If the airplane is in straight-and-level flight, which statement best describes the forces?

• A. The airplane is accelerating
• B. Lift is greater than weight
• C. Drag is greater than thrust
• D. The four forces are in balance

Answer: (D)

In straight-and-level flight, the aircraft moves with constant speed in a straight line, so there is no acceleration. That means the forces acting on the airplane must sum to zero. The vertical forces balance with lift equaling weight, and the horizontal forces balance with thrust equaling drag. When these pairs are equal, the overall four forces are in balance, and the airplane maintains altitude and airspeed. If lift exceeded weight, the airplane would climb; if drag exceeded thrust, it would slow down. None of those scenarios occurs in steady, straight-and-level flight, so the balanced-forces condition is the correct description.