Macsboss The OP came here looking for arguments to challenge that Vmcg answer. Some folks above share the concern, namely people who had seen the inside of performance software and/or earn their living by flying aircraft on the wrong side of V-speeds.
Originally Posted by
vilas
My answer was to post #6 by FD.
And thanks for that.
I was looking for a similarity:
V2 is a chosen speed. When using paper charts
on my type the
printed figures observe all the limits. Once case you start applying corrections for non-standard influences (QNH, bleeds, MEL) it is possible the manually adjusted V2 becomes too low invalid. In such a case, the instructions say to disregard the whole FLEX(=ATM) thing and
use the TOGA paper charts. Which is funny for us the nerdy types.
- V2 is limited on the low side by Vmca(controllability) or Vmu(airframe geometry),
- for FLEX(ATM), it is a regulatory requirement to account for full TOGA when calculating the limiting parameters
==> If you cannot FLEX for breaching a low speed limit, how does using TOGA improve your odds? In real life, it would worsen them significantly in the controllability area. But on the inside, the calculation uses TOGA on both accounts anyways. Is actually using it for the take-off better?
I think it is not, but you get a certified way out like that. Constrained by the already printed numbers, there is no other option.
The manually adjusted values from the paper FLEX table provided an invalid V2. The TOGA tables will give a good one, so off you go. If there were a chance to print the paper FLEX charts again, forcing a calculation with all the restrictions so that no manual adjustments to the speeds are required, there would be a valid V2 making FLEX possible.
Speculation towards the OP's dilemma:
V1 is also a chosen speed. If the calculation for FLEX perhaps hit the limit of Vmcg in an iterative manner, could it be the SW defaulted into TOGA results (similar to the fixed printed scenario above) instead of re-doing the FLEX values properly? It sounds like a far shot...
Gut feeling what happened: The TOGA displayed was for a De-rated result.
The goal of crunching TKOF performance numbers is enabling a departure with the lowest permissible thrust setting. On a certain number of occasions - when Vmca/g limited - a De-TOGA
will give you better (lower N1) value, because it is calculated using reduced thrust for Vmca/g (and all other) purposes. Whereas FLEX needs to absorb the full adverse kick from the live engines. My argument here is that FLEX
was possible, but the SW gave a
de-rated TOGA showing a more desirable result.
Question (general): Can Vmcg sometimes be actually
higher than Vmca?
My understanding is that both "G" and "A" are calculated using aerodynamical means only (no pavement gear friction for "G"), the difference being
G: gear down, level wings
A: gear up, 5° helpful bank permitted.
How close / apart are the values in real life? What about L/G down ferry, or aircraft versions with a short fuselage and strong engines?