About 50% of the time i find airports do not activate take off runways into headwind. I checked the AFCAD, and rechecked the ATIS and weather(ASV6) and i can't figure out why.For example..KEWR 212@8, but ATC directs you to take off 4L, instead of 22R. Theres surface wind, and winds aloft, is there another that directing ATC to T/O against Pilots wishes? Any clues or help would be great. thanksKeyno

The FAA and FS use the concept of preferred runways and light and variable winds.Most of the time in FS you need more than 10 kts wind to turn a runway away from a preferred runway direction in FS.In the real world turning an airport like KEWR is a lengthy complicated task and has to be coordinated to ensure changing landing and takeoff patterns don't have a negative impact upon traffic at other airports it the area.The decision of landing / taking off with a light tailwind is made more on traffic patterns at busy airports than the weather.

Similar rules apply in the UK. At Heathrow the limiting tailwind component is 5kt though and applies if the runway is dry.

Ahh that helps in understanding, need 10+ winds to change preferred T/O RWY.thanks