A Mayday radio call should be reserved for life threatening situations. These may include, but are not limited to:
◾Loss, or imminent loss of aircraft control for any number of different reasons
aircraft upset by turbulence;
pilot incapacitation;
spatial disorientation;
control surface or structural failure;
engine failure that will lead to a forced landing/ditching/ejection/bailout;
I've always wondered exactly what ATC was supposed to do, or were capable of doing to assist the crew or fix the situation? They're not going to join the crew, after all. So, unless the pilot needs traffic priority or the fire brigade, what's the point?
GF