Definition
The apparent movement of one aircraft as seen from another aircraft, determined by the difference in their flight paths, speeds, and directions. In collision avoidance, an aircraft showing no relative motion against the windscreen — staying in the same spot relative to the canopy — is on a collision course.
Plain English
How another aircraft appears to move when you watch it from your own aircraft. If it seems to stay still in your window while getting larger, you and that aircraft are heading for the same point in space.
Context Anchor
Seen when scanning for traffic outside the airplane and judging whether another aircraft may become a collision threat.
Derivation
From Latin relativus, meaning 'having reference to,' and motio, meaning 'movement.' The motion is described relative to the observer rather than relative to the ground — what the pilot actually sees out the window.
Why Pilots Care
If another aircraft shows no relative motion and remains fixed in your field of view, it is on a collision course.
Analogy
If you walk through a parking lot and a person crosses in front of you, you see them move sideways. If they stay in the same spot in your view while getting closer, you are walking toward each other.
Grounding Statement
In flight, the important question is not just where the other aircraft is, but how its position is changing in your view.
Intuition Check
Do not assume “relative motion” means the other aircraft’s actual path through the sky. Here it means the motion you see from your aircraft, while you are moving too.
Example Sentence 1
During the scan, the instructor pointed out that the traffic at two o'clock was showing no relative motion against the windscreen, and they immediately altered course.
Example Sentence 2
During pattern work, the student learned to watch for any aircraft that showed no relative motion.