How do aircraft stall warning systems handle (or not) asymmetric-stall situations?

Question

Most, if not all, new airplanes are required to have stall warning systems to (as the name should make clear) provide the pilots with a warning when the airplane is about to stall. Most stall warning systems work not by directly detecting the rapid increase in airflow separation near the stall point, but, rather, by measuring the airplane’s angle of attack and giving an alarm if it is greater than some threshold value. However, as the attack angle at which the wing stalls can vary by a great deal, these older stall warning systems have multiple thresholds; for instance, a particular system might sound an alarm at 15º in a clean configuration, but lower the threshold to just 5º if icing is detected (as icing promotes airflow separation, and, thus, lowers the wing’s stall angle), while waiting until 30º if the slats are extended (as leading-edge devices, such as slats, delay airflow separation, and, thus, increase the allowable attack angle).

With all this in mind, how do stall warning systems handle situations in which one wing is stalled, or close to stalling, but the other wing is not?

I can think of a number of scenarios under which this might occur:

• Category 1 - The two wings have different stall angles, such as could result from...

  • Scenario 1: ...asymmetrical icing, or damage to the leading edge of one wing but not the other, which would increase the roughness of the damaged or more-iced wing, thereby increasing the tendency for the airflow to separate from said wing, and, therefore, cause it to stall at a lower attack angle than the undamaged or less-iced wing.

  • Scenario 2: ...asymmetrical leading-edge device (non)deployment, such as from failure during takeoff of the hydraulic system powering one wing’s outboard slats, resulting in the retraction of said slats, which would decrease the stall angle of the wing with non-extended or less-extended leading-edge devices.

• Category 2 - The two wings are at different angles of attack relative to the airflow around them, such as could result from...

  • Scenario 3: ...violent, fine-grained turbulence, which could result in the airspeed of one wing being considerably lower than that of the other wing, thereby causing the horizontal component of the airflow around that wing to be considerably decreased while leaving the vertical component of said airflow unchanged, thus resulting in the lower-airspeed wing encountering the oncoming air at a much steeper angle than the higher-airspeed wing, and, as a result, possibly causing the lower-airspeed wing, but not the higher-airspeed wing, to exceed its stall angle; for instance, if an airplane is flying at an indicated airspeed 30 knots above its stall speed, and is hit by an asymmetric gust that causes the left wing to experience a 45-knot tailwind while the right wing sees a 15-knot headwind, the left wing will be 15 knots below its stall speed at the same time as the right wing is 45 knots above its stall speed, and, therefore, the left wing will stall, but not the right wing.

  • Scenario 4: ...a rapid yaw to one side, possibly as a result of overenthusiastic piloting or a mechanical failure, causing the airspeed of one wing to increase and that of the other to decrease, possibly causing an asymmetric stall via the same mechanism as in scenario 3 (for instance, a sudden rudder hardover would produce an extremely high yaw rate, and, thus, airspeed differential, before the pilot(s) had time to react, which could easily bring one wing, but not the other, below its stall speed).

Answer 1

They don't. The Stall Protection Computer only considers overall body AOA as measured by vanes on the left and right side of the nose. AOA is just one bit of data, and the SPC also is measuring G loads, yaw rates, speed wise acceleration or deceleration, pitch rates, etc. and also getting inputs on flap/slat position etc to allow for all those effects. It computes a solution for presenting the barber pole on the speed tape, and the shaker and pusher (if installed) firing points based on the body AOA and all those other factors, not local wing AOA.

If you yaw a swept wing jet really hard with rudder, it will roll on its back, stalled or not, just from the asymmetric lift with the sweep (full aileron may or may not be enough to counteract it - you learn this the first time you do a V1 cut in training).

In any case you don't normally touch the rudder pedals on a jet except during takeoff and landing or if an engine quits. Yawing the plane also makes the vanes show radically different angles because of the changes in airflow around the nose when skidding or slipping, and you will get some kind of mismatch related error message.

On a jet with heated leading edges, if you had ice on the wings, the SPC will be fooled and can't account for this, and the wing will stall at a lower AOA than the SPC's computed solution and the airplane may depart when the barber pole still shows good margin, and before the shaker goes off. A nasty surprise. Ice on one wing and not the other just means when it goes, before shaker, it will roll really hard.

Answer 2

The pilot handles asymmetric stalls, this is what a pilot is there for. Along with all of the other corner cases that glorified calculators just can't be relied on to handle. A pilot with adequate manual flying practice can easily detect abnormalities in the response of control inputs and (in the USA)they are actively trained for these different configurations and conditions. I can't count the number of stalls and spins I have purposefully initiated during my training, in many different configurations. (flaps no flaps, level, in turns, full thrust, idle thrust, different altitudes) The typical student private pilot will have stalled in at least two configurations by the the third hour of training as part of the curriculum.

A spin is just an asymmetric stall allowed time to fully develop. Suspected icing calls for different configuration and speed range, generally includes reduced angle of attack. The selection of the icing AOA is a pilot decision, in small aircraft this is done simply with different flap settings and minimum speeds, in advanced aircraft the pilot must set the stall calculator its icing mode. In any case pilots have been handling this for 100 years.