The Doctor holding an aerial to send a mayday. (TV: The Sea Devils)
"Mayday" was a codeword, used to signal distress.
When the Doctor and Jo Grant became trapped on a sea fort the Doctor improvised a radio transmitter from a receiver to send a mayday. (TV: The Sea Devils)
Supervisor Lowe sends out a Mayday after the Swarm attacks Titan Base. (TV: The Invisible Enemy)
Munro sent out a Mayday call when the Prince Charlie oil rig was attacked by a Skarasen. (TV: Terror of the Zygons)
Discovering that Titan Base was under attack from persons infected by the Swarm, Supervisor Lowe sent out a Mayday distress call. (TV: The Invisible Enemy)