I do not fly, not because I know it is very risky and I scare to death but because simply I do not like to (I am a liar!).
Mayday-Mayday (in opposition to Pan-Pan) when broadcast three times from a plane in the emergency frequency 121.5 MHz alerts everybody about the big crash you are about to have.
Fortunately I have not ever needed to use it and if I would ever have to use it I will remember what my instructor used to tell me:
"In spite of wasting your time saying Mayday-Mayday you’d better use that time to pray!"