Stall (flight) - Accelerated and Turning Flight Stall

Accelerated and Turning Flight Stall

An accelerated stall is a stall that occurs while the aircraft is experiencing a load factor higher than 1 (1g), for example while turning or pulling up from a dive. In these conditions, the aircraft stalls at higher speeds than the normal stall speed (which always refers to straight and level flight).

Considering, for example, a banked turn, the lift required is equal to the weight of the aircraft plus extra lift to provide the centripetal force necessary to perform the turn; that is:

where:

= lift

= load factor (greater than 1 in a turn)

= weight of the aircraft

To achieve the extra lift, the lift coefficient, and so the angle of attack, will have to be higher than it would be in straight and level flight at the same speed. Therefore, given that the stall always occurs at the same critical angle of attack, by increasing the load factor (e.g., by tightening the turn) such critical angle - and the stall - will be reached with the airspeed remaining well above the normal stall speed, that is:

where:

= stall speed

= stall speed of the aircraft in straight, level flight

= load factor

The table that follows gives some examples of the relation between the angle of bank and the square root of the load factor. It derives from the trigonometric relation (secant) between and .

bank angle
30°	1.07
45°	1.19
60°	1.41

For example, in a turn with bank angle of 45°, V_st is 19% higher than V_s.

It should be noted that, according to Federal Aviation Administration (FAA) terminology, the above example illustrates a so-called turning flight stall, while the term accelerated is used to indicate an accelerated turning stall only, that is, a turning flight stall where the airspeed decreases at a given rate.

A notable example of air accident involving a low-altitude turning flight stall is the 1994 Fairchild Air Force Base B-52 crash.