In steady unaccelerated flight, the forces acting on the aircraft are in which state?

• A. Equilibrium
• B. Acceleration
• C. Velocity
• D. Momentum

Answer: (A)

In steady unaccelerated flight the forces are in equilibrium. That means the sum of all forces acting on the aircraft is zero, so there is no net force to cause a change in motion. Practically, lift exactly balances weight, and thrust exactly balances drag, keeping the airplane’s speed and altitude constant. The fact that the airplane is moving at a steady speed doesn’t change this balance—the velocity is a consequence of the motion, not the state of force balance. If the forces were not balanced, the aircraft would accelerate, which is not the case here. Momentum is related to motion, but the key point is that the forces are balanced, producing zero net force.