Definition
An ATC clearance that changes an aircraft's previously assigned route, altitude, or speed, thereby altering its planned flight path through the airspace system.
Plain English
A new instruction from air traffic control that changes where, how high, or how fast you were going to fly.
Context Anchor
You may see this term in ATC and flight planning discussions when a controller changes an aircraft’s route, altitude, speed, or heading from what was previously expected.
Derivation
Trajectory comes from the Latin 'trajectoria,' meaning the path of a moving object. In aviation it refers to the aircraft's planned path through space — laterally, vertically, and over time. A clearance that 'alters' that trajectory is one that changes any part of it.
Why Pilots Care
Compliance keeps the aircraft within the authorized flight profile, maintains separation from other traffic, and supports efficient routing.
Intuition Check
Do not read “trajectory” as only a curved line on a chart. In this term, it means the aircraft’s overall planned movement, including route, altitude, speed, and timing. Do not read “clearance” as casual permission. In ATC use, a clearance is an instruction or approval that the pilot is expected to follow if accepted.
Example Sentence 1
After receiving a trajectory altering clearance to descend and turn direct to a new fix, the crew updated the FMS and read the new clearance back to ATC.
Example Sentence 2
The controller gave us a TAC to descend earlier so we could make the arrival window.