Performance Q.: Vmcg, V1

[OneIn60rule]

I think I'm puzzled because from what I've learned about those two is that you cannot take off if your Vmcg exceeds your V1.

At the same time though on page 62 in the CAP I see that you are advised to make V1 equal Vmcg if V1 is less than Vmcg.

So if I see a question where I'm asked if I can take off with a Vmcg which exceeds my V1 and the answers are>

A) You can take off if you make V1 equal your Vmcg
B) You cannot take off
C) You can always take off
D) Yes if Pigs fly.

Would in this case answer B be more accurate?

[Old Smokey]

Tread very carefully through this minefield, it will depend heavily upon which type of aircraft you refer to and how it was certified.

V1, as you know, is the latest speed at which you can stop, but also the earliest speed at which you can go.

In the rejected takeoff case, most of the run up to V1 will be below Vmcg, and, if a stop becomes necessary the throttles / thrust levers are chopped rapidly (removing any future problems with directional control below Vmcg) and the aircraft stopped.

Beyond V1 (and most operators specify at or beyond V1), rejected takeoff is impossible and you are commited to 'GO'. This may involve considerable acceleration to Vr on the ground, but, if below Vmcg, such operation would be impossible due to insufficient directional control. This is why V1 must be at or above Vmcg (the margin above depends on the certifying authority).

The simple solution is to increase V1 to Vmcg (as you've alluded), and manuals for some aircraft I've flown such as B777 advise to do this. Here comes the tricky bit.........

V1 normally provides for either a safe stop, or a safe continued takeoff. If the V1 is arbitarily raised by a pilot, then his rejected takeoff capability within a restricted field length just went out the window - Can't have this No.1! If the pilot arbitrarily used a lower V1 than certified, and Vmcg was not a problem, rejected takeoff performance would be much improved, but the much longer (slow) acceleration from V1 to Vr and / or V2 would require a Takeoff Distance beyond that available - Can't have this No.2!

So what now? If the Approved Flight Manual advises to raise V1 to Vmcg, the manufacturer has factored this in to the complete calculation package to ensure that, with this pilot modification, both the rejected and the continued takeoff can be accomplished. DO go looking for performance penalties which may apply!

If V1 is below Vmcg, and the Approved Flight Manual makes NO mention of raising V1 to Vmcg, the aircraft is in a 'no-fly' zone and takeoff must not be attempted. If faced with this, your only solution would be to find a lower takeoff weight where V1=Vmcg, and use this as your Maximum Takeoff Weight. Now, both the rejected and the continued takeoff can be accomplished.

I hope that this clears the air.

[Mad (Flt) Scientist]

From a certification aspect, we cannot schedule a V1 which is less than "V1min". One of the constraints on V1min is what we call V1mcg, which is Vmcg plus a little bit to account for pilot reaction time etc. So in that regard you should never be presented with a situation where V1 is less than Vmcg, and you should not attempt such a takeoff as the consequences in the event of engine failure after V1 may be severe.

That would indeed imply that B is correct.

What happens in developing the takeoff speed schedules is that V1 would be adjusted in this case to ensure the Vmcg requirement is met, and the other takeoff speeds also adjusted as required so A is the action which the performance specialist takes to allow a takeoff. So A isn't entirely incorrect either.

If I were going to be pedantic, C is also correct, since it uses "can" - you can always try to takeoff - doesn't make it sensible or legal, of course...:)

For a pilot I'd expect B to be the expected answer, since A is only part of the story...there's a bunch of other stuff you'd also need to do if you changed V1.

[Keith.Williams.]

If your purpose in asking this question is related to preparation for the JAR ATPL Performance exam, then the answer is option A. This is the instruction given in the CAP 698 which was produced by the CAA for this exam. The real world may well be different in some respects but the exam is based on JAAWORLD.