Definition
Movement of an object in only one direction at a time, either purely linear (along a straight line) or purely rotational (around a single axis), without combining the two. It is the basic building block used to describe and analyze more complex motion.
Plain English
Motion that goes in just one way at a time — either straight along a line, or turning around a point — but not both at once.
Context Anchor
Seen in basic aircraft mechanics when describing how a part such as a control rod, cable end, or cylinder rod moves.
Derivation
From Latin simplex, meaning 'single' or 'uncombined.' Here it describes motion that has not been combined with any other motion — a single, clean type of movement.
Why Pilots Care
Understanding simple motion helps a pilot or mechanic picture what a control or mechanical part is supposed to do, which makes abnormal movement easier to notice during inspection or troubleshooting.
Intuition Check
Simple does not mean the movement is unimportant or easy to ignore. Here it means the motion being described is one basic straight-path movement, not a combined or rotating movement.
Example Sentence 1
A piston traveling up and down inside a cylinder is an example of simple motion, because it moves in a straight line only.
Example Sentence 2
During the preflight check the mechanic confirmed the trim tab moved in simple motion without binding.