According to the FAA's 2008 Aeronautical Information Manual Chapter 6
Section 3, Mayday is to be used for Distress situations, and Pan-Pan
is to be used for Urgency situations: See
http://www.faa.gov/airports_airtraff...6/aim0603.html
for further details.

-John

Chris Reed wrote:
PAN PAN in Europe, which would have the desired effect. I've heard this
is not fully recognised in the US - true or not?