Effect of wind on takeoff performance

[falcon71]

I'm still trying to make sense of the takeoff performance figures. Let's take it step by step and only look at the effect of wind for now.

This image assumes a tailwind of 5 knots:

And this image is under the same conditions, but with a headwind of 5 knots:

I would expect engine thrust to have an effect on V2, but not the wind? Why does the wind have such an effect on V2 while the thrust and flaps stay the same?

Since the flaps, the weight and the thrust settings are the same, I'm curious why the wind affects VR? I would have expected VR to stay the same in these conditions.

It does make absolute sense, the V1 is very much influenced by the wind, since it changes the GS. It also make sense that it is higher with a headwind. However, since VR was already possible at 157 knots in a tailwind, I would have assumed that V1 would also remain at 157 knots, since it must be less than VR. Is there a bug, that the higher V1 pushes VR instead?

 

[CF104]

7 minutes ago, falcon71 said:

I'm still trying to make sense of the takeoff performance figures. Let's take it step by step and only look at the effect of wind for now.

This image assumes a tailwind of 5 knots:

And this image is under the same conditions, but with a headwind of 5 knots:

I would expect engine thrust to have an effect on V2, but not the wind? Why does the wind have such an effect on V2 while the thrust and flaps stay the same?

Since the flaps, the weight and the thrust settings are the same, I'm curious why the wind affects VR? I would have expected VR to stay the same in these conditions.

It does make absolute sense, the V1 is very much influenced by the wind, since it changes the GS. It also make sense that it is higher with a headwind. However, since VR was already possible at 157 knots in a tailwind, I would have assumed that V1 would also remain at 157 knots, since it must be less than VR. Is there a bug, that the higher V1 pushes VR instead?

 

Wind will affect the V1 speed along with available runway length. This is due to runway stopping distance. With a headwind the V1 can be higher as your stopping distance required will be less. Conversely with a tailwind your V1 can be less as your stopping distance required will be longer. V1 can never be higher than VR so if the calculated V1 with the tailwind is 157, the VR can never be lower than this. VR will always be greater or equal to V1. V2 can be influenced based on VR although it's basic calculation is thrust, flap setting and weight. Per the A300 flight training manual "VR = Speed at which rotation is initiated to reach V2 at an altitude of 35 feet by the end of the runway." In the headwind example the V1/VR is delayed and therefore the V2 is likely to be higher as the A300 needs to get to the required 35 foot safety altitude while at the same time it's accelerating. In the tailwind example with the lower V1/VR the V2 is lower as well as it will reach the 35 foot safety altitude at a lower airspeed. The one thing you don't see is the absolute V2 number based only on thrust, flaps and weight. One can only surmise from these calculations in your screenshots that V2 is no less than 162 knots. 

Cheers,

John (A300 Mechanic AME)

[falcon71]

Thank you very much for you explanation. I ran a few more tests and I think I understand it now.

This example is the same as the headwind example, but with the runway length reduced to 3000 meters.

What put me off was that, if for flaps 15/0, flex 58° a rotation at a 3000 meter runway was possible at 158 knots, then it should surely be possible at the same speed for a 4000 meter runway as well. I assume the iniBuilds takeoff calculator uses up as much runway as possible to gain as much speed as possible, which then results in a higher climb gradient?

If this is the case, then it would very well explain why a head- and tailwind have such large effects that I'm seeing, as this is not much different than changing the runway length.

 

[Crabby]

Think of take offs and landings this way...

EVERY takeoff is planned as a rejected take off.  Becoming airborne is just another acceptable outcome of the takeoff roll. 

EVERY landing is planned as a go-around.  Landing and taxiing to the gate/fbo is just another acceptable outcome.  

A takeoff performance calculator (or manual calcs) is designed to get you to V1 with enough runway left to safely stop or get to Vr.