1955, near Feltwell,I once heard a fellow (Piston) Provost course trainee call "Switching to 121.5 for Practice Pan"; knowing there was a Royal Flight on at the time, I switched too, just out of interest. The conversation went:
"Practice Pan, Practice Pan,Practice Pan,this is XXXX"
"XXXX this is XXXX Centre, there is a Royal Flight, no Practice Pans permitted"
"XXXX Roger................Practice Mayday, Practice Mayday,Practice Mayday."
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Years later in a Valley Gnat, on TR&G heard a Navy Hunter near Brawdy:
"Mayday, Mayday,Mayday, this is XXXX"
"Mayday aircraft this is XXXX your position is XXXX, what is the nature of your emergency, can you maintain altitude?"
"XXXX this is XXXX, the Mayday's not for me, it's for him".
Turned out to be an ACM exercise & the leader of the pair had ejected]