Many moons ago, I, like many other readers of these fora, I am sure, had to learn swathes of the JSP 318. For our non-military and overseas friends, JSP 318 was the RAF's 'Miitary Flying Regulations'.
Of course, it was laden with definitions including those of PAN and MAYDAY, the definitions of both I shall undoubtedly carry with me to the grave.
MAYDAY - 'an aircraft is threatened with serious or imminent danger and is in need of immediate assistance'.
I would suggest that the flight in this scenario fitted the bill to the letter and a MAYDAY, irrespective of your location around the globe, would be an unambiguous call to make.