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If I have read and understood the question correctly, disregarding Transponder codes (hijack situations), there are two "basic" emergency calls, firstly "MAYDAY" used for serious emergencies concerning the safety of the aircraft/crew/passengers. A MAYDAY call will give that aircraft priority over all others with the specific Air Traffic Control Authority. Secondly "PAN PAN" which is the emergency code for less grave or serious problems. A "PAN" call will take precedence over everything but a "MAYDAY"!
