“Declaring an emergency” is US specific. The rest of the world doesn’t recognise this phrase you have to use either “Mayday” for immediate danger to life or “Pan Pan” for less severe problems. As I understand it, “Declaring an emergency” is the equivalent of a Mayday and the US is very slowly switching away from this non standard phraseology.

"Declaring an emergency" isn't even correct in the US. The FAR/AIM only lists "mayday" and "pan pan". (Yes, it's oddly common here in the US for some reason. I think some pilots are afraid to say the "m" word.)

That said, controllers are human. As long as you get the message across somehow they'll do everything they can to help, even with non-standard phraseology.

Of course, the risk of non-standard phraseology is that you might be misunderstood - especially in countries where English isn't the primary language. It's still good to stick to standard ICAO phraseology whenever possible.

For sure, I think in an English speaking country you would probably get your point across. They would most likely come back with asking for confirmation that you are declaring 'mayday', but you would probably cause varying degrees of confusion in many other countries.

I suppose the aim is to make it absolutely clear as quickly as possible that you want ATC to press the big red button labeled 'crash' which sets off the alarms in the airport fire station and causes other controllers to start diverting flights away and telling planes on approach to go around.

These two podcast episodes about the last major crash at Heathrow are pretty interesting if you want to know what happens in ATC during something like this. The fire appliances were already en route before the plane hit the ground. http://airlinepilotguy.com/adam-spink-and-speedbird-38-part-... http://airlinepilotguy.com/adam-spink-and-speedbird-38-part-...