Dear Tom,
My job requires me to fly often. I have noticed that the "takeoff time"—the time
elapsed between when takeoff power is first applied and when the jet finally lifts off
—is greater during hot, humid weather. What is the explanation?
William Frederick
Dear William,
Air density is the issue. The wings of an aircraft generate lift that is
proportional to the density of the air through which the craft is flying, as well as its
forward speed. Reduce one and you must increase the other in order to compensate.
When attempting a takeoff in less-dense air, the pilot must bring the craft to a
greater speed to attain lift sufficient to get the craft airborne. This takes more time
on the runway.
Weather conditions that reduce air density are precisely the factors that you have
noticed: heat and humidity. Hot air is less dense than cold air and, although it seems
counter-intuitive, humid air is less dense than dry air.
