That is all well and good in an English speaking environment, with everybody focused on your particular problem. However it does rely on the nuances of language, which are less honed in non native English speaking environments.
If
Avianca flight 052 had reverted to standard international (and this was an international flight) terminology in order to convey the deteriorating nature of his fuel situation, that disaster might well have been avoided. Words such as "priority" often become subject to translative interpretation.
An American pilot flying in the UK or a UK pilot flying in the USA, will readily tell you how very different are the R/T procedures and realities that they experience in the other environment. Without doubt, the USA has a massive domestic air transport structure, and it has clearly evolved in a manner that makes only minimal consideration for the wider global theatre. The UK is a small island surrounded by many other nations with an enormous variety of different native languages. In this environment the vital necessity of standardisation and adherence to more defined protocols has evolved.
The use of emergency R/T prefixes, should minimize the confusion that often results from the nuance of intonement or the use of language that may not convey the same imperative, when being used by pilots or controllers for whom English is not a first language. Even as in the UK/USA example, the speed, accent, subtlety and nuance of a (basically the same) language, has the ability to cause much confusion when applied to the alien culture.
Many pilots will adapt and change as they cross 30 West. UK pilots flying West will often adopt the clipped and rapid style of their hosts, often quite badly. conversely many US pilots flying East will slow down their more common "auctioneers patter" to a pace that sounds like the "batteries are running out!"
In theory, the use of a
mayday call should convey the same imperative in London as it does in L.A or Lagos, Libreville, Lima, or anywhere else. It may be less customary or more unusual in the USA, but if anbody uses it, or indeed had Avianca used it approaching JFK that unpleasant evening, I doubt the imperative would have been lost on the receiving controllers.
An emergency isn't about your ability to accurately convey the correct choice of prefix. It is about dealing with the emergency! Nevertheless communication at many levels will play a vital part in the sucessful outcome of that emergency. As the necessity to communicate with ATC and possibly others over the radio fits into the priority, the use of either of these phrases will start the application of the appropriate procedures. Time may not allow an accurate assesment of the emergency, and the situation can be upgraded, or downgraded as necessary. It is also worth remembering that individuals will revert back to what is instinctive (albeit modified by training or reinforcement) when confronted by situations of high stress. In these situations the communication of an emergency should be simple and standardised.
I am making no criticism of what works in the USA, or of what has evolved into standard practice. But given the global nature of the industry, it is important that procedures generally, and certainly emergencies, are likely to be properly understood and appreciated by those involved, wherever they occur in the world.