Plain English Meaning

This formula tells you how much spare power the airplane has at any moment. If thrust is bigger than drag, you have extra power to climb or speed up. If drag is bigger than thrust, you're running an energy deficit and must give up altitude or speed.

Full Definition

The specific excess power equation. PS is specific excess power, T is thrust, D is drag, V is true airspeed, and W is weight. The equation expresses how much power, per unit of weight, is available to either climb, accelerate, or both. When thrust equals drag, PS is zero and the airplane can only maintain its current speed and altitude. When thrust exceeds drag, PS is positive and energy can be added (climb, accelerate, or both). When drag exceeds thrust, PS is and energy must be lost (descend, , or both).