What must be adjusted for an airplane to maintain altitude while airspeed is decreasing?

To maintain altitude while airspeed is decreasing, it is essential to increase the angle of attack to compensate for the decrease in lift produced by the wings. As the airplane slows down, the airflow over the wings decreases, resulting in a reduced lift. To counteract this loss of lift and keep the airplane at a constant altitude, the pilot must increase the angle of attack.

By increasing the angle of attack, the wings capture more airflow, enhancing lift generation even at lower airspeeds. However, it is important to keep in mind that there is a limit to how much the angle of attack can be increased before reaching the critical angle, which could lead to a stall. Thus, careful adjustment of the angle of attack is crucial in this scenario to maintain altitude.

Other options indicate adjustments that would not be effective or appropriate in this context. For instance, decreasing the angle of attack would further reduce lift, and maintaining a constant angle would not address the loss of lift associated with decreasing airspeed. Therefore, increasing the angle of attack represents the correct approach to ensure the airplane can sustain its altitude despite the lower speed.