At least here in Canada, the PAN would be used more as a way of breaking in on a congested frequency than as a declaration of emergency.
In 16 years of ATC, I have NEVER heard a PAN call! A pilot will simply announce that he has an emergency, state the nature of it, then what response is required is based upon his statements.
A MAYDAY call is obviously different in that it implies that the aircraft is in dire need of immediate assistance.
To sum up, for example:
- Gear problem:
"We have an unsafe gear indication"
"Are you declaring an emergency?"
"Affirmative"
- In flight fire:
"Mayday, Mayday, Mayday!!!" and so on.
Cheers,
LX