Warning, Caution and Alerting Systems
Stall warning
A stall warning system uses a lift transducer or angle-of-attack vane to trigger a horn, light or stick shaker before the stall. It must be tested before flight. Traps: the warning may be inhibited with flaps retracted on some types, and a stall warning in cruise can indicate a blocked pitot or a faulty AoA sensor rather than an actual stall.
Overspeed warning
An overspeed warning (clacker or aural warning) activates near Vmo/Mmo. Understand that Vmo is indicated airspeed and Mmo is Mach; both may be displayed with barber poles on the PFD.
GPWS/EGPWS
Ground proximity warning systems use radio altitude, barometric altitude, vertical speed, gear and flap status. Modes include excessive descent rate, excessive terrain closure, altitude loss after take-off, unsafe terrain clearance and glideslope deviation. Enhanced GPWS adds a terrain database and GPS position for a forward-looking warning. Traps: a false warning can occur with an incorrect barometric setting or an unusual approach; correct response is to initiate the escape manoeuvre and verify.
TCAS
Traffic alert and collision avoidance system uses transponder replies to display traffic and issue traffic advisories (TA) and resolution advisories (RA). An RA requires immediate response, even if it conflicts with ATC. Traps: TCAS does not provide separation from terrain, and RAs are inhibited below a set altitude.
Master caution/warning
A master warning (red) and master caution (amber) draw attention to the alert system; the specific message is then read and actioned.
Worked rule of thumb
For a GPWS "PULL UP" warning, apply maximum available thrust, wings level, and pitch to the escape attitude - do not delay to diagnose.