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 both.
Plain English
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.
Context Anchor
Seen in performance and energy-management discussions when comparing whether the airplane has more thrust than drag or more drag than thrust.
Why Pilots Care
T-D directly governs whether the airplane can maintain level flight, climb, accelerate, or must descend at a given speed and power setting.
Grounding Statement
On takeoff roll with full power, T is much larger than D, so T — D is strongly positive and the airplane accelerates. In level cruise at a steady speed, T equals D, so T — D is zero and nothing changes.
Intuition Check
Do not read T — D as two separate items listed together. The dash means subtraction: thrust minus drag.
Example Sentence 1
When the instructor reduced power in level flight, T — D went negative and the airplane began to slow.
Example Sentence 2
In level cruise the pilot trims for the speed where T-D equals zero at the chosen power.