I am not sure about this - does flex takeoff saves fuel or not, considering the thrust reduction altitude to be 500ft?

I know it saves engine life. but not sure about fuel saving?

In most circumstances it does save fuel. Takeoff thrust is typically at N speed above Optimum Thrust Specific Fuel Consumption (TSFC), thus any reduction to or towards Optimum TSFC N will use less fuel per unit of thrust produced.

Even if the thrust reduction is below optimum TSFC N, you should still save fuel even though TSFC is now worsening again, as the rate of TSFC degredation for engine speed increments below optimum is significantly less than for increments above optimum.

A lot of "It all depends" in this one, it all depends upon your specific engine characteristics. The reply above is generic.

A higher thrust rating, from the start of the take off roll to level at cruise allows you to reach more efficient, fuel saving cruise altitudes earlier thus saving fuel on a sector basis.


On flights pushing the maximum range of the Aircraft I fly I always use a full power take off and a climb to altitude using maximum continuous power which equates to climb power in the high 20's anyway.


It did make a difference in our time to climb and saved a few critical hundred pounds of fuel when they were most needed.

Saves wear and tear on the motors. Does not save fuel from takeoff to altitude though.

I used assumed temp when the first level off is 2 to 3 thousand feet. Much less dramatic and even traumatic for the sponsors in the back who make my paychecks possible.

BOEING says it DOES NOT save fuel.
Derate/assume=higher fuel burn.

However not using derate or Ass temp will decrease your engine efficiency after some time therefore INCREASING its overall fuel burn.

However not using derate or Ass temp will decrease your engine efficiency after some time therefore INCREASING its overall fuel burn.

That's if maintenance hasn't fixed/replaced the engine first.

........yes Sir and the cost related to it would decrease that same fuel saving profit..wouldnt it?

Check out "Coming to grips" on performance. Actually there is not much in the numbers in favor of fuel savings on flex take off. In most cases the burn is increased. However the cost saving in terms of saving engine life is significant due to lower thrust take offs.
Cheers

I'm not sure if I read it somewhere. I seemed to remember it burns more fuel rather than saving but like everyone said, the engine wear is the whole point of flex take off.

........yes Sir and the cost related to it would decrease that same fuel saving profit..wouldnt it?

yeah but the question was not about Cost as such. it was about derating/flexing and how that affected fuel consumption.

ie keep the variables the same.. same engines wear same conditions. for a particular flight the fuel consumption would be less ...

It goes without saying that the overall costs would increase with constant use of full rated thrust. That's why most operators reduce takeoff thrust when they can.

InSoMnIaC:That's if maintenance hasn't fixed/replaced the engine first.

Right. But that's someone else's budget, so it doesn't concern you. :rolleyes:

Reduced thrust (derate, flex, assumed) doesn't save fuel, it reduces engine wear only. Using max climb thrust instead of reduced climb thrust wherever possible on the other hand does save fuel. In the case of our 737s it is around 50 to 60 kg of fuel per take off. Doesn't sound like much, but saves well over 5 million $ of fuel a year at current prices.

The assumed temp is normally only used until 1000' in most cases. So I would ASSume that the small amount of time at a lower thrust may save a kilogram or two of fuel. Once into the climb phase, a reduced thrust climb ( nothing to do with Assumed Temp now), will use more total fuel.

I've always believed (or been told) that FLX saved engines, not fuel, and when I really wanted to save fuel, went for TOGA, now that Old Smoky has weighed in, time to rethink my strategy.

FLX has quite a long history for saving engines' lives. I was told about it in the late 1940s by the senior pilot at my first job (as a very new Commercial Licence holder). He used it when flying a Rapide DH89a. He had some 8000 hours on type at the time ( all hand flown, no A/P !).

da facto:However not using derate or Ass temp will decrease your engine efficiency after some time therefore INCREASING its overall fuel burn.

That's the whole point. Reduced thrust >> more fuel burned in the first few minutes, BUT reduced deterioration >> better SFC (lower fuel burn) in cruise.

So it's a tradeoff, but with a big payback.

Longer engine life is the only reason for reduced thrust take offs. It is recorded and used for it's next maintenance cycle. Fuel burn is negligible.

I normally use derated thrust, as per our SOPs. If, for whatever reason, fuel is getting really tight before departure, I remove the derates for TO and CLB as for this particular type it gives the lowest overall fuel burn for the sector at CI=0, assuming an unrestricted climb.

Yes, it produces more engine wear than normal but the other option of returning to stand, shutting down, refuelling and starting up again probably puts more stress on the mechanicals. The delay to the flight and the knock-on costs need to be taken into account as well.

If you're actually at the limit of your payload-range envelope very often, then I'll argue you need better equipment.

BUT - if instead of cancelling Flex, is you just reduce your assumed temp to Flat Rate +5C, you get most of the benefit of thrust reduction, AND still be close to min. trip fuel burn. :)

Old Smokey, you are correct when you say that an engine running at flex thrust will burn less fuel that an engine at maximum thrust, but if the OP was talking about the whole procedure of flex thrust versus maximum thrust, then the answer is that Flex Thrust will burn more fuel to the same altitude due to the longer amount of time to get there.

Mutt

Hi Mutt,

Thanks for the correction, it seems that I'm correct for the 'small picture' of the Takeoff in isolation, but didn't address the 'big picture' of the entire flight profile. I must admit that the OP's reference to 500 ft thrust vreduction altitude had me thinking that after 500 ft both scenarios are the same. No argument from me that a 'Thrust Reduced Climb' will cost more fuel.

Best Regards,

Old Smokey

http://www.srs.aero/wordpress/wp-content/uploads/2010/08/SRS-TSD-005-Rev-0-A380-Flex-Take-Off-Analysis.pdf

From memory, book figures show a 200kg saving to TOC for the 767 at intermediate weights using reduced T/O and clb thrust.
Grey matter is a bit dusty ..... last flew it in 2001.... but sure of figures.

'#19 (permalink)
barit1

Join Date: Feb 2005
Location: flyover country USA
Age: 70
Posts: 3,669

If you're actually at the limit of your payload-range envelope very often, then I'll argue you need better equipment.'


As a mere line Pilot I don't get to select the equipment !

Right. But that's someone else's budget, so it doesn't concern you. :rolleyes:


:} but if the engine blows during take-off, that becomes our (the pilots) major problem:}.