alexious85
At LHR we anticipate the duration of the reaction time and take off roll.
For a two minute wake turbulence separation, we time the rotation of the first aircraft, add two minutes to that time, and then take away what we anticipate will be the time taken from issuing the take off clearance to the rotation of the follower aircraft (45-50s for a Medium, 60-65s for a Heavy).
So we would give take off clearance 1:15 after the lead aircraft rotates, in an effort to achieve 2:00 at rotation.