ground Idle EGT values are greater that normal.
737-200 did not required cooldown after landing. Hot stream thrust reverser installations. Correct me if I am wrong

Diabolo, I'm trying to connect the dots here - The JT8D on the 737-200 is a bypass engine (albeit low bypass ratio) and the clamshell reverser reverses both hot and cold flows.

And the high bypass engines I'm familiar with do not "simply cut the cold stream of air that cool the core and the casing" when in reverse.

Expand on what you're saying, please.