In straight and level flight, which statement describes the net force on the aircraft?

• A. The net force equals zero
• B. The net moment equals zero
• C. Lift equals weight
• D. Thrust equals drag

Answer: (A)

In straight and level flight the aircraft moves with constant velocity and altitude, so there’s no translational acceleration. That means the forces acting on the airplane must add up to zero. The four forces—lift upward and weight downward, thrust forward and drag backward—balance each other when lift equals weight and thrust equals drag, producing a zero net force.

This is the direct way to describe the condition of equilibrium for the airplane’s motion. The other statements describe parts of that balance (vertical or horizontal) or the rotational balance (net moment), but they don’t by themselves guarantee there’s no overall translational acceleration. The essence in steady straight-and-level flight is that the net force is zero.