Which of the following conditions is necessary for an aircraft to achieve level flight?

For an aircraft to achieve level flight, two key conditions must be met: lift must equal weight and thrust must equal drag.

When the lift generated by the wings equals the weight of the aircraft, the forces acting vertically are balanced. This balance allows the aircraft to maintain a constant altitude. If lift were greater than weight, the aircraft would climb, while if lift were less than weight, it would descend.

Simultaneously, for the aircraft to maintain its horizontal path without accelerating or decelerating, the thrust produced by the engines must equal the drag, which is the resistance the aircraft faces as it moves through the air. If thrust exceeds drag, the aircraft would accelerate forward, and if drag exceeds thrust, the aircraft would decelerate.

Therefore, both conditions—lift equaling weight and thrust equaling drag—are essential for achieving and maintaining level flight.