Plain English Meaning
All the blades on a spinning rotor or propeller need to follow exactly the same circle as they go around. When they do, the system runs smoothly. When one blade rides a little higher or lower than the others, it causes vibration, and the blades are said to be 'out of track.'
Full Definition
The condition in which all blades of a helicopter rotor or propeller follow the same path through the air as they rotate. When blades are properly tracked, each one passes through the same plane of rotation, producing smooth operation free of vibration.
Context Anchor
Seen during propeller or helicopter rotor maintenance, especially when checking for vibration, bent blades, or incorrect blade adjustment.
Derivation
From 'track' meaning the path something follows. In this use, the 'track' is the circular path each blade traces through the air. Blades 'in track' all follow the same path; blades 'out of track' don't.
Analogy
It is like watching the tips of fan blades as they spin. If one blade tip is not following the same circle as the others, the fan may shake instead of running smoothly.
Intuition Check
Tracking does not mean navigation tracking here. In this maintenance use, it means checking the path followed by rotating blade tips.
Example Sentence 1
After replacing a rotor blade, the mechanic checked the tracking to make sure all three blades were following the same path.
Example Sentence 2
During the ILS approach the crew maintained precise tracking on the localizer by making small heading adjustments.