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I read this also in the FCTM green operating procedures that doing a flex takeoff increases fuel consumption as we have a longer takeoff phase

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  • $\begingroup$ When you ask your instructor about this you may share the answer here :) $\endgroup$
    – vasin1987
    Commented Nov 29, 2017 at 8:51
  • $\begingroup$ I’d change the question to something more along the lines of “how does assumed temperature takeoff affect fuel consumption.” Flex is not the most common term for the concept. The question also asserts that reduced thrust takeoff causes an increase in fuel burn without reference or establishing that as a fact. $\endgroup$
    – ryan1618
    Commented Nov 29, 2017 at 21:10
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    $\begingroup$ I would like to run numbers to be sure (don’t have time at the moment), but I believe in general it’s because flex takeoff doesn’t make most efficient use of fuel when viewed for the whole flight. Assuming standard day, no wind, jet transport aircraft trip fuel is minimised when the optimum cruise altitude (maximum specific range) is obtained as quickly as possible after takeoff. Choosing to use less than maximum thrust will make the aircraft stay lower (and in dirtier configuration) for longer, thus increasing fuel consumption over the theoretical optimum. Comment instead answer, as no proof. $\endgroup$ Commented Nov 29, 2017 at 22:52

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Fuel consumption for the flight will be slightly higher when carrying out a reduced thrust takeoff.

Simply put, the flight is operated at lower, less efficient altitudes for a tiny fraction longer, so less time is spent at more efficient altitudes. It’s a rather small amount, but nonetheless measurable.

From Airbus‘ Getting to Grips with Fuel Economy:

Compared to a full thrust take-off, flex thrust will generally increase fuel burn. The increased time at low level offsets the slight reduction in fuel flow induced by the lower thrust.

The source goes on to list typical fuel burn increases, e.g. for an A320 it’s 1kg (!) when taking off in CONF 1+F and 5kg in CONF 2.

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I am afraid it is just the opposite, even though the take off phase is longer the fuel consumption is reduced. The slope of the delivered power to fuel consumption goes decreasing well before reaching full thrust.

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