Definition
At a right angle (90 degrees) to a given line, surface, or reference direction.
Plain English
Pointing straight across something at a square angle, like the corner of a book.
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, lift, and turning much easier to follow.
Intuition Check
Perpendicular does not just mean that two lines cross. It means they meet or act at a 90-degree right angle.
Example Sentence 1
The wings of an airplane in level flight are perpendicular to the vertical axis of the aircraft.
Example Sentence 2
On the crosswind leg the airplane flies perpendicular to the runway centerline.