From another thread, with regard to the weather in Melbourne yesterday.
JB is Vr increased when operating in this type of weather to provide some sort of buffer against change of widespeed/direction during the takeoff roll?
Windshear on departure is handled by....choosing another runway, delaying/not going, using full power only (no derating allowed). An abort as a response to unacceptable speed variation on the take off roll is quite possible, even likely. The real answer is to not go!
The take off performance applications are somewhat cryptic in the way they vary their results as you change the input conditions. Basically, they can vary more of the factors than we did when using charted data, so numbers don't necessarily relate as you might expect. That means the answers are better, but it also requires a little more thought to ensure that you're happy with them. We'll choose the worst (most tail) wind component for departure calculations.
Using 34 (and some heavy, but not overly so 380 numbers):
Using a 0 knot headwind component (we can input the actual wind (direction and strength) or just a component value). We use both, but pick a worst case wind...the most adverse. Power, V1, Vr, V2, configuration.
Changing just dry or wet... gives a very small change.
Dry: Flex 37, 130, 154, 159, flap 2 Wet: Flex 36, 130, 153, 159, flap 2
Wet, 15 kt headwind component: Flex 39, 137, 161, 167, flap 1
Wet, 15 kt tailwind component: TOGA, 125, 147, 154, flap 3
So, you can see that all sorts of things are being changed with regard to speeds and configuration and power in response to changes in the conditions.
At the approach end of the runway, with windshear being reported, we may change the configuration to use less flap for the landing. The target IAS can also be adjusted upwards by up to 15 knots. Increasing the speed by more than a small value has its own issues. The pitch attitude will be reducing by about a degree for every 10 knots, and the aircraft will be much more inclined to float. The aircraft itself will vary the current speed target as it looks at the difference between the wind currently being experienced, and the wind loaded into the approach performance page of the FMC (the ATIS wind).