The minimum safe airspeed at which an airplane can stay in flight is called stall speed, which varies with factors such as airplane weight, wing loading and altitude. A stall occurs when the wings of an airplane stop generating enough lift to counteract aerodynamic drag and the weight of the airplane.
When the speed of an airplane drops below the stall speed, the airflow over the wings separates from the surface and becomes turbulent. High airplane weight, banking maneuvers and altitude influence the stall speed.
For example, during banking maneuvers, a force called wing loading can raise the stall speed of an aircraft by increasing the apparent weight of the airplane, which means that the airplane can stall at a higher speed. This requires the generation of more lift to counteract the wing loading in a banking maneuver. In a heavy airplane performing a banking maneuver, the stall speed can be dangerously low, especially in low-speed flight, such as during takeoff, approach and landing.
At increasing altitudes, the density and temperature of the air generally tend to decrease in the troposphere, the region of the atmosphere in which most airplanes fly. Due to the decreasing amount of air molecules available to generate lift with increasing altitude, the airplane can stall at a higher speed than at lower altitudes and higher density. Most aircraft are equipped with a stall warning indicator, such as a stall warning horn or, in the case of commercial aircraft, a stick shaker, that buffets the pilot's controls to warn the pilot of an impending stall.