Full power or reduced t/o, which saves fuel?

[Carbon Life Form]

Aside from any other considerations with takeoff planning,which power setting do you think saves fuel? <for the entire flight>

[BOING]

The reduced thrust take-off burns more fuel but supposedly saves money overall by reducing maintenence cost over the life of the engine. I suppose that if you keep the engine in "better" condition by using reduced thrust take-offs then the engine will be more efficient in cruise for a longer time so you recover the extra fuel costs from the reduced power take-off.

Interestingly, most of the benefits of reduced thrust on maintenence costs are achieved at a fairly small thrust reduction because the stress on the engine peaks rapidly at t/o power settings. A big thrust reduction does not have a proportional big saving in engine wear.

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[AIRLIFT]

According to the boing flight trainning manual if your objective is to save the maximum amount of fuel for the entier flight (extra long distance to cover) the full thrust T/O and full climb is a better choice. Provinding you are not leveling off until you reach your final cruising altitude. I belive it's because you'll be reaching the said cruising level faster so you'll be burning less fuel. Although I agree with BOING on the long time affect on the engines. The savings from a full thrust would be only profitable for a tight flight fuel wise.

[twistedenginestarter]

I once worked with a bloke who had been at BP studying roof insulation. The amount of enery used to make it is greater than the total lifetime savings in house heating energy.

[static]

Twisted,

Same goes for using your own coffeemug instead of styrofoam.
But then again, we all need to be educated, don`t we? We need to be made more environment conscious. ;)

[bunyip]

For Boing. I made the same statement to a P&W rep about most of the saving being made in the top few percent of thrust reduction, and he said that was true of the older engines but not for the current ones. The later engines have a straight line wear and tear reduction with thrust, thus every bit you can squeeze out will help, right down to minimum EPR.

I also believe that time is important; the less time you can spend at takeoff thrust the less the wear and tear. This means that (if I am right) an early thrust reduction to Climb Thrust (avoid Noise Abatement procedures unless they are required at the particular airport, or use Procedure B) and the use of minimum flap for the runway length available (within reason) will be of benefit. These two techniques will reduce the time spent at high EGT (which is the killer) and so baby the engines.
And since I brought up Noise: A full power takeoff will get you up faster and higher over the noise sensitive area and the noise footprint will be lower.

[jonno]

Just a related question, if I can.
We operate RB211/C2 engines, and use the assumed temp method of calculating reduced power for t/o, we used to use +44C as our top limit, but now are to use +54C,(limited by climb thrust at amb. temp), does anybody else go to this limit, any problems?
We operate the Classic B747, all models.

[quid]

jonno-

And that raises the next question. If there is no engine savings at the higher settings, then why the additional reduction?

I think it is pretty much agreed that there is a slight fuel cost with the lower power setting. Why then, burn more fuel if there isn't a payback somewhere?

[Atlas]

Reduced thrust T/O's and climbs consume extra fuel. Generally with jets, lower RPM = less efficiency. The savings are presumably derived on the maintenance side, with reduced engine wear. One confidently assumes the bean-counters have figured the cost of the extra fuel to be more than offset by increased engine life :rolleyes:

bunyip - I believe that the actual peak EGT attained in the cycle is the most important factor in the possible savings, ie slightly lower temp is beneficial even tho' for a longer time. That would be why there is little engine-condition related benefit to reducing below climb thrust. Same for cold Wxx T/O's, where cruise EGT at altitude may match that at T/O anyway.

quid - Many operator's are now justifying reduced thrust for noise reasons (usually climbing initially with a significant post-T/O thrust reduction). While this does lengthen the footprint, the overall noise level is lowered noticably, presumably to an acceptable level for those additional people exposed to it. Sort of like moving gently thru' the gears on a motorcycle takes it further down the street to top gear than full bore acceleration, but maybe lets the neighbors sleep none-the-less :).

Cheers, Atlas

[CaptainSandL]

Article on the assumed temp thrust reduction is available at

http://www.b737.org.uk/assumedtemp.htm

S & L