Which condition results in constant airspeed?

• A. Drag equals weight
• B. Lift equals weight
• C. Thrust equals weight
• D. Thrust equals drag

Answer: (D)

Constant airspeed occurs when there is no net force along the direction of motion. In steady, level flight, thrust acts forward and drag acts backward. When thrust exactly balances drag, the forward and backward forces cancel out, so there’s zero net tangential force and the airspeed remains constant. Lift balancing weight keeps the aircraft at a constant altitude, but it doesn’t directly affect the speed along the flight path. Other options mix vertical forces or pair forces that don’t create a zero net along-path force, so they don’t guarantee constant airspeed.