Plain English Meaning

It is the leftover push after drag has taken its share. If thrust is bigger than drag, the airplane has spare energy to speed up or climb. If they are equal, nothing changes. If drag is bigger, the airplane is running an energy deficit and must give up speed or altitude.

Full Definition

In the energy-system view of an airplane, T — D is the difference between thrust (T), the forward force produced by the engine and propeller, and drag (D), the rearward force resisting motion through the air. When T — D is positive, surplus energy is available to accelerate the airplane or climb. When T — D is zero, the airplane is in energy balance and holds its current speed and altitude. When T — D is negative, the airplane has an energy deficit and must lose speed, lose altitude, or .