This means that if we do our 20% derated takeoff with 25% ATM thrust reduction on top, if we have an engine failure on takeoff we are limited to the maximum derated thrust. In other words, we can remove the assumed temperature reduction to give us a bit more thrust if we desire, but we must not firewall the remaining thrust lever as we may well be below Vmca for the full rated thrust and would very swiftly lose control of the aeroplane. ![]() When we de-rate, we are reducing the maximum possible thrust output and so the Vmc numbers will also change (reduce, because there is less thrust asymmetry for the rudder to overcome). The sticking point is that Vmcg/Vmca are calculated based on the maximum thrust output. ![]() This is clearly much less than 75% of the full rated thrust (with no derate) but this is OK because we are pretending we have strapped on smaller engines. As an example, if we were to apply a 20% de-rate we could then, given sufficient runway etc, apply a further 25% assumed temperature reduction on top of that. ![]() ![]() This distinction is very important because there are Vmc implications.īecause de-rating effectively re-sets the maximum takeoff thrust to a different (lower) value, we can apply an assumed temperature reduction to a derated thrust takeoff. First up to clarify the terminology for anyone unsure:Īssumed Temperature (ATM): Advance the thrust lever a little bit less than full for takeoff, but no less than 75% of the full thrust availableĭerate: Strap on (by means of software) a less powerful engine.
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