Stall, Overspeed and Configuration Warnings

The stall warning system

The function of a stall warning system (SWS) is to warn the crew that the wing is approaching the stall, early enough for the recovery to be a small correction rather than an upset. The warning must be unique: among every sound the flight deck can make, this one may be confused with nothing else, because it demands one immediate reflex, reduce the angle of attack, and any time lost identifying the sound is taken straight out of the margin the warning exists to provide.

The types scale with the aircraft:

Light aircraft — use a simple sensor at the leading edge, a vane or flapper switch that the shifting stagnation point flips as the angle of attack rises, sounding a horn;

Transport aircraft — compute the margin from the angle-of-attack sensors module 2 taught and warn through a stick shaker, motors that vibrate the control columns, unmistakable in the hands even in a loud flight deck.

The main components are the same three jobs in every type:

the sensor (vane, flapper or AoA probe).

the judging element (a computer on transports; in the simplest systems the switch itself does the judging).

the warning device (horn or stick shaker).

The two speeds must not be confused: the stall warning speed is deliberately higher than the actual stalling speed: the warning is set to fire with a certified margin above the stall (at least 5 kt or 5% CAS, whichever is greater), so the crew acts while the wing is still flying. At the warning the aircraft is approaching the stall; at the stalling speed it is in it. The margin is the whole point, the aerodynamic story the stall lessons told, now delivered by a system instead of the airframe.

Stall protection

The function of a stall protection system is to go one step further: not to tell the crew but to act, preventing the stall itself when the warning has not been enough. The types follow the flight controls:

on mechanical controls — a stick pusher physically pushes the column forward at the critical angle, forcing the nose down through whatever is holding it up;
on FBW aircraft — the protection lives in the control laws, the angle-of-attack limit the envelope-protection lesson taught: the computers simply refuse to command a stalling angle, whatever the pilot asks for.

The difference between an SWS and stall protection is exactly the difference between the two verbs: the warning system informs and the crew acts; the protection system acts itself.

The overspeed warning

At the other end of the tape, the overspeed warning exists to announce that the aircraft has reached its maximum operating speeds: its visible half is the VMO/MMO pointer, the barber’s pole module 2 put on the speed display, marking the currently limiting speed, VMO low down, MMO at altitude. The examples are the two faces it wears:

the barber-pole pointer on a round dial — riding the rim of a mechanical airspeed indicator;
the barber-pole band on a vertical speed tape — descending from the top of an electronic display.

And the regulation adds a sound: for large aeroplanes with electronic displays, an aural warning must accompany the overspeed warning (AMC 25.11), because a band on a tape can be missed in the very manoeuvre that is breaking the limit, and the aural (traditionally a clacker) cannot.

The take-off warning

The take-off warning system exists for one narrow, unforgiving moment: it warns the crew, as take-off thrust is applied, that the aircraft is not in a configuration safe for take-off (AMC 25.703). The typical triggers are the classic fatal misconfigurations:

flaps or slats — outside the take-off range;
pitch trim — outside the take-off band, the green band of the trim lesson now seen from the warning’s side;
speedbrakes or spoilers — not stowed;
the parking brake — still set.

The logic arms on the ground and checks at thrust application, the last moment the error is easily corrected.

The altitude alert

The altitude alert system guards the selected altitude: its function is to alert the crew when the aircraft approaches the altitude selected on the FCU/MCP, and again if it then deviates from it. The displays and alerts come in the two familiar channels:

a visual alert, a light or a highlighted altitude readout.
an aural chime, typically as the aircraft closes to within roughly a thousand feet of the selected level, so the level-off is prepared.

And both again, more insistently, if the aircraft leaves an altitude it was holding, the deviation nobody commanded. It is the everyday guardian of the clearance: level busts, not crashes, are its business.

The stall warning system

Check you understood

Stall protection

The overspeed warning

The take-off warning

The altitude alert

What the exam asks