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.
Cheers,
Smokey