Plain English Meaning

Pointing straight across something at a square angle, like the corner of a book.

Full Definition

At a right angle (90 degrees) to a given line, surface, or reference direction.

Context Anchor

Seen in explanations of level turns, especially when describing the direction a force acts compared with the wings or the airplane’s path.

Derivation

From Latin perpendiculum, meaning a plumb line — a weighted string that hangs straight down. A plumb line forms a perfect right angle with level ground, which is why the word came to mean 'at 90 degrees to a reference.'

Why Pilots Care

In a turn, knowing what is perpendicular to what helps a pilot picture where forces are acting, which makes the explanation of bank, , and turning much easier to follow.