Equilibrium in aerodynamics describes a condition when what occurs?

• A. The sum of moments about the center of gravity is zero
• B. The sum of forces is zero
• C. The aircraft maintains zero altitude
• D. The rate of climb is constant

Answer: (B)

Equilibrium in flight means no net external force acts on the airplane, so there is no acceleration. In steady, level flight, the upward lift balances the downward weight, and the forward thrust balances the backward drag, giving a zero resultant force. That zero force condition is what keeps the aircraft from speeding up, slowing down, or changing its translational state.

While keeping the aircraft from rotating also matters for true stability, the essential description of equilibrium in this context is the balance of forces leading to zero net force. The other statements describe specific situations (no altitude change, or a constant rate of climb) that do not define the fundamental force balance.