The distress message, element by element
The message consists of as many as possible of these elements, spoken distinctly and, if possible, in this order:
The station addressed — time and circumstances permitting; normally the station already working the flight.
The identification of the aircraft — the call sign, in full.
The nature of the distress condition — what has gone wrong.
The intention of the person in command — what the aircraft is about to do.
Present position, level and heading — where to find it, with level as flight level or altitude as appropriate.
The urgency message uses the same elements in the same order, as many as required, and closes with any other useful information. Around the fixed shape, the annex adds latitude: the distress message may go out on 121.5 MHz or another frequency if that seems better, though not every station keeps a continuous guard on the emergency channel; it may be broadcast; maritime calling frequencies may be used; and the aircraft uses any means at its disposal to attract attention, including the SSR codes: 7700 for emergency, alongside 7600 for communication failure and 7500 for unlawful interference.
Who does what: the distress machinery
The moment a distress message is acknowledged, duties attach to every receiver of it:
The station addressed, or the first station acknowledging, immediately acknowledges, takes control of the communications or clearly transfers that control, telling the aircraft of any transfer; informs the ATS unit and the aircraft's operator; and warns other stations so no routine traffic transfers onto the distress frequency. Informing the operator never outranks an action involving the safety of the flight.
Every other station — distress communications have absolute priority: a station aware of them does not transmit on the frequency unless the distress is cancelled or ended, the traffic has moved to other frequencies, the controlling station gives permission, or it must itself render assistance. A station that cannot help keeps listening until it is evident that assistance is being provided.
Silence — the aircraft in distress, or the station controlling the traffic, and only those two, may impose silence on all stations or on one: "all stations, stop transmitting, MAYDAY".
Traffic stays where it began — the distress traffic remains on the frequency where it started unless better assistance argues for a transfer; one form that transfer can take is a discrete emergency frequency (DEF): a frequency ATC designates for the emergency traffic alone, keeping the working and guard frequencies clear, and at an aerodrome emergency able to link the crew, the fire-fighting incident commander and the tower directly.
The ending is as regulated as the beginning. When the aircraft is no longer in distress, it cancels the distress condition itself; the controlling station, once authorized, ends the communication and the silence with a message including the words DISTRESS TRAFFIC ENDED, and passes the news to the ATS unit and the operator.
The urgency machinery
Urgency runs on the same rails with a lighter touch. The station addressed acknowledges the message, informs the ATS unit and the operator, and exercises control of the communications if necessary, rather than by default. Other stations take care not to interfere: urgency traffic has priority over everything except distress. And one special voice rides the urgency signal: a protected medical transport under the Geneva Conventions announces itself with PAN PAN, preferably three times, followed by the signal MEDICAL, pronounced MAY-DEE-CAL as in the French, the PAN PAN MEDICAL of the first lesson's priority table; then come its identification, position, the number and type of transports, the intended route, and its estimated times.