Before any individual warning can be studied, the grammar they all share has to be learned: what makes something a warning rather than a caution, which colour and sound belong to which urgency, and what a crew does in the first seconds after a master light comes on. That grammar is this lesson.
The alerting rulebook
Alerting is regulated, not styled: for aeroplanes, CS-25 and its AMC 25.1322 define the categories, criteria and characteristics of alerting systems (CS-29 does the same for helicopters). The rulebook sorts every alert into three categories by the urgency of the response:
a warning — for conditions requiring immediate crew awareness and immediate response;
a caution — for conditions requiring immediate awareness with a response to follow;
an advisory — for conditions requiring awareness where a response may become necessary later.
The categories are assigned by those urgency criteria; beyond them, the rulebook demands characteristics every alert must have: it must get attention reliably, tell the crew unambiguously what condition exists, and never be outranked improperly, the alerts being prioritised so the worst problem speaks first.
The flight warning system
The machinery that enforces the rulebook is the flight warning system (FWS): it collects fault and exceedance signals from across the aircraft, sorts them into the three categories, and annunciates master warning, master caution and advisory. The typical locations put the urgency where the eyes are:
the master warning and master caution lights — on the glareshield, directly in front of each pilot, one pair per side;
the advisories — on the central annunciation display, read when the crew looks rather than demanding that they do.
The three levels
Master warning — annunciator colour red; aural alert continuous, sounding until cancelled, because the condition cannot wait: typical triggers are an engine fire, an excessive cabin altitude, or the continuous horn of an unintended autopilot disconnect, the autopilot lesson’s warning now placed in its hierarchy.
Master caution — annunciator colour amber or yellow; aural alert an attention-getter, a single chime or tone that announces and stops: typical triggers are single-system failures that need action soon rather than now: a hydraulic pump, a generator, a bleed fault, a fuel imbalance.
Advisory — annunciator colour anything other than red, amber, yellow or green (green would say normal, the alert colours would overstate), commonly white or cyan; no aural alert at all: typical scenarios are status conditions the crew should know about without being interrupted: a system in a fallback mode, a minor anomaly logged for the ground.