As altitude increases, what must an aircraft do to maintain the same amount of lift?

To maintain the same amount of lift as altitude increases, an aircraft must fly faster. Lift is generated by the wings and is a function of airspeed, air density, and wing area, as described by the lift equation: ( L = \frac{1}{2} \rho V^2 S C_L ). Here, ( L ) is lift, ( \rho ) is air density, ( V ) is the velocity of the aircraft, ( S ) is the wing area, and ( C_L ) is the coefficient of lift.

As an aircraft climbs to higher altitudes, the air density decreases. This reduction in air density means that for the same amount of lift needed (to keep the aircraft level), the aircraft must compensate by increasing its speed. Flying faster allows the aircraft to generate sufficient lift despite the lower density of the air at higher altitudes.

While changing weight, flying slower, or changing wing shape are considerations in aircraft performance, they do not directly address the immediate need to compensate for lower air density at higher altitudes to maintain lift. Thus, the correct response is to increase speed in order to maintain lift.