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CaptainMongo
30th Jul 2019, 18:22
Engine warm up if shut down for less than two hours is 2 minutes at idle, if shutdown for more than two hours warmup is 5 minutes at idle.

Scenario: Engines have been shut down for 3 hours. Push back from gate, start both engines because it is required for initial taxi. After clear of congested area, shut down engine two (it has been running for less than 2 minutes) and continue taxi. What will be the warm up required upon restart of engine 2?

763 jock
30th Jul 2019, 18:30
Are you serious? Really?

airseb
30th Jul 2019, 19:10
He seems to be🤯

Jim_A
30th Jul 2019, 19:31
Engine warm up if shut down for less than two hours is 2 minutes at idle, if shutdown for more than two hours warmup is 5 minutes at idle.

Scenario: Engines have been shut down for 3 hours. Push back from gate, start both engines because it is required for initial taxi. After clear of congested area, shut down engine two (it has been running for less than 2 minutes) and continue taxi. What will be the warm up required upon restart of engine 2?

For you, 15 minutes, because one can never be too safe..... ;)

Atlas Shrugged
31st Jul 2019, 04:19
Scenario: Engines have been shut down for 3 hours. Push back from gate, start both engines because it is required for initial taxi. After clear of congested area, shut down engine two (it has been running for less than 2 minutes) and continue taxi. What will be the warm up required upon restart of engine 2?

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/500x286/bloke_e62bc6391d4ec372695c46d7112dcff792675bc0.jpg

Intruder
31st Jul 2019, 06:01
Why not keep it running for 5 minutes and THEN assess whether you need to shut it down?!? Do you REALLY need to make things MORE complicated?!?

ScepticalOptomist
31st Jul 2019, 09:32
Ask yourself what you are trying to achieve - a warm engine prior to takeoff.

If your manual states 5 minutes to warm up a “cold” engine, then unless you have run the engine for that long prior to shutting it down again, I’d suggest you start the warm up timer again on the next start. Be conservative.

I would be a little suspect of the reason for shutting it down again so soon after pushback, hypothetical aside.

B2N2
31st Jul 2019, 09:45
What’s the increase in cost for nosewheel replacements ?

Tu.114
31st Jul 2019, 10:19
OP, as You did not mention the type or general situation you are referring to, maybe You could write a few words on why a shutdown after clearing of the congested area is necessary or beneficial? Are you taxiing out for departure or are you referring to a mere relocation of the aircraft on the apron that is for whatever reason done under its own power and not by tug?

One thing that needs to be remembered as well is that not only fuel costs money, but so does an engine. A startup is not exactly easy on the engine and it may well be that the additional maintenance costs due to the number of startup/shutdown cycles racked up by this practice will be way larger than the possible fuel and brake wear savings...

pineteam
31st Jul 2019, 11:20
In case of a doubt, just be conservative: I will definitely go for the 5 min. 2 min more is not going to kill anyone.

misd-agin
31st Jul 2019, 17:34
What’s the increase in cost for nosewheel replacements ?

Probably close to zero, especially after viewing all the skid marks going around tight taxiway turns or lining up on the runway. The additional side load on the nose gear is minimal in comparison. The amount of corrective inputs by the rudder tiller to compensate for the asymmetrical thrust is minor.

misd-agin
31st Jul 2019, 17:51
What’s the increase in cost for nosewheel replacements ?

We know there’s a fuel savings. What’s the additional cost in brake wear on arrival when many of the aircraft will accelerate to 20+, or even 30+ kts, with all engines turning and burning at arrival weight?

CaptainMongo
31st Jul 2019, 21:45
Several gate areas where we operate require a two engine taxi from the gate. After leaving the congested area, the taxi route can be 15 - 25 minutes, thus the engine shutdown. Monday I had a taxi which was 97 minutes gate to runway. How about a show of hands who would run two engines during that length of taxi. Once during that taxi we even shut down both engines for about 20 minutes, think of that. Then, to top it all off, we did a cross bleed to start number two.

Our outfits bean counters have run the numbers and have determined any engine shutdown, regardless of length, is money saving. I fly airplanes, the bean counters count beans, if that’s what they have determined that’s good enough for me. To those of you who have provided a thoughtful response, thanks.

Check Airman
31st Jul 2019, 21:59
I don't consider 20 minutes a long time at all. I'd keep them both running. Having the PTU or yellow pump running is incredibly annoying from a passenger perspective, particularly for those seated nearest to those pumps (A320).

Obviously for a 90 minute taxi at JFK, I'd consider shutting one down, but again, the noise in the cabin for that length of time may give me pause. If I know the taxi will be that long, I'll request extra fuel. We may be too heavy to make a SE taxi practical anyway.

FlightDetent
31st Jul 2019, 23:02
Get a tug to pull you out a bit.

According to a knowledgeable CFM gentleman, up to 15 minutes warmup is still beneficial to the longevity of the engine, which translates well into direct fuel savings due to low(er) burn for long(er) time.

Though I understand you speak of IAE engines, which at least on the small bus really have a lot of ground idle thrust, pitty you cannot make it out on just one.

Towards the core of your question, I have nothing to add. It is an odd situation hence you came here with it. The suggestion with a tug is serious - beancounter's business as you have already pointed out.

tdracer
31st Jul 2019, 23:20
I thought I posted this earlier, but it didn't show up so I'll risk repeating myself.
The bottom line is that running at/near idle for a few minutes then shutting down for 90 minutes does not meet the requirements for having a 'warm' engine. Perform a minimum of a five minute warmup.
Setting takeoff power on an engine that isn't thermally stable is hard on the engine. The impact can be anything from a tip seal rub (permanent performance loss) to in extreme cases it can cause an engine surge.
Saving a couple of gallons of fuel by not doing a proper warmup will be quickly swamped by even a small increase in engine fuel burn until the next overhaul.

Check Airman
1st Aug 2019, 03:29
I thought I posted this earlier, but it didn't show up so I'll risk repeating myself.
The bottom line is that running at/near idle for a few minutes then shutting down for 90 minutes does not meet the requirements for having a 'warm' engine. Perform a minimum of a five minute warmup.
Setting takeoff power on an engine that isn't thermally stable is hard on the engine. The impact can be anything from a tip seal rub (permanent performance loss) to in extreme cases it can cause an engine surge.
Saving a couple of gallons of fuel by not doing a proper warmup will be quickly swamped by even a small increase in engine fuel burn until the next overhaul.

Yea but what do you know? You're not a beancounter :ugh:

Thanks as always for your insightful contribution.

I've flown with people who must pride themselves in applying takeoff thrust and shutting down the engine right at the limits. Never understood it.

Uplinker
1st Aug 2019, 07:44
Mechanical sympathy applies.

I once got to the staff carpark at the same time as a group of ground handlers. They obviously had some sort of private race going on, or were trying to beat the traffic. They each started their car engines and as soon as the engines caught, they all accelerated away like mad things.

Every one of their engines sounded like a bucket of bolts, i.e. worn to buggery because of working the engines hard before the oil was warm enough to circulate fully. Now, think of a gas turbine engine, parts of which are under much higher stress than a car engine, especially at take-off power.

As well as seals, and thermal stresses within the engine, the oil needs to be warm enough and fluid enough to get around the engine easily and quickly.

Tu.114
1st Aug 2019, 10:31
Even at the risk of some thread drift:

Monday I had a taxi which was 97 minutes gate to runway.

Now, the operational area I am familiar with is Europe and its closer surroundings, and I may not have come across the yet unmentioned airport that this took place on. However, this does strike me as exceptionally long - possibly longer than the subsequent flight. Does ATC allow startup and pushback without taking runway queues into account? Is there no CDM or "first come-first serve" queue along which startups are approved while both allowing sufficient taxi time and keeping the queues at the holding point somewhat within a reasonable limit?

Surely, such a system would be desirable from an efficiency, fuel saving and engine lifetime saving standpoint and beneficient to ATC and airlines alike? If you are not going to depart for another 90 minutes, why not wait at some parking spot with, possibly a remote one, with ground power and aircondition plugged in?

oggers
1st Aug 2019, 11:48
Engine warm up if shut down for less than two hours is 2 minutes at idle, if shutdown for more than two hours warmup is 5 minutes at idle.

Scenario: Engines have been shut down for 3 hours. Push back from gate, start both engines because it is required for initial taxi. After clear of congested area, shut down engine two (it has been running for less than 2 minutes) and continue taxi. What will be the warm up required upon restart of engine 2?

Ok then. 5 minutes because whether or not you can 'credit' any of the "less than two minutes at idle" towards the requisite 5 minutes, is information you do not have.

Uplinker
2nd Aug 2019, 10:22
In the OP’s scenario, my [unofficial] advice would be to look at the engine oil temperature gauge after said engine had been running for a couple of minutes.

If after a few mins, the oil temp was well above the manufacturer’s published minimum, then you are good to go. If it is near or at the limitation, I would give it another few minutes before demanding take-off or flex power.

EcamSurprise
2nd Aug 2019, 13:52
Most conservative wins - 5 minutes. And if you have a 97 minute Taxi out, a 5 minute warm up before departure will hardly be noticeable.

For what it’s worth, my Airbus CFM Operator asks for a 3 minute warm up on a warm engine and a 5 minute warm up on a cold engine (>6 hours). In your case I’d still apply the 5 minute limitation.

misd-agin
2nd Aug 2019, 20:46
“Right at the limits” is acceptable. If it wasn’t they’d change the limits.

“Takeoff as as soon as the oil temperature is within limits”? Please don’t follow that advice. The manufacturers would say that if it was acceptable. The oil temperature is typically within limits by the time the engine reaches idle. The most it takes is until you do the flight control check. Both are much shorter than the 5 (reduce able to 3) minute limitation.

As tdracer says there’s more to the warm up then just achieving normal oil temperatures.

Tomaski
2nd Aug 2019, 21:49
I tend to be fairly conservative about warm up times. The only times I even get close to min warm up time is when I've got engines shutdown on the taxiway awaiting a departure slot and I get one of those short notice launch or lose it windows to get airborne. Gas is cheap compared to engines so treat them nice and hopefully they will return the favor.

tdracer
2nd Aug 2019, 22:11
“Takeoff as as soon as the oil temperature is within limits”? Please don’t follow that advice. The manufacturers would say that if it was acceptable. The oil temperature is typically within limits by the time the engine reaches idle. The most it takes is until you do the flight control check. Both are much shorter than the 5 (reduce able to 3) minute limitation.
Agreed - I can't speak for all installations, but I know of several engine installations where the min oil temp is based on the ability of the oil to 'deice' the fuel (via the fuel/oil heat exchanger). It has nothing to do with the engine being 'warm'.

Dave Therhino
3rd Aug 2019, 09:47
The engine manufacturer's minimum warm up time and the minimum oil temperature limit together are intended to ensure several things, as tdracer has discussed. My understanding is that the main intent is:
1) Minimum oil temperature for the lubrication to be adequate for the bearings to roll properly and withstand the loads of high power operation
2) Minimum oil temperature to ensure that, when fuel flow goes way up at power set, the temperature of the fuel downstream of the fuel oil heat exchanger does not dip below the freezing temperature of water for longer than the servo circuits of the engine control system hydromechanical unit have been demonstrated to withstand, and
3) The engine case has warmed up enough so that, when the centrifugal forces of high rotational speed expand the rotor at power set, the engine does not experience rub at the blade tip seals or stator and nozzle seals because the case hasn't yet expanded enough.
4) The rotor has warmed up and expanded enough so that, when the takeoff roll progresses and the case heats up and expands quickly, the clearances don't get too large a minute or two later before the rotor thermally starts catching up to the case, which can cause a stall around rotation or initial climb.

There is a combination of thermal effects, pressure effects, and centrifugal effects that determine the radial clearances between the rotor and the static parts as changes in cycle temperature occur. The rotor drum and disks have more thermal mass than the engine case, and they respond to changes in cycle temperature more slowly. On top of the effect of thermal expansion and contraction of the rotor (which lags behind cycle temperature changes), the centrifugal force on the rotor causes it to immediately grow and shrink with changes in rotational speed. The case grows and shrinks with changes in cycle temperature more quickly than the rotor (and turbine case cooling systems obviously also affect the turbine case). There is also a pressure driven expansion of the case, but I have understood this to be a relatively small effect compared to the thermal effects (maybe tdracer can confirm this). If you set takeoff power before the case has had a chance to warm up and expand some, you may get rubbing of the blade tips seals and stator and nozzle inner seals right at initial power set when the rotor expands immediately from centrifugal force. Once takeoff power is set, the case expands from the temperature increase more quickly than the rotor, and the maximum clearances in the compressor can be right about at rotation or initial climb. As tdracer said, if the clearances get too large or are not uniform, a compressor stall can occur.