Engine warmup.

[Matt48]

Could anyone enlighten me as to how long it takes to warm through a jet engine before takeoff power can be applied.


Was wondering why airlines , in search of economy of operation, don't tow aircraft from the ramp to a holding point that gives enough time to warm/check engines before takeoff.

[Piltdown Man]

Each engine varies. GE, our engine manufacturer recommends 2 minutes (CF 34). Rolls Royce recommends between 2 and 4 minutes, depending on oil temperature.

Towing aircraft is a good idea and it has been used from time to time, assuming a long distance tow is approved at takeoff weight with passengers onboard. But you start to incur other costs and complexities. Who is in charge? How many ground crew? Some believe you have to have wing-walkers whenever you have a tug attached. Wing-walkers will reduce your towing speed. How many more tugs will be required? How does the tug get back to the apron? What processes will be in place for emergencies? Figure that lot out and let us know.

[Matt48]

Thanks for your interesting replies gents , another question I have also wondered about, again with operating costs in mind is why don't aircraft at the ramp use 'shore power', why run up hours and fuel running the APU to provide aircon and power when alternate power is available.

[PDR1]

1. Jet engines need very little (seconds) warming up except where there are specific issues like engine hydraulics whose fluids and seals may need to be warmed in very low ambient temps. Piston engines need to be warmed up for two basic reasons. Firstly to bring the pistons, rings and cylinders to a stable design temperature so that the various fits and clearances have closed to the intended design values, risking gas blow-by and consequent damage to the rings and cylinder bores if run at more than minimal power. Secondly it is to warm the lubricant to within its intended operating range - when too cold the oils won't "wet" properly and so lubrication is poor, risking metal-to-metal contact (and subsequent engine damage/failure) if run at more than minimal power. In the case of liquid-cooled engines it is also necessary to bring the coolant to working temperature to avoid the risks of shock-cooling parts of the block and heads with cold coolant.

2. It would definitely be a bad idea to have lots of tug vehicles mixing with aeroplanes on taxiways and runways. The risk of collisions would be unacceptably high for an un-necessary risk. Fuel consumption whilst taxiing and holding is often reduced by only using one engine, starting the other(s) just before final take-off checks (depends on the aeroplane, the operator's procedures and the ambient conditions - single engine taxiing in high winds and/or on a slippery runway is generally avoided). This rather proves the point about rapid warm-up times for jet engines.

The amount of fuel used in taxiing and holding is generally negligible compared to the amount used at cruise power, so it's not THAT big an issue.

3. "Shore Power" is fine where the services required are electrically driven. Cabin conditioning usually isn't. There's also the detail that it would need to be metered and billed, which just adds more administrative bumph which would be difficult to justify when the aeroplanes all have perfectly good APUs. Also who pays the bill if "bad" shore power damages an aeroplane? How would you prove it?

[Denti]

Well "shore power" can be seen as ground power and that is comonly used and airlines pay for it, so billing in some way is already there. However, it is quite often very expensive so that the APU is cheaper. Add to that that you need both ground power and external AC commected to replace an APU in anything warmer than 20 degrees C or colder than 10 and it becomes complicated. The 787 is the only excpetion as it only need electricity, just lots of it.

[Chris Scott]

Quote from PDR1:
"Shore Power" is fine where the services required are electrically driven. Cabin conditioning usually isn't. There's also the detail that it would need to be metered and billed, which just adds more administrative bumph which would be difficult to justify when the aeroplanes all have perfectly good APUs."

I retired many years ago, and am not au-fait with curremt regulations at airports. However, during the 1990s the major European airports gradually introduced swingeing restrictions on the running of APUs, due to local noise and air pollution regulations.

On arrival it became the norm to shut down the APU as soon as ground-electrics had been established. At some airports the APU was not even started, which meant that one engine had to be left running instead. That would be costly on occasions when the dispatcher (or other competent ground crew) was late arriving at the gate. In extremes of ambient temperature, "ground [air] conditioning" would be supplied (eventually) for pax comfort during disembarkation.

A similar logic was, and presumably is, applied for departure. The APU was started only a few minutes before departure, to facilitate engine start during push-back.

I'd be very surprised if these airport restrictions on APU running have been lifted since I retired. More likely, they've been increased! From a pilot's point of view, the only redeeming aspect is that, if the APU is u/s, the ground services are more likely to be geared up to greet you on arrival.

[DaveReidUK]

On arrival it became the norm to shut down the APU as soon as ground-electrics had been established. At some airports the APU was not even started, which meant that one engine had to be left running instead. That would be costly on occasions when the dispatcher (or other competent ground crew) was late arriving at the gate. In extremes of ambient temperature, "ground [air] conditioning" would be supplied (eventually) for pax comfort during disembarkation.

A similar logic was, and presumably is, applied for departure. The APU was started only a few minutes before departure, to facilitate engine start during push-back.

Barcelona is a good example.

At pier-served stands, the APU must be shut down within 2 minutes of arriving on stand and cannot be started until 5 minutes before departure time.

For remote stands, the limits are 10 and 15 minutes, respectively.

[larryboy]

@ pdr1, jet engines do require warm up periods, for almost the exact same reasons as piston engines, except clearance tolerances are measured at compressor and turbine blade tips. Starting a jet and applying thrust immediately will surely result in stall/ surge/ blade cracking. Depending on how long the engine has been shut down, ie if the engine is already warm, will determine length of warm up period

[lomapaseo]

I don't believe that the limit section of the engine's type certificate prescribes a warm up period.

It's nice to baby an engine as some do in their car in the belief that it will lengthen the time between overhauls.

As always follow the manufacturer's recommendations and fly safe

[eckhard]

RR Trent 1000:
5 minutes warm-up time from start until take-off, unless the engines have been shut down for less than 2 hours, in which case the warm-up time is 2 minutes.

Similar on the RB.211-524G/H except I believe it is 1.5 hours and then 3 minutes.

Very similar on the IAE V2500.

5 minutes cool-down after landing before shutting down on the RR Trent 1000.

[NSEU]

another question I have also wondered about, again with operating costs in mind is why don't aircraft at the ramp use 'shore power', why run up hours and fuel running the APU to provide aircon and power when alternate power is available.

My old airline switched to ground power and conditioned air on almost all transits. On the surface, it does look cheaper and cleaner. However, here are a few factors which add to the costs (sorry for the off topic thread creep):

Additional manpower required to install ground power/conditioned air (e.g. on A380s, our airline insisted on installing 4 power leads and numerous conditioned air supplies... very labour intensive).
Medical expenses due to injuries incurred during the fitmentment of such. Overtime costs when replacing injured staff.
Occupational Health and Training course expenses (when the company realises so many people are suffering injuries)
The provision of expensive (sturdy/safe) steps at almost every gate for installing power/air leads (sometimes the aircraft connections can be quite high off the ground).
Aerobridge operating training costs (where power/air is supplied by connections on the aerobridge). On this type, there are interlocks preventing you using power/air connections when the aerobridge is in motion, so arriving flight crews have to start the APU anyway until the Aerobridge has been fully docked with the aircraft.
Delay costs due to engineering manpower being diverted to the removal of power/air when they could be fixing/servicing/certifying the aircraft.
Maintenance may require APU operation anyway (bleed air may be required for engine ground runs, airconditioning pack faults, etc).
Ground power/air isn't always serviceable.
Ground conditioned air isn't always sufficient to cool a large aircraft on hot days.
The switching from external power to APU electrical power is not always smooth and can cause glitches (which can cause delays if the APU is started only 5 minutes before departure).

I've probably missed a few.

[Centaurus]

In the case of liquid-cooled engines it is also necessary to bring the coolant to working temperature to avoid the risks of shock-cooling parts of the block and heads with cold coolant.


Interesting observation. The Pilots Notes for aircraft equipped with Rolls Royce Merlins (Mustang and Lincoln for example, which I flew) in the Limitations section mentioned a minimum of 6o degrees C for coolant. I was never aware of the term "shock cooling" when I flew these types in the Fifties. The term is relatively more modern I believe.

From recollection (always a bit suss with age :ok:) coolant temperatures would rise rapidly after engine start probably within one minute and so the 60C lower limit was not an issue. Probably because all of my experience on these aircraft was in the Australian tropics.

I am not sure if the point about shock cooling of RR Merlin cylinders is valid. My reading of WW2 history where RAF Spitfires and Hurricanes did "scramble" take-offs in Britain mid-winter soon after engine start, would suggest that shock cooling was not an issue for Rolls Royce Merlin liquid cooled engines.

[tdracer]

What larryboy said:D
In a turbine engine, the compressor rotor and blades heat up (and expand) much quicker than the compressor case. Set high power too soon after a cold start and best case is you'll rub the compressor blades resulting in permanent performance loss (there was an accidental accel on a cold engine while we were doing an engine ground test many years ago - the resultant compressor rub resulted in a permanent 20 deg C increase in EGT :eek:). In severe cases it can result in a surge/stall and even break compressor blades. In short proper warmup is critical to the health of turbine engines.
At Boeing, we also do a minimum 5 minute warmup before setting power above approach idle, and similarly 5 minutes idle cool down before shutdown if the engine has been above approach idle.

[riff_raff]

There were some recent issues with start procedures of the P&W GTF engines.

New P&W President Has ?Nothing To Hide' On GTF Starting Issue | Commercial Aviation content from Aviation Week (http://aviationweek.com/commercial-aviation/new-pw-president-has-nothing-hide-gtf-starting-issue)

[A37575]

For pistons, no takeoff before oil temp in the green

From the Cessna Model 152 POH Section 5 normal Procedures page 4-23.
Quote:
"During cold weather operations no indication will be apparent on the oil temperature gauge prior to take off if the outside air temperatures are very cold. After a suitable warm up period (2-5 minutes at 1000RPM) accelerate the engine several times to a higher RPM. if the engine accelerates smoothly and oil pressure remains normal and steady, the airplane is ready for take off."

In other words no requirement to have the oil temp in the green. In fact, if you operate the engine on the ground for an extensive period in order to get the oil temperature in the green, it introduces another problem and that is lack of effective cooling airflow over the engine which can result in long term heat damage to leads and components in the engine bay.

From the above, it is probably unwise to generalise "for pistons, no take off before oil temp in the green." :ok:

[Uplinker]

It's nice to baby an engine as some do in their car in the belief that it will lengthen the time between overhauls.


+1:ok:

I remember at one base there were some days/nights when I arrived back at the carpark from flights at the same time as a shift of airport workers. These guys were obviously racing each other or under the impression that they were saving money becuse as soon as their engines fired, they set off down the road, accelerating madly. Made me wince every time, and all their engines sounded like buckets of nails....

Years ago I rebuilt a Rover V8 engine - rebore, new crank, pistons, new camshaft etc, etc, and before I fired it up for the first time after the rebuild, I made a tool to prime the oil pump with my electric drill via the distributor drive to pump oil through the engine before starting it. I would say it took about 20 seconds of pumping for the oil to reach the top of the engine (I had the rocker covers off so I could see how the oil was progressing). Since then, I try to let any car engine idle for at least that sort of time before putting any load on it, and even then use it as gently as possible until it is fully warm. So far, my car's engines have sounded smooth like sewing machines even after 10000s of miles.

I also wince when I see people banging on electrics, air and both packs, the very second the APU says 'available'. I try to make subsequent selections no quicker than once every 30 seconds or so.

Our CFMs need 3 mins to warm up, our IAEs need 3 mins if warm, 5 mins from cold, and both types need 3 mins at idle to stabilise before shutting down.

[PDR1]

Interesting observation. The Pilots Notes for aircraft equipped with Rolls Royce Merlins (Mustang and Lincoln for example, which I flew) in the Limitations section mentioned a minimum of 6o degrees C for coolant. I was never aware of the term "shock cooling" when I flew these types in the Fifties. The term is relatively more modern I believe.

From recollection (always a bit suss with age :ok:) coolant temperatures would rise rapidly after engine start probably within one minute and so the 60C lower limit was not an issue. Probably because all of my experience on these aircraft was in the Australian tropics.

I am not sure if the point about shock cooling of RR Merlin cylinders is valid. My reading of WW2 history where RAF Spitfires and Hurricanes did "scramble" take-offs in Britain mid-winter soon after engine start, would suggest that shock cooling was not an issue for Rolls Royce Merlin liquid cooled engines.

The liquid-cooled WW2 fighters are an interesting case. There was a clear operational requirement for rapid start-up for scrambles, and this requirement was addressed in design features of both the oil & glycol cooling systems and the engines themselves. I'm told it was very, very easy to overheat a spitfire (especially the early ones up to mkV) because the cooling systems were barely adequate at zero airspeed - this was a design feature to speed the warm-up. Similarly the engines had wide clearances to allow them to operate with colder oil than might be ideal. A consequence of this was higher oil consumption* when cold. This was partly mitigated with operational procedures in which aircraft on "standby" would be initially warmed up and then given a few minutes running every 20-30 mins to keep the oil warm (the oil was the bigger concern) and ready for "scrambles".

Of course the main driver for proper warm-up procedures would be the reduced wear and longer engine life. This was not really a consideration in 1940 when few spitfire or hurricane engines ever reached their TBO (a huge number of these aircraft never even got to the 100 hour checks).


* Anyone who ever owned a Coventry Climax engine (whether in an Imp, an Elite or any of the range of racing cars which used them) would be thoroughly familiar with this. The Climax was originally a firepump engine designed to be dragged next to a nearby pond to suck water for fire hozes. So it was very light and also designed to be started and immediately run at a governed 9,000rpm from stone cold. To achieve this reliably required very wide clearances, so these engines were all prodigious oil-burners. It wasn't a fault - it was a feature.

[Willit Run]

Turbine engine warm up is nothing new. Can you apply full thrust shortly after start up? Sure, but engine longevity is greatly increased by an adequate warmup period. Now, for all the EPR vs N1 arguments, a cold engine may very well benefit from an EPR setting due to input vs output pressure, but that doesn't account for any blade tip erosion. A cold engine setting TO thrust by N1 ain't gonna produce the required or anticipated thrust that a properly warmed up engine will.
For any relevant basis, I have only operated High bypass ratio engines. Never flown a turbo prop. However, I have a a lot of time flying BIG radial engines. Most of us are paid by the minute/hour. Why not make the engines more reliable? Why not reduce your real V1 failures
, hence your time in front of the long table with no ashtrays. All the big three engine manufactures recommend a 5 minute warm up before applying max thrust, at least on the big thrust engines.......some recommend 10 minutes if really nipply......

[megan]

From some old time flight manuals.

P-51B (Merlin)
Emergency Take Off
Use oil dilution to obtain proper oil pressure at moderate power, and as soon as the engine will take the throttle, taxi out, and take off.
Warning
Apply throttle slowly but steadily. Sudden application of full throttle greatly affects torque.

Normal Take Off
Min Oil Temp 15°C
Max Oil Pres 90 lb/sq inch
Max Coolant Temp 60°C

C-54
Scramble Tale Off
Dilute engine and prop feathering oil in accordance with existing atmospheric conditions.
Check oil temperature rise. Temperature must rise at least 10°C above residual oil temperature prior to take off.

Spitfire V
Normal Take Off
Oil Press Normal 60/80 Minimum 45 lb/sq inch
Oil Temp Min 15°C
Coolant Min 60°C Max prior to commencing take off 100°C

Spitfire makes no mention of emergency take off. From the notes it implies when at readiness engines were kept warm.

[msbbarratt]

Alright, so no one has yet powered an aircraft with one. However, they too have to be gently warmed up. Superheated steam could easily be 600 degrees C, and you'd have to dribble some through the turbine to let the blades warm up to prevent damage caused by rotor bow. Yep, same problems.

Nuclear steam plants are much less fussy - they've not got superheated steam available in the first place. In the early days of nukes their turbines were distinctly old-school in comparison to the last of the oil fired steam driven warships.