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When one chooses a DERATE for takeoff (TO-1 or TO-2 in the case of a B737), does the system actually reduce the available thrust of the engines, or is it computing a N1 that corresponds to the reduced (de-rated) thrust?

Referring to a B737NG for example, does a setting of "TO 89%" produce the same thrust as a TO-1 or TO-2 with the same N1 of 89%?

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    $\begingroup$ Do you want a general answer or a 737 specific answer? $\endgroup$
    – John K
    Jan 27, 2021 at 16:07
  • $\begingroup$ A specific B737 answer would serve fine :-) $\endgroup$
    – Stamatis Vellis
    Jan 27, 2021 at 18:58

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I cant' give you a 737 specific answer, but how DERATE or Flex Thrust is applied generally depends on whether the engine is FADEC or not.

If a non-FADEC, it's just an FMS computed bug target setting on the N1 indicators and you move the thrust levers as required to match N1 to the bug. So it's still the pilot actually setting thrust to a lower computer-provided target than normal (if the airplane has an autothrottle system, it's the autopilot doing that job on behalf of the pilot through thrust lever servos).

If FADEC, where you (or an autothrottle system) just move the thrust levers until they click into a detent of some sort for takeoff and let the FADEC computers directly control the engines to achieve a computed N1 setting, it's the computers controlling the engines in accordance with your thrust rating selection when you ask for TO thrust. So in that case it's the pilot telling the computers what he/she wants, and the computers do the actual work.

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  • $\begingroup$ Understood, this is how I thought it works. But my specific question was and is whether the same N1 setting produces different thrust depending on whether TO (26k), TO-1 (24k derate) or TO-2 (22k derate) has been selected. Forgetting about Assumed Temperature for this example. $\endgroup$
    – Stamatis Vellis
    Jan 27, 2021 at 19:32
  • $\begingroup$ No on a turbofan the N1 value is the primary indication of thrust and if thrust is reduced, the N1 value, the fan being the primary thrust source, will be lower. Turbofans are just turboprops with fixed pitch many-bladed propellers (and even "propeller" reduction gearboxes on the newest ones), and like an O-200 with a fixed pitch prop, fan rpm is the primary power indicator. $\endgroup$
    – John K
    Jan 27, 2021 at 23:07
  • $\begingroup$ The 737 is somewhere in between the non-FADEC and the FADEC cases you mentioned. It has EEC, but no detents for the thrust levers. The autothrottle servos set the exact thrust lever position for you on takeoff, but the pilots could overwrite it by pushing the levers further forwards. $\endgroup$
    – Bianfable
    Jan 28, 2021 at 7:59
  • $\begingroup$ @Bianfable i made some edits in response to your comment but you should perhaps provide that as an answer no? $\endgroup$
    – John K
    Jan 28, 2021 at 13:50
  • $\begingroup$ @JohnK Thanks, that looks fine. I didn't answer because I wasn't really sure what OP wanted to know. I think your first comment actually answers what they wanted to know. $\endgroup$
    – Bianfable
    Jan 28, 2021 at 14:00

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