Definition
Unintended deviations from the assigned or selected heading during straight-and-level flight, typically caused by uncoordinated flight, allowing a wing to drop, or improper rudder use. In instrument flying, heading changes are detected by reference to the heading indicator, magnetic compass, and turn coordinator, and are corrected with coordinated aileron and rudder inputs to return to the desired heading.
Plain English
When the airplane's nose drifts off the direction you were trying to fly, instead of staying pointed steadily on your chosen path.
Context Anchor
Seen during instrument straight-and-level flight, especially when checking whether the airplane is staying on the desired or assigned heading.
Derivation
Heading comes from the idea of the direction the aircraft's head, or nose, is facing. That helps because a heading change is about where the airplane is pointed, not necessarily where it is drifting over the ground.
Why Pilots Care
Unintended heading changes reveal control coordination problems and can cause course deviations or spatial disorientation in instrument conditions.
Intuition Check
Do not read heading as the path the airplane makes over the ground. Heading means the direction the nose is pointed, so heading changes are changes in where the airplane is aimed.
Example Sentence 1
During the cross-country leg in the clouds, the pilot caught small heading changes early by including the heading indicator in every scan.
Example Sentence 2
Consistent heading changes during level flight often indicate the pilot is using too much aileron without balancing rudder.