For an airplane to maintain lift at higher altitudes, what must occur?

To maintain lift at higher altitudes, the airplane must achieve a higher true airspeed for any given angle of attack. As altitude increases, the air density decreases, which directly impacts the generation of lift. Lift is created by the flow of air over the wings and is proportional to the air density, the square of the true airspeed, and the wing area.

At higher altitudes, since the air is thinner (less dense), the airplane must increase its speed through the air to ensure that the lift generated by the wings is sufficient to counteract its weight. This means that for the same angle of attack, which is the angle between the wing's chord line and the oncoming air, the true airspeed must be higher to produce enough lift.

By understanding the relationship between air density, speed, and lift, it becomes clear why increasing true airspeed is necessary to maintain altitude in thin air. As such, maintaining lift in these conditions is not simply a matter of adjusting angle of attack; the increased speed is crucial for generating the lift required to keep the airplane aloft.