A dead radio hands the flight a script, and the script has two authors. Annex 10 writes the communication procedure: which channels to try, and how to keep transmitting into the silence. The rules of the air write the flight procedure: what the aircraft then does with itself, taught in full by the air law course and walked here in its radiotelephony clothes.
The channels to try
Failure to raise the station on the designated channel starts a fixed search: try the previous channel used, then another channel appropriate to the route. If those fail, attempt contact with the appropriate station, any other aeronautical station, or other aircraft, by every available means, advising that contact on the assigned channel could not be established; an aircraft in an HF network also monitors the VHF channel for calls from nearby aircraft relaying on its behalf.
Transmitting blind
When nothing answers, the aircraft assumes it can still be heard: it transmits its message twice on the designated channel, preceded by the phrase TRANSMITTING BLIND, naming the addressee where needed; in network operation the blind transmission goes twice on both primary and secondary channels, with each channel change announced. This is the blind transmission the first lesson defined: two-way cannot be established, but the called station is believed able to receive.
A receiver failure sharpens the procedure. Knowing it cannot hear any reply, the aircraft transmits its reports at the scheduled times or positions on the channel in use, preceded by TRANSMITTING BLIND DUE TO RECEIVER FAILURE, sends each message followed by a complete repetition, and adds two things a deaf aircraft owes the ground: the time of its next intended transmission, and, when under ATC or advisory service, the commander's intentions for the continuation of the flight.
An aircraft whose equipment has failed also tells every radar screen at once: it selects code 7600, the SSR code for radio communication failure. The ground side has its own discipline: other stations and aircraft are asked to relay, but ATC clearances are never transmitted blind to an aircraft, except at the specific request of the clearance's originator.
What the flight then does
With the procedure running, the flight rules fork three ways, and the figure holds the whole script: