Distress, urgency and safety
Distress, urgency and safety calls share the same shape. Only the gravity of the situation differs, and each has its own spoken prefix, spoken three times at the start of the message.
| State | Spoken prefix | Meaning | Priority |
|---|---|---|---|
| Distress | MAYDAY, MAYDAY, MAYDAY | Threatened by serious or imminent danger and requiring immediate assistance. | Absolute priority over all other communications. |
| Urgency | PAN PAN, PAN PAN, PAN PAN | Concerns the safety of the aircraft or a person on board or in sight, but immediate assistance is not required. | Priority over all except distress. |
| Safety | SECURITE, SECURITE, SECURITE | The least urgent state, used mainly by coast stations and marine vessels for navigational or meteorological warnings. | Below distress and urgency. |
The same conditions can be signalled without voice. The distress signal in Morse is SOS, and the urgency signal is the group XXX. A radiotelephony distress or urgency message may also be sent by data link carrying the intent of the word MAYDAY or the words PAN PAN.
PAN PAN MEDICAL, spoken in the French form PANNE PANNE MAY-DEC-CAL, marks a message about a protected medical transport under the 1949 Geneva Convention. MINIMUM FUEL is not an emergency: it tells ATC that the aircraft is now committed to a specific aerodrome and that any further delay could produce an emergency. MAYDAY FUEL is a full distress condition.
Frequencies, transponder and ELT
Distress traffic stays on the frequency it started on until a better option is clear. Alongside voice, the transponder and the ELT carry the emergency to radar and to search and rescue.
| Channel | Use |
|---|---|
| 121.5 and 243 MHz | International VHF and UHF distress frequencies. Not every station keeps a continuous guard on them. |
| 2182 kHz | International distress frequency in the MF band. |
| 500 kHz | International maritime distress frequency. |
| 7700 / 7600 / 7500 | Special SSR codes: emergency, radio communication failure, and unlawful interference. |
If the aircraft is not in direct contact with an ATS unit, the pilot switches the transponder to 7700 with Mode C where available. An ELT, the emergency locator transmitter, is self-contained and self-powered and transmits on 121.5, 243 and 406 MHz. The 406 MHz signal is picked up by the search and rescue satellite network.
If the emergency is resolved, the ELT is turned off and ATC is advised as soon as possible. If the ELT is left off and ATC is not advised, it is assumed that the aircraft has crashed and search planning begins.
The MAYDAY message
The distress message has a fixed order. Time and circumstances permitting, the pilot sends the items in this sequence so that the receiving station can act and, if needed, launch a search.
| Order | Item |
|---|---|
| 1 | MAYDAY, MAYDAY, MAYDAY |
| 2 | Name of the station addressed, when appropriate and time permitting |
| 3 | Callsign |
| 4 | Type of aircraft |
| 5 | Nature of the emergency |
| 6 | Intention of the pilot-in-command |
| 7 | Present or last known position, flight level or altitude, and heading |
| 8 | Any other useful information, for example endurance remaining and persons on board |
An accurate aircraft position can be critical to any later search and rescue action. In addition to the message, the pilot switches the transponder to 7700 and activates the ELT if not in direct contact with an ATS unit.
Handling distress traffic
Once a distress call is heard, the frequency belongs to that traffic. Acknowledgement, silence and termination all follow set forms.
The station addressed acknowledges a distress message by giving the identification of the aircraft in distress three times, its own identification three times, and then ROGER MAYDAY. If the station addressed does not acknowledge, other stations that have heard the message render assistance.
The station in distress, or the station controlling the distress traffic, may impose silence. The phrase STOP TRANSMITTING MAYDAY is reserved for those two stations. It can be addressed to all stations or to one station that is interfering. QRM means interference, and imposing silence deals with the same problem.
Other stations aware of distress traffic must not transmit on that frequency unless the distress is cancelled, the traffic is transferred to another frequency, the controlling station gives permission, or they must render assistance. A station with knowledge of the distress that cannot itself help keeps listening until it is clear that assistance is being provided.
When the aircraft is no longer in distress it transmits a message cancelling distress. The controlling station then ends the distress working and the radio silence with ALL STATIONS, its callsign, and DISTRESS TRAFFIC ENDED, spoken three times if communication is difficult.
Urgency, relay and unlawful interference
Urgency traffic uses the same message content as distress with a different prefix and a lower priority. Any station may relay an emergency for an aircraft that cannot get through.
An urgency condition covers difficulties that compel a pilot to land without requiring immediate assistance. It can be signalled visually by switching the landing lights, or the navigation lights, on and off repeatedly. The PAN PAN message carries the same items as the MAYDAY message: station addressed, callsign, aircraft type, nature of the problem, intention, position and level, and other useful information. Urgency communications have priority over everything except distress.
A relayed emergency message is one that any aeronautical station or aircraft may transmit when it is needed to get help for an aircraft in distress or urgency. The relay must make clear that the station transmitting it is not itself in distress, for example MAYDAY, [addressee], [callsign] INTERCEPTED DISTRESS CALL FROM [aircraft], POSITION, LEVEL, HEADING.
For unlawful interference the pilot squawks 7500 and, if able, reports the situation. If circumstances prevent a report, the pilot acts consistently with the situation and the published contingency procedures. The aerodrome emergency plan coordinates the fire, medical and outside services that a landing aircraft may need.