During which flight condition are the four forces in equilibrium?

• A. During unaccelerated flight
• B. During constant-speed climbs
• C. During level turning flight
• D. During deep stall

Answer: (A)

The four forces are in equilibrium only when there is no net force acting on the airplane, meaning no acceleration. That occurs in unaccelerated flight, where lift exactly balances weight and thrust exactly balances drag, so the aircraft flies straight at constant speed. In other flight conditions, such as when the aircraft climbs or descends, turns, or is in a stall, there is acceleration (change in speed or direction), so the forces do not sum to zero. For example, a level turn requires extra lift to provide the centripetal force, so lift and weight are not in a simple 1:1 balance, and the thrust–drag balance may also shift.