In steady-state level flight, thrust must balance which aerodynamic force?

• A. Drag
• B. Weight
• C. Lift
• D. Inertia

Answer: (A)

In steady, level flight the aircraft moves at a constant speed and altitude, so forces along the flight path must balance and produce zero net forward acceleration. The engine’s thrust provides the forward push, and it must counter the aerodynamic drag that resists that forward motion. If thrust matches drag, the airplane maintains its speed; if thrust were less, it would slow down, and if more, it would accelerate forward. Lift and weight balance each other vertically to hold altitude, and inertia isn’t a separate aerodynamic force to be balanced—it's the resistance to changes in motion. So the forward thrust must balance the drag.