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V2min calculation when flap data not entered

Started by Hardy Heinlin, Thu, 16 May 2019 02:37

Hardy Heinlin

Good morning,

the FMC can compute V1min and VRmin when it knows the pressure altitude and OAT.
But to compute V2min it also needs to know the planned flap setting (and gross weight).

A question for the astronauts:

When the gross weight data is in big font and the flap data is boxed ...

(A) The FMC calculates V2min for flap 20 (lower V2min value).
(B) The FMC calculates V2min for flap 10 (higher V2min value).
(C) The FMC allows any V2 entry down to 100 kt.
(D) The V2 line is not dashed but completely blank and doesn't allow any entry.


A question for the training captains:

What is worse? When the miscalculated V2 is too low -- or too high?

Re (A) - When an engine fails and the V2 bug is set too low, you may stall.
Re (B) - When an engine fails and the V2 bug is set too high, you may reduce your climb rate and hit an obstacle.

In other words, it's hard to determine the safer side when the FMC has to guess the flap setting.

For example, when the fuel reserves data is boxed, the FMC's fuel alert will internally refer to 5 tons.


Regards,

|-|ardy

emerydc8

QuoteA question for the training captains:

What is worse? When the miscalculated V2 is too low -- or too high?

If Vr is correct, you aren't really paying much attention to V2 during the first part of the V1 cut. No one is that good. You are flying pitch attitude exclusively and bringing it up to 12-degrees with the wings level. With an engine out, this pitch puts you right in the ballpark of V2  to V2+10 anyway. Maybe a fine adjustment one way or the other, but you're not going to stall and you're not going to hit anything. You can make the escape turns starting as low as 50' AGL but you can't exceed 15-degrees of bank until you reach V2+10. I would error on the high side versus the low, as long as you have a positive rate of climb. Control of the airplane is better on the high side as well.

Britjet


Hardy Heinlin

Thank you. So I will use flaps 10 as a default reference for V2min when the flap data is boxed.


|-|