Hi,
Just thought I would add a little story to this thread. Happened back in the late '80's. 0100Z Flying across the desert from Dubai to Cyprus. No VHF then just poor HF.
Had loss of cabin pressure requiring emergency descent to FL140 (MSA), and diversion to CAI.
Tried to get clearance from Jeddah on HF but no reply to Mayday call, so made Mayday on 121.5 advising all traffic that I was making emergency descent. Almost immediately an american voice said " Get off the frequency,you are on guard". Just what I needed!!
A very nice Speedbird pilot explained to him that I was making an emergency transmission and what didn't he understand about the word Mayday repeated three times. He then copied the rest of my message and gave me a Mayday relay to Jeddah.
Just wish that I had got the americans callsign.