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De-rated power settings question

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De-rated power settings question

15th Feb 2008, 13:11

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De-rated power settings question

I can just not get my head around de-rated take offs.

If you are taking off from a 1c in temperature airport, then you need less power to take off as the air is denser.

If you are taking off from a 38c in temperature airport, then you need more power to take off as the air is less dense.

Therefore by my calculation the rotation of the engine will need to be a lot more to push more air into the compressor to get the necessary thrust in a hotter environment.

So I must be mis-reading things when a de-rated take off is one where the engine power is reduced in hotter climates?!

Help - I am wondering lonely as a confused cloud...
15th Feb 2008, 18:19

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The engine power rate is related always to the ISA temperature. Temperatures below ISA will give you max power. From ISA to ISA+40º engine performances are gradually degraded because of EGT limits, that is to say "You can not use full power" then you use reduced or derated power always function of the EGT MAX.
Considere the same air mass. As its temperature increases you may add less temperature (=fuel burned) to reach max EGT.
Aerodynamics works the same with ISA. So, when you TO below ISA to ISA you can always use full TO power and have more lift, resulting on LESS RWY needed for a given TO weight. If you adjust your performances to the full lenght of the RWY and the obstacles at the TO path then you will use less power, less EGT and more rwy.
15th Feb 2008, 18:25

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de-rated take off is one where the engine power is reduced in hotter climates?
Not quite right, we operate in a summer environment where temperatures of 40-50C exist. We dont reduce the engine power because of these temperatures, in fact we generally seek the highest installed engine power available.

However, when the takeoff weight is not limited, we have the ability to reduce takeoff thrust, we do this by fooling the engine into thinking that the outside air temperature its hotter than it actually is, ie, its 45c and we tell the engine that its 50c, the power delivered is therefore less than it would be at 45c... hence reduced.

Make sense?

Mutt
15th Feb 2008, 18:30

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Temperatures below ISA will give you max power
What engine type are you talking about? I'm used to getting max rated thrust upto ISA+15.

From ISA to ISA+40º engine performances are gradually degraded because of EGT limits, that is to say "You can not use full power" then you use reduced or derated power always function of the EGT MAX.
Not really correct, if the OAT is 10/20/30/40C, that is still considered FULL POWER, its only reduced if there is a difference between OAT and the temperature used for the takeoff/thrust calculation.

Mutt
15th Feb 2008, 19:46

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Ok I am getting some of this....

one of the limiting factors is the Exhaust Gas Temperature, so putting very hot air into an engine you cannot add as much heat to the air compared to if it was cold - ok got that seems to make sense.

Now this "fooling the engine into thinking..." part... well why does an engine have to be fooled? Surely modern Flight Management Computers you can say "I am at this weight today, which is lighter than usual, the outside temperature is this, so work out how much power/thrust I need to get off the ground..." Why do you have to start putting in false figures to fudge the result so you get what you want?
15th Feb 2008, 23:21

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Hi Andy. To get the idea of de-rate lets forget about FMC and clever software.

First lets think about a modern Gas Turbine.
It has a normal TO thrust.
This thrust lets say is 20000lb.

So the engine is designed such that when the pilot or autothrottle system puts the thrust levers to the TO position the engine accelerates to a speed that deliverers 20000lb.

However if the air temperature is too high(density too low) the engine will reach its maximum operating temperature (TGT, ITT, EGT) before it reaches the speed that equates to 20000lb thrust. The engine is now temperature limited and the maximum thrust it can produce reduces as the air temperature increases(density reduces).

lets say that at all temperatures below and up to ISA (+15 at sea level) our engine can produce 20000lbs but as the temperature becomes higher than ISA the thrust reduces, Say 19000lb at isa +5 (+20) 18000lb at ISA +10 (25).

Today the temperature is below ISA so I will get 20000lb thrust when I set Take Off thrust.
My runway is 10000ft long.

So I calculate that today I can take off with a weight of 35000kg and achieve all the take off requirements.

I find that my actual take off weight today is in fact 30000kg.

I have more thrust than I need to achieve all the requirements. So I can choose to use all 20000lb of thrust and have very very good performance or I can use less than normal TO thrust and just meet all the requirements.

I choose to reduce the thrust to save engine life (money)

How much thrust do I need?

I calculate that at my weight I only need 19000lbs of thrust.

How do I set 19000lbs of thrust? If I put the thrust levers to TO because the temperature is low I will get 20000lb. I could put the thrust levers somewhere below the TO position but the problem with this is it's tricky to set them exactly and it will take time to adjust them to the correct setting.

But I know that if the temperature was isa +5 (20) if the thrust levers are at TO I would get 19000lbs.

So I cheat and tell the engines its ISA +5. So I can put the thrust levers to the TO position and I will get the 19000lb thrust I need.

This way I always use the TO position for take off. And I can use this method to always set just the amount of thrust I need to take off no matter what my weight is.

Notes.

1) The engine has to do a bit of clever calculation because even though I told it the temp was 20, the air entering the engine is 15.

2) There are maximum limits to how much I can reduce thrust so some time I will have a lot more thrust than I need.
16th Feb 2008, 06:27

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I'll have what he's having! FE Hoppy is bang on the mark.

FE Hoppy,

Without a doubt that is one of the easiest and clearest answers to a question that i have seen on the prune yet. I knew what needed to be said(even had a crack at typing a draft) but sometimes getting what's in your head and onto a computer screen for a wordwide audience to read can be a little difficult. Well done. Clear, concise and to the point without going too deep and getting too technical.

I'll just add that the method of de-rating the engines desribed above is called the "assumed temperature" method of de-rate.

Some aircraft/engine combinations have settings for specific de-rate levels. 767 for example. You'll find settings for "de-rate 1" and "de-rate 2" which limit thrust levels by a set percentage. This de-rate can also be used for climb thrust de-rate levels in addition to take-off.

Last edited by Spanner Turner; 16th Feb 2008 at 06:48.
16th Feb 2008, 06:40

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We haven't discussed the differences between de-rated thrust and assumed temperature method.

De-rate is a fixed thrust setting. Eg. T/O, rating 1 and rating 2 thrust levels.
That means if you use a de-rated thrust such as rating 1 or 2, it is almost like strapping on a smaller engine. You then have associated limits for that thrust rating e.g V speeds and maximum thrust.
Some airlines use de-rated T/O, but almost all use de-rate for the climb.

Assumed temperature method (ATM) is a thrust reduction based on not needing a full thrust T/O. Again you trick the engine into thinking that the outside air temperature is hotter than it really is. Therefore you have a lower thrust.

ATM can also be used with de-rates.

People sometimes use de-rate when they acutally mean ATM. Big difference!
16th Feb 2008, 06:51

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oz in dxb,

i was editing my post to touch on the difference between the two methods - you beat me to it.

16th Feb 2008, 15:11

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Ok folks. First of all sorry for my poor English. I would be clearer using Spanish, French or Italian.
All of you have explained it really good and easy.
Mutt, you should read a little more about engine performances. ISA reference temperature is 15º celsius measured at Alicante (a city in Spain at sea level, crossed by the Greenwich meridian) in a special day of the year. The ideal atmosphere considers that temperature is reduced from there 2º celsius each 1000 feet elevation (Aprox).
When Rolls Royce says that an engine gives you a max of 20000 lb. is related to that point in Spain at SL, that special day of the year with 15 degrees Celsius. You can use full power every time and every place. Even at 40º or 80º Celsius. but RR does not guarantee 20000lb. Let say 15000lb. FE Hoppy has explained it easy and clear, better than me. So you are using full power but you are not using full thrust. There it is the difference. Full power but less thrust (reduced due to EGT limits)
The second point I would like to explain is that my friend Andy Rylance is mixing some concepts and I was trying to explain him the differences. He said:
If you are taking off from a 1c in temperature airport, then you need less power to take off as the air is denser.
Less power, only if you considers same atmosphere, elevation, rwy length used, TO weight etc. Less power than if the temperature with the same conditions is 20ºC and you use the same length of rwy.
From another point of view. Same airport, same conditions (TO weight, airplane etc.) and 1ºC (lets suppose ISA) In real life you may use full power (TO pwr) obtaining full thrust. If with the same conditions there is 20º C you will use also full TO pwr but you will not obtain full thrust, even you have seen the same EGT, and you will lift off at a point well beyond the one at 1ºC.
The use of derated power and how it works has been very well explained by my colleges.
Nice flights to everybody.
16th Feb 2008, 18:38

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ATM is also known as flex thrust, because assumed temperature can be adjusted to fit a variety of conditions, runway & TOGW permitting.

Derate is a fixed decrement (-4%, -10%, etc) and separate AFM performance charts are provided giving certified aircraft performance for each derate.

Derate may be used whenever TOGW will allow.

Flex or ATM may NOT be used with contaminated runway.

Here is a prior thread with more detail.
17th Feb 2008, 09:22

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Barit1 you are right. I have been flying B757 and it has 3 options for the TO. TO pwr, Derate, and flexible. TO power states for full power. There are two kinds of derated power TO1 and TO2, reducing TO power by a 12% and a 20% respectively. Flexible is used as usual, regarding on the current rwy performance analysis, introducing the max assumed temperature possible in the FMS.
You also have 3 options for the climb. Max climb power, derated climb1 or climb2 that reduces the climb power by a 6 and 12% respectively.
The A340 has only full TO pwr or flex, but for the climb the ultimate fms has derated1 or d2.
I have used other TO options as CLB power under certain conditions on the CRJ200 and water&glycol injection on the Fairchild metroliner.
17th Feb 2008, 09:41

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spelling

For the 340 you have; de-rate, flex and TOGA (Take-off Go Around) thrust.

De rate is used in very very cold conditions to produce LESS thrust so in case of an engine failure on the ground you can control the swing...so less thrust (slower acceleration) allows for a heavier take off weight. Ask the Finns and the Swedes. Pop VMCG into a search..NB your operator MUST have authority for this.

Flex thrust is thrust corrected for temp. Ie the maximum take off weight at 46 deg is 350 000KG. If we weigh 350 tons on a 25 deg c day we tell the engines to produce power as per a 46 deg c day..accelerate slower but quiet safe.

TOGA if runway is contaminated or required for performance... TOGA take -offs in lightweight big jets are reasonably exciting!
17th Feb 2008, 10:35

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Have a look at the thrust chart for your B757, tell me at what temperature does the thrust start to decrease? ISA or ISA+15C?

What do you understand by the Flat Rating of an engine?

If you are used to A340's, what is Tmin?

Mutt
17th Feb 2008, 11:16

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Mutt, you should read a little more about engine performances. ISA reference temperature is 15º celsius measured at Alicante (a city in Spain at sea level, crossed by the Greenwich meridian) in a special day of the year.

Alicante, Spain
Latitude: 38°16' 55" N
Longitude: 0°33' 29" W
Elevation: 142
Hmmm, before lecturing an aircraft performance specialist about engines, perhaps ppppilot might perfer to get the facts straight....
17th Feb 2008, 12:10

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Most (but not all) modern engines are flat-rated up to approx. ISA+15 - above that temp, thrust is dialed down to protect EGT.

And ISA is a purely arbitrary definition - I don't see why longitude has anything to do with it. It dates back to an early definition of worldwide average SL air temperature (15C or 59F).

BREAKING NEWS!!! think the latest worldwide average temp promulgated is 14.9 C or thereabouts -
17th Feb 2008, 19:12

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Have a look at the thrust chart for your B757, tell me at what temperature does the thrust start to decrease? ISA or ISA+15C?
indeedy, Mutt is on the money, my 757 charts show the engines (rb211s) flat rated until isa (or isa +3 - can't remember - haven't got any performance charts to hand - but certainly not ISA +15)

TOGA if runway is contaminated or required for performance... TOGA take -offs in lightweight big jets are reasonably exciting!
oooh yeah baby - hold onto the tail and enjoy the wild ride.

Windshear reported is a case were we use TOGA - took a 757 with about 10 pax and 10 tons of fuel the other day (70 tons with 39 tons of thrust), - the VSI only goes up to 6000fpm and it was pegged - for quite some time.
17th Feb 2008, 20:21
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ISA reference temperature is 15º celsius measured at Alicante

a chap learns something new every day ...

It (ISA) dates back to an early definition of worldwide average SL air temperature (15C or 59F)

I suspect (from aging memories of reading on the history of the matter) that the stats upon which the ISA is postulated are based on North American profiles dating back quite a few years, now.

It is important to accept that it doesn't make any difference what standard atmosphere you use (and there are a number in common use) so long as everyone talking the same subject are using the same standard atmosphere. In general it makes good sense to base discussions on a standard atmosphere which bears some resemblance to the typical atmosphere of interest but, at the end of the day, it is just a convenient means to the end of reducing performance from a given set of ambient conditions to some (arbitrary) standard conditions for the purpose of discussion and comparison ...
18th Feb 2008, 08:39

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Thank you, thank you, thank you, gracias, merci etc. to all... it has helped a great deal. I was just getting confused but the clear explanations have really helped work my head around this one.

18th Feb 2008, 09:12

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Besides, if you try to put a huge fan and compressor (rigid hub, no variable pitch) rotating at the N1 and N2 and N3 limit speeds in, say, ISA-65 Celsius conditions at Yakutsk or Yellowknife at -50 Celsius and sea level, the dense cold air propelled by the fan might, apart from pushing the plane off the runway in one-engine-out case, break something else. Like generate excess pressure in combustion chamber, or break pylon, wing etc. The engine, pylon and wing strength are fitted to the rated thrust - so if the air is denser than ISA+15, the fan and compressor must be slowed down to keep the thrust within what the engine can endure.

Do turbopropellers also have derate to prevent generation of excessive forces in cold and dense air?