UlanBatori wrote:How do you save fuel by using full throttle at takeoff? Violates 2nd law of thermo. Unless they were trying to use the rocket launch equation and use up all the fuel in one big impulse on the ground..
yak herder saar,
depending on the number of engines one buys, one is offered a variety of options to choose from as part of the standard fit for one's engines.
Larger customers always get the better (best) deal in terms of these optional modifications and upgrades that are given to them free as part of their large purchase.
This sometimes includes derating the engine from max power to some other professionally and safety decided value concurred by the manufacturer as well as the regulatory authority.
On a derated engine, max power is lower anyway thus "saving" fuel over another similar engine not derated.
This saves on needless wear and tear as also lesser fuel burn, lower maintenance reflected in the reduced total cost of ownership of the derated engine
Many operators, (who don't have the derating option) by virtue of company training and company mandated operating procedures restrict take off power to about 75% max setting, this is again TLC for the engine because of lower stresses, lower temps and probably more life in terms of engine cycles, maybe TBO, lower fuel burn ityadi. Generally, after about 1500 feet or so of climb, the max power anyway kicks in via the flight management system/auto throttle/FADEC whatever but then again, the air is now cooler at height, so the thrust, as well as fuel burn, is better for the climb to cruising altitude.
This 75% max allowed power is not set in stone but determined by altitude, temp, weight, runway condition, ityadi but 99.99% of the time it will fit the 75% power constraint.
the difference in fuel burn during take off and cruise can vary by more than 300%. So fuel management is a vital issue for these guys.