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PAXboy
1st Jul 2002, 21:43
I understand that heavy duty diesel engines need a certain amount of running time before service. This can be to build brake pressure and so forth.

Other engines may need to be at full operating temperature, before being placed at maximum demand.

Is there a minimum time that a typical modern high bypass jet turbine (Engineers will say that there is no such thing as a typical engine) [others will say that that I have the nomenclature mixed up] needs to be operating before take off roll?

Also, if you have a long slow crawl out to departure (such as EWR :rolleyes: ) can the engines overheat? This is based on them being designed to operate primarily at high altitudes. Or, are they able to keep themselves cool due to the nature of the bypass fan?

<I sit to be corrected :p >

basil fawlty
1st Jul 2002, 22:08
piston engines require a warm up period because the oil is too viscous to lubricate properly when cold with the "wet sump" arrangement that exists on such engines.
gas turbines are pressure lubricated, and the oil is synthetic and lower viscosity (as indeed some reciprocating engine oils are now...) and therefore lubricate effectively straight from cold.

no, gas turbines don't "overheat" with prolonged ground operation in hot climates. The limiting factor is the turbine inlet temperature, (i.e the maximum temp the turbine blades, disk etc. can cope with) and this is only potentially achieved in warm ambient air when the engine is at its maximum thrust setting. Idleing EGT will be a few hundred degrees lower!!

Max Angle
1st Jul 2002, 22:51
The limits for the V2500 say that the oil temp. must be +50 before takeoff. Also if the a/c has been on the ground for more than 2 hours you must run the engine for 3 mins before takeoff.

lomapaseo
2nd Jul 2002, 00:33
>(Engineers will say that there is no such thing as a typical engine) [others will say that that I have the nomenclature mixed up] <


I like the idea of the multiple choice opinion poll but I didn't see where to enter a vote;)

Regarding the V2500. It had/has a "bowed: rotor spoolup problem that causes uncomfortable vibrations. The oil damped bearings need to work and of course the temperature effects need to be controlled to reduce the bow. Not much more than a uniqueness to this model.

Capt Claret
2nd Jul 2002, 03:12
PAXboy

ALF502/LF507 as fitted to BAe146 require a minimum oil temp of 30 degrees C and if the engine has been shut down for more than 30 minutes, 3 minutes warm up.

After landing the engines require 2 minutes warm down @ less than 60% N2.

moleslayer
2nd Jul 2002, 17:47
Sure you don't mean 5 mins MAX :D

Capt Pit Bull
2nd Jul 2002, 18:16
Although you can get problems with nacelle overheating on turboprops.

CPB

Max Angle
3rd Jul 2002, 00:13
moley,

doh!, 5 mins. it is.

SuperRanger
3rd Jul 2002, 05:51
for most turbine engines, there is a recommended idle time of 3-5 mins before setting takeoff power and 1-3 mins before shutting down. this is to prevent thermal shock to the turbine blades.

SR

Elevation
3rd Jul 2002, 10:11
For RR Trent 800 series engine,
+50 C oil temp before takeoff power can be set.
During cold wx ops on ground, engine run-up to 50% N1 every hour.

lomapaseo
3rd Jul 2002, 13:22
>During cold wx ops on ground, engine run-up to 50% N1 every hour.
<

That one I just don't understand. Just how long do you operate on the ground?

I can't see how after an hour of operation on the ground that a runup is going to change the thermals much inside an engine.

On the other hand a runup to 60% N1 is recommended on most similar size engines every 15 mins to remove any ice built up on the vanes behind the fan.

pigboat
3rd Jul 2002, 17:47
If you ever get the urge to go operate a large piston powered aircraft sometime, here's part of the procedure for when it got chilly.
If the forecast overnight low temperature was below, say, -10 Fahrenheit, you would consider using oil dilution to aid the start the following morning. (Oil dilution is a procedure whereby avgas is added to the engine oil via a metering system to raise oil viscosity in cold temperatures.) There were two types of dilution, hot and cold. Hot dilution was when the engine oil temp was over 50*C, and cold dilution was for oil temps below 50*C. The 50*C cut-off point was because above that temperature, the added gas boiled off, while below 50*C it remained mixed with the engine oil. You carried out a hot dilution immediately prior to the last shutdown of the day. With the oil temp over 50*C, you ran the engine at 1500 RPM for thirty seconds, exercising the prop several times to allow diluted oil to the prop dome, then you shut down and waited for the oil temp to drop below 50*C. While waiting, you consulted your dilution charts and decided how many minutes of cold dilution you would need, based on the forecasted OAT next morning. If the forecasted temp was -20*F, you would need say, three minutes cold dilution. At an oil temp of say 40*C, you fired up and ran the engine at 1000 - 1200 RPM, all the while holding the oil dilution switch engaged. At the end of three minutes you shut down, holding the switch engaged until the prop stopped. Your engine was now "diluted."
The following morning, you had to burn off the dilution before take-off. This meant ground running the engine until the oil temperature had exceeded 50*C, to evaporate the gas in the system. You needed about five minutes of ground run with an oil temp greater than 50*C for each minute of cold dilution you'd given the night before. If your aircraft was equipped with a Janitrol heater, all was well. If not, it got downright frosty.
Ahh..nostalgia just ain't what it used to be.:)

PAXboy
6th Jul 2002, 21:13
My thanks to all, your answers most helpful and interesting. Also, following pigboat's story, I shall decline all offers of pistons with low temperatures! I have read those stories of draining the oil when it was hot and then thrawing it out on the stove the next morning, I didn't realise how true they were!

To sum up: Given the time that it takes from Start to turning onto the departure runway, you always have enough time to warm the engines through. The last part of the question is - following Start, at what stage are you comfortable that the engine/s are stable and you don't mind asking them to get you off the ground? OR must you presume that an engine can fail at any time for any reason and NEVER take it for granted? Which, I presume, is the correct view!

lomapaseo
7th Jul 2002, 04:43
> an engine can fail at any time for any reason and NEVER take it for granted? Which, I presume, is the correct view!
<

Yup, that's about it except for Murphy's law.which tend s to shade the stats a little.;)

OzExpat
7th Jul 2002, 08:42
Better you than me, pigboat! Brrrrrrrrrrrrrr :eek: :D

pigboat
7th Jul 2002, 23:13
Yeah Oz, like I said, nostalgia ain't all it's cracked up to be.:D

OzExpat
8th Jul 2002, 11:57
It might've been... when we were younger mate! :D

mcdhu
9th Jul 2002, 13:28
..............and once you got into the air, wasn't there a mystical art to keeping the oil temps within limits thus avoiding the phenomena of 'gulping' or 'coring' which would very quickly cook the oil if your dexterity with the oil cooler shutters wasn't all it might be!
Ah, all those levers!
Cheers,
mcdhu
PS Maybe it is better now!

OzExpat
10th Jul 2002, 12:00
Maybe it really IS better now mcdhu, but what you've cited is a pretty good reason to stay awake... :D Seems to me that there's lots more boredom these days - aside from the occasional moments of stark panic, of course!

In between those times, I don't even have "PPRuNe in Cruise"... :(

100BMEP
26th Jul 2002, 23:51
PigBoat:
Not to mention the joys of starting said cold soaked oil/diluted engine :p

N2
27th Jul 2002, 01:24
I believed as mentioned here, warm up times have
different meanings for different engines. For the
Trent engines, Rolls is a bit concerned about the
oil system (heat exchangers ect) when the oil is
excessively cold and viscous requiring a 50 deg C
oil temp prior to higher power. Their counterparts at General Electric are not quite so concerned,
-10 deg C for the CFM(if memory serves me right).
But have a look at the oil pressures involved,
normal idle pressure for the Trent is around 70
psi where for the GE (CFM 56, CF6-50, CF6-80)
its' around 20 psi. Viscous oil will have more of
an impact on the Trent when advancing power.

The other reason for warm up more common to all
engines, is to allow the turbine section to
thermally stabilize prior to high power. A good
example at the moment is the CFM56 on the A340,
a great engine but it works hard to get airborne.
For CFM's getting tired, EGT margin becomes
critical and a good warm up period can help
prolong life on wing.

N2

pigboat
29th Jul 2002, 03:35
Yeah BMEP, particularly if Herman Nelson was on strike. Blow pot time!:p

Johnman
29th Mar 2004, 22:34
EGT margin, what are the acceptable limits? Are you provided with the data and the OATL?