What radio signal word indicates an imminent distress condition, and how should it be transmitted?

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.
e. Normally, the station addressed will be the air traffic facility or other agency providing air traffic services,
on the frequency in use at the time. If the pilot is not communicating and receiving services, the station to be called
will normally be the air traffic facility or other agency in whose area of responsibility the aircraft is operating,
on the appropriate assigned frequency. If the station addressed does not respond, or if time or the situation
dictates, the distress or urgency message may be broadcast, or a collect call may be used, addressing "Any Station
(Tower)(Radio)(Radar)."
f. The station addressed should immediately acknowledge a distress or urgency message, provide assistance,
coordinate and direct the activities of assisting facilities, and alert the appropriate search and rescue coordinator
if warranted. Responsibility will be transferred to another station only if better handling will result.
g. All other stations, aircraft and ground, will continue to listen until it is evident that assistance is being
provided. If any station becomes aware that the station being called either has not received a distress or urgency
message, or cannot communicate with the aircraft in difficulty, it will attempt to contact the aircraft and provide
assistance.
h. Although the frequency in use or other frequenc ies assigned by ATC are preferable, the following
emergency frequencies can be used for distress or urgency communications, if necessary or desirable:
121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by
direction finding stations and some military and civil aircraft. 243.0 MHz is guarded by military aircraft. Both
121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, and radar facilities. Normally
ARTCC emergency frequency capability does not extend to radar coverage limits. If an ARTCC does not respond
when called on 121.5 MHz or 243.0 MHz, call the nearest tower.