Plain English Meaning
A point on the route where the chart tells you exactly what altitude (or range of altitudes) you must be at when you fly over it.
Full Definition
A waypoint on a published instrument procedure that has a mandatory altitude restriction the aircraft must satisfy — at, at or above, at or below, or within a specified altitude window — when crossing that point.
Context Anchor
Seen on published arrival and approach charts and in the aircraft’s navigation computer when planning a climb or descent.
Derivation
‘Constrained’ comes from the Latin constringere, meaning ‘to bind or tie tightly.’ The waypoint is ‘tied’ to a specific altitude requirement — the aircraft is not free to cross it at any altitude.
Why Pilots Care
Allows the FMS to build a vertical path that meets all restrictions while maintaining terrain and traffic separation.
Analogy
Think of it like a checkpoint on a road trip where you not only have to pass the checkpoint, but also have to arrive there at a required time. The point matters, and the condition attached to it matters too.
Intuition Check
Constrained does not mean the waypoint is blocked or difficult to reach. It means a required condition, usually an altitude, is tied to crossing that point.
Example Sentence 1
The crew programmed the descent so the aircraft would meet the constrained waypoint at or above 11,000 feet.
Example Sentence 2
Before activating VNAV the pilot verified every constrained waypoint on the arrival to confirm the profile would work.