Bill Clinton, like him or not, is a shrewd fellow. When asked a
question that put his political life at risk, he made damn sure just
what the questioner meant. And the crux rested on "is." Now the more
astute of you will realize that "to be" is a tautology: state of being.
While some would think this a foundational word, a skilled speaker can
imbue it with all sorts of meaning, including confusing its tense.

If you understanding of flight depends on words (think back to your
time as a student), then should we be any less certain of the meaning
of those words since our lives may depend on them? Think of how many
times loose language in the cockpit led to accidents (take off power,
for example)... Why shouldn't equally loose language in textbooks be
just as hazardous?

Remember, it's winter. We can afford to look behind the words while we
wait for the weather to improve. (a norther point of view)