FAA AIM
6-3-1. Distress and Urgency Communications
a. A pilot who encounters a distress or urgency condition can obtain assistance simply by contacting the air traffic facility or other agency in whose area of responsibility the aircraft is operating, stating the nature of the difficulty, pilot's intentions and assistance desired. Distress and urgency communications procedures are prescribed by the International Civil Aviation Organization (ICAO), however, and have decided advantages over the informal procedure described above.
b. Distress and urgency communications procedures discussed in the following paragraphs relate to the use of air ground voice communications.
c. The initial communication, and if considered necessary, any subsequent transmissions by an aircraft in distress should begin with the signal MAYDAY, preferably repeated three times. The signal PAN-PAN should be used in the same manner for an urgency condition.
d. Distress communications have absolute priority over all other communications, and the word MAYDAY commands radio silence on the frequency in use. Urgency communications have priority over all other communications except distress, and the word PAN-PAN warns other stations not to interfere with urgency transmissions.
CAA CAP 413 Radiotelephony Manual
States of Emergency
1.2.1 The states of emergency are classified as follows:
a)
Distress A condition of being threatened by serious and/or imminent danger and
of requiring immediate assistance.
b) Urgency A condition concerning the safety of an aircraft or other vehicle, or of
some person on board or within sight, but does not require immediate assistance.
The pilot should start the emergency call with the appropriate international RTF prefix
as follows:
a)
Distress ‘MAYDAY, MAYDAY, MAYDAY’
b) Urgency ‘PAN PAN, PAN PAN, PAN PAN’