A metaphor that the user needs to explain.

"Magic words" (real ones, which btw do not exist) are words which, when
uttered, do things that are beyond the laws of physics. They are
incantations of, well, magic. Simply saying the magic words cause
things to happen which ordinarily could not take place.

"Please" and "thank you" are metaphorically "magic words" because by
using those words, people respond differently than when the same
situation is presented, but without these words. They are (of course)
not =really= magical and do not invoke the other world, but because they
are powerful social lubricants, they have been given this metaphorical
moniker.

"Mayday" and "this is an emergency" are also words which elicit a
powerful response when otherwise such a response would not be
forthcoming. While they do not have the power of actual magical
incantations (and thus cannot put fuel in your tank) they will in
general garner much more assistance than would normally be gotten
without them.

"Unable" is similar in this respect. By conveying that the requested
action is not just inconvenient, but either unsafe or impossible, the
other party is more likely to try to find a workaround, if it is possible.

As with all incantations, metaphorical or otherwise, abuse leads to the
wrath of the Dark Side, which may be vented on those later to come.
Therefore, these magic words should be used with care. But they do have
a powerful effect, even if it is not truely magical.

Controllers can use these words too. "Request straight in to runway 5"
(runway 5 is full of men and machines, seventy parked cars, and a large
tent). "Unable runway 5, I can give you left base to runway 35. Report
three miles."

Generally however it is the pilots who will have the more stringent
operations requirements, and will be more likely to use magic words.

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.