I am flight mechanic, and I would like to know about this performance limitation:

  1. What is the definition of alternate take off?
  2. Why is the alternate take off temperature limit (ITT) lower than max. cont. ITT?

Engine Operating Limits

  • 1
    $\begingroup$ You need to ask these questions separately $\endgroup$ May 3, 2021 at 14:29
  • 4
    $\begingroup$ @Abdullah, these 2 questions are inexorably linked. $\endgroup$ May 3, 2021 at 14:38
  • $\begingroup$ How should I correct this question ? @MichaelHall $\endgroup$
    – Richard
    May 3, 2021 at 15:50
  • 2
    $\begingroup$ @Richard, I don't think you need to, it is a good question. You may want to clarify whether or not this is for a specific aircraft. I have flown two planes with different versions of the PT6 and I have never taken off at anything other than max power... $\endgroup$ May 3, 2021 at 16:41
  • $\begingroup$ MichaelHall Thank you for your answer. I just suspect and need to understand about this subject because every summer the ITT will reach 825 Of Alternate Take off but still not over max. power ITT. So I need any information to clarify our team, sir. $\endgroup$
    – Richard
    May 4, 2021 at 9:13

1 Answer 1


Alternate Take-off is a reduced power option feature, similar to flex thrust on turbofans. The aircraft manufacturer will provide performance data applicable to the reduced power setting that can be used when circumstances permit. It's all about reducing wear and tear by not running the engine as hard if it's not essential.

The Max Continuous power setting is intended for emergencies (mostly single engine climb, or both engine climb in some marginal situations, or maintaining altitude single engine where you are one engine and high terrain makes it imperative to not descend) and as such is rarely used, so there is a higher limit to allow even a tired engine to make Max Continuous HP in a critical situation.

The reduced AT ITT limit reflects the objective of limiting wear and tear with a power setting used regularly, whereas the MC limit reflect the balls-to-the-wall limit the engine can take for extended periods when the power is desperately needed.

  • 1
    $\begingroup$ Note that the alternate take-off ITT limit is lower than for MCT, but the torque limit is higher. Normally you don't hit both limits at once, but one or the other depending on the weather (mainly temperature), so it might make alternate take-off unavailable at high temperatures. $\endgroup$
    – Jan Hudec
    May 3, 2021 at 17:02
  • 1
    $\begingroup$ Or on a worn out engine that's running out of ITT margin. $\endgroup$
    – John K
    May 3, 2021 at 18:07
  • $\begingroup$ @JohnK Thank you very much, sir. $\endgroup$
    – Richard
    May 4, 2021 at 4:14
  • $\begingroup$ @JanHudec Do you have information that why Alternate T/O torque limit is higher than MCT ? $\endgroup$
    – Richard
    May 4, 2021 at 8:04
  • 1
    $\begingroup$ Specifically the Alternate Takeoff is a 10% derate, based on the numbers in the table. $\endgroup$
    – GWP
    May 4, 2021 at 8:41

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