Slow flight  Slow flight is a portion of an airplane's performance envelope above the speed at which the plane will stall, but below the aircraft's endurance speed. This part of the performance chart is also known as "the back side of the power curve" because when flying in this area, "more power is required to fly at a speed lower than the minimum drag speed" and still maintain straight and level flight. A large angle of attack is required in order to maintain the altitude of the aircraft. At such low speeds, aircraft flight control surfaces begin to lose their effectiveness. Ailerons, in particular, are susceptible. The rudder remains the most efficient flight surface and the adverse effect of yaw which alters bank angle is useful for altering the direction of the aircraft without the need for aileron inputs. If the ailerons are used excessively it is probable that one wing will stall (due to the increased angle of attack of the wing with the downward aileron) and send the aircraft into a spin. In modern aircraft, flight envelope protection in the aircraft flight control system prevents a pilot from controlling their aircraft into doing this.