View Full Version : De-rated thrust take-off

30th Aug 2004, 13:07
Hi guys
I have two contradictory answers for the same question in two books:
Will a reduced thrust take-off result in more or less trip fuel burn?
One says more because you don't get as fast to the altitude of your best SFC. The other one (which I agree with) says less trip fuel because you burn less on T-O, and then climb on the normal climb profile.

Thanks for clearing things up.

30th Aug 2004, 14:18
As to Reduced Climb thurst,it is mentioned in my AOM(B737-500)
Reduced Climb thrust get better fuel mileages,but reduce its ENG
life.So I guess that it is applied to Reduced T/O thrust as well.

30th Aug 2004, 14:31
You got it slightly backwards but you knew what you meant to say.

A reduced thrust takeoff BURNS extra fuel (longer to accelerate, less efficiency of the engine at lower thrust) in exchange for dramatically extended engine life.

Max thrust saves gas, but turns your engine into a pile of rubble in no time at all. As the engine degrades efficiency will go down and fuel consumption will increase (probably where the theory that reduced thrust saves fuel comes from, then engine will stay in pristine most efficient shape longer)

So on a leg by leg basis, just to rehash, reduced thrust burns more fuel. BUt on a const basis over the long haul reduced thrust saves buckets of money.


30th Aug 2004, 16:05
Very interesting topic.

Just one quick question; are there any differences in the fuel consumption and engine life depending on the method of reducing T/O thrust (Derated or Assumed Temperature)


30th Aug 2004, 17:51
The engine doesn't know the difference.

Derate vs Assumed temp...

Derate can be used in conjuction with assumed temp to get even further reductions than would be available to you through assumed temp for takeoff, and then you get further temp reductions and protections throughout the rest of the envelope for flight.

Effectively an aircraft with Derated engines on them has engines that are too large for the aircraft. So its an efficiency loss. HOWEVER, that efficiency loss provides for a much larger thermal margin again extending the life of the engine on the wing and saving maintenance dollars.


30th Aug 2004, 21:25
Totally depends on aircraft type AND sector length.


31st Aug 2004, 03:15
I wrote opposite thing!
Reduced climb use more fuel,but good for the engine life.

31st Aug 2004, 08:33
Will this also apply to turbo-prop aircraft engines?


31st Aug 2004, 16:23

The efficiency of turbine engines goes up the higher the front to back temperature and also with higher pressure at the compressor discharge (both of which go up with increasing RMP and power) In a geared turboprop like the Garret the pressure (RPM would be the same) but the front to back temp would still increase.

Of course it is that temperature at the turbine that wears em out so quick.


Old Smokey
31st Aug 2004, 16:49
I'm in complete agreement with Mutt, it "Totally depends on aircraft type AND sector length".

However, to answer from VERY general principals, I would favour the argument that implies a small fuel saving. Considering the quite short time that Takeoff thrust is applied (about 2 minutes), it really comes down to splitting hairs to find any fuel advantages, although the maintainance savings are enormous and very obvious. So too is the increase in safety commensurate with operating an engine somewhat below it's critical limits.

The optimum Thrust Specific Fuel Consumption (TSFC) engine speed typically occurs in the vicinity of MCR or MCL in the classic "Text Book" aircraft (if such an aircraft exists). The lower limit of reduced thrust allowed is typically in the vicinity of MCL, there is certainly no engine stress reduction advantage in reducing below MCR.

Any variation in engine speed above or below optimum TSFC speed will result in worse than optimum fuel used for each unit of thrust produced. Full Takeoff thrust has relatively poor TSFC, and, if performance permits, operation down to lower engine speeds approaching optimum will yield improved fuel use for achieving Takeoff and the subsequent climb to the point where thrust is reduced to climb. The saving is SMALL, due to the short time involved.

Yossy, You've mentioned that for the B737-500 "Reduced Climb thrust will get better fuel mileages". It would seem that for this engine best TSFC occurs at MCR, and whilst MCL will give an obviously better performance, MCR in combination with optimum range climb speed will give you the best ANMPK, in short a long term cruise-climb. The obvious disadvantage is that at medium and high levels, climb performance is weaker than most pilots like to see. Our company faced a lot of pilot resistance to this when we introduced the A340. Sector length will have a large bearing on this, on a long sector, go for it. On a short sector, shallower climb and longer climb distance will mean being 'stuck' with lower than optimum cruise levels, which may well negate any advantages gained in a fuel economical climb.

Wino, you make good points, every word you say in respect of Derated thrust and Reduced Thrust holds good in respect of fuel inneficiency and prolonged engine life. The other side of the Derate coin is that, in allowing lower V1s, in about 20% of our operations, we can lift greater payload on shorter accelerate-stop limited runways. Thus, when we start talking of economics, whilst reduced engine maintenance costs are advantageous, the benefit of carrying several Tonnes more passengers and freight cannot be ignored, and commercially, negate the inneficiencies of increased fuel use.

POL.777, Thrust is thrust is thrust, no matter how you obtain a particular desired thrust, be it Assumed Temp (Flex) against Full thrust or Derated thrust, Wino's arguments about maintenance costs, reduced efficiency, increased safety hold good. Bear in mind though, my remarks regarding commercial advantage, where we might invent a new term in lieu of TSFC - PSFC - Payload Specific Fuel Consumption.

ECJ, Reduced Power / Torque will certainly reduce engine fatigue, and therefore maintenance costs, but fuel specifics will really depend on the type of engine. You will see very different fuel specifics for constant speed engines, free turbine engines, variable speed engines, or God forbid!, does it use Water Methanol. I did exhaustive performance analysis and flight testing on a constant speed turbine aircraft about 20 years ago, and the results, apart from reduced maintainance costs, were inconclusive.

As a footnote to the unconvinced, I once worked for an airline that strongly resisted reduced thrust on the principal of reduced safety due to reduced performance. The opposition airline, using the same aircraft and engines, heavily and strenuously advocated it's use. Our rate of engine failure and unscheduled engine change was 2.4 times that of the opposition. I rest my case.

P.S. Sorry Wino, I think we were posting at the same time. I think we said the same thing anyway.



24th Sep 2004, 21:03
hello everybody,

derating an engine & assumed temp method are effectively used to save on engine life duration, so you save money.

it all boils down at finding the lowest allowable t/o rpm=t/o n1, which saves the most wear & tear on the engines, which equals to money in the bank.

leaving the engine at 23.5k(b734) & assuming a very high temp approx. 60c for example will most probably produce a lower t/o n1 than derating & then assuming a possible temp of only 49c eg.

fuel considerations are of less importance in this case, although you will have a reduced climb thrust & reduced fuel flow as well.