Flight Safety Foundation:
"International Civil Aviation Organization procedures for declaring mayday or pan pan eliminate ambiguity about an aircraft in distress or an aircraft in an urgency condition, respectively. Declaring an emergency generates maximum assistance from air traffic controllers worldwide, but delay in declaring an emergency may create confusion or narrow the pilot’s options."
http://www.flightsafety.org/pubs/ao_2000.html
http://www.wingfiles.com/files/comms/aomarapr00.pdf
cheers
