Originally Posted by
topsy1970
I would dare say 30 -45 seconds on average. Most jetliner aircraft become airborne between 240-300km/h. (130-160knots). How fast they get there depends on weight, temperature, altitude, pressure altitude, flap setting and reduced thrust settings.
www.charlesbronson.ca
Those are the big variables. Given the same aircraft and same weight, the time to reach a certain speed would vary based on the density altitude (the pressure altitude) of the airport the aircraft is taking off. The higher and hotter the airport is, the slower and longer the takeoff role because the engines produce less thrust and the wings produce less lift.
A 744 taking off from DTW when it's 25F outside is going to use up alot less runway and time than a 744 departing Denver on a 90F day.
Runways have distance markers along the side, so you can count the number of feet being used during the takeoff roll, count the time and then you can calculate a rough estimate of the speed at rotation.