Definition
A sequence of speed reductions along a procedure in which each successive speed restriction is equal to or lower than the one before it, never increasing as the aircraft progresses along the path. In VNAV path construction, monotonic decelerations ensure the aircraft can continuously slow down through the procedure without ever being asked to speed back up between fixes.
Plain English
Speed restrictions that only go down or stay the same as you move along the route — they never go back up. Once you've slowed to a given speed at one fix, the next fix won't ask you to be faster.
Context Anchor
Seen in Vertical Navigation planning, especially when describing how a descent path is built from cruise flight toward approach and landing speeds.
Derivation
From Greek 'monos' (one, single) and 'tonos' (tone, direction) — literally 'one direction.' In math, a monotonic sequence moves only one way: always down or always up, never reversing. Applied to decelerations, it means the speeds only step downward (or hold steady), never reverse to a higher speed.
Why Pilots Care
Ensures the aircraft maintains proper energy management and meets all altitude and speed restrictions without creating unstable approach conditions.
Grounding Statement
Picture a descent where the planned speeds step down from higher to lower as the airplane gets closer to landing.
Intuition Check
Do not read “monotonic” as meaning “boring” or “unchanging.” Here it means the speed changes in only one direction: slower, not faster again.
Example Sentence 1
The arrival was designed with monotonic decelerations, so the crew could plan a continuous slow-down from 250 knots through each successive crossing restriction.
Example Sentence 2
Planners verify that all speed reductions remain monotonic so the descent stays predictable through each waypoint.