Hey
Pugilistic Animus, great to see your name in the lights again. Stand by for E-mail.
John_T, likewise, pardon my rudeness in not responding to your recent PM, stand by for incoming.
Fredairstair, to add to
John_T's wise words, actually your question in many ways summarises what this entire thread is all about.
"
Can anyone tell me why in the engine failure case a fixed derate is considered a limitation whereas an assumed temp reduction is not? (Boeing FCTM)"
The Derated Thrust
IS a limitation. V1, Vr, and V2 are based upon Vmcg and Vmca (as appropriate). V1 must not be less than Vmcg, Vr must not be less than 1.05 X Vmca, and V2 must not be less than 1.1 X Vmca. Any increase in thrust beyond the Rated Thrust used to calculate the V speeds leads to higher Vmcg / Vmca, and the entire V1/Vr/V2 package is out the window, and the performance is invalidated. Thus, when using V speeds based upon a Reduced Thrust Rating, it is an absolute limit....end of story.
Now we come to Reduced Thrust (Assumed Temp / Flex). Reduced Thrust is calculated in accordance with the Thrust Rating being used, be it Full Thrust, or any of the Derated Thrusts available. Although the actual thrust when using reduced thrust is less than the Rated Thrust being used (the limit thrust for the particular operation), Vmcg and Vmca are still based upon that for the Rated Thrust from which the Reduced Thrust is derived. There is NO CREDIT for reduction in Vmcg / Vmca at the Reduced Thrust, thus, if you wished to, you could push the thrust to the Rated Limit in use with safety. You most definately should not exceed the Rated Thrust in use, as a whole new set of VMCs come into play.
If numbers make a more convincing argument, consider the following V1 speeds (AS LIMITED BY MCG) for the B777-200ER/Trent892 engines. (admittedly these speeds are for low weights, but low weights are the VMC danger zone).
For 30°C/86°F at Sea level, comparing V1 speeds for Full Takeoff Thrust, 8% Derate (TO-1), and 20% Derate (TO-2).
TO : V1(MCG) = 122 Kt
TO-1 : V1(MCG) = 117 Kt
TO-2 : V1(MCG) = 109 Kt
So let's say that I'm doing a low weight Takeoff using TO-2 and the V1(MCG) of 109 Kt. An engine fails right at the 109 Kt V1, and I GO. If I leave the thrust alone at no more than TO-2, I'll have a good day
. If I advance the thrust to Full TO at my V1 speed of 109 Kt, I'm a full 13 knots below VMCG, my rudder effectiveness is only 80% of that required, directional control would be totally lost, and I'm going to crash.
Do the numbers and the words require any further development? I don't think so.
A FIXED DERATE IS A LIMITATION.
Regards,
Old Smokey