Wolfgang Schwanke writes:

That's not the point. The point is that an American accent, even
"standard" American without any noticeable regional dialect, can be an
obstacle to foreigners who were taught British English, especially if
their command of the language is not perfect.

This is incorrect. If they are sufficiently fluent in English, the accent
will not be an obstacle. Any accent strong enough and different enough to be
a problem would also be a problem for native English speakers. In this case,
the native speakers understood easily, and so the problem was the incompetence
of the pilot. Rest assured, he wouldn't understand any British accent,
either.