I read an article earlier today which referenced mayday and pan pan. It described which term was used in different scenarios. I've never heard of pan pan and got to thinking about the origins of these terms. Where did mayday and pan pan originate?
They're both based on French words: mayday is from "m'aider", which means "help me" and panpan is from "panne" which means "breakdown".
http://www.itu.int/dms_pub/itu-s/oth/02/01/S02010000144002PDFE.pdf page 81 contains the description and source of "mayday". I don't have a source for "panne" right now, but I'll try and dig one up.