Definition
The point along a flight path at which an aircraft begins its planned descent from cruise altitude toward a lower altitude or the destination. In RNAV and FMS-equipped aircraft, the TOD is calculated automatically based on cruise altitude, target altitude, descent angle or vertical speed, groundspeed, and any altitude constraints along the route.
Plain English
The spot on your route where you stop flying level and start coming down.
Context Anchor
Seen in instrument flight planning, vertical navigation planning, and GPS or flight management displays that help plan when to descend.
Derivation
Top refers to the starting or highest point; descent comes from Latin 'descendere' meaning to go down. Together they mark the exact beginning of the downward phase of flight.
Why Pilots Care
Proper TOD calculation ensures efficient fuel use, smooth descent rates, and compliance with altitude restrictions.
Grounding Statement
Picture TOD as the point where level flight changes into the planned descent toward the next altitude goal.
Intuition Check
TOD does not mean the highest altitude of the flight. It means the planned starting point for the descent.
Example Sentence 1
About ten miles before top-of-descent, the crew briefed the arrival and reviewed the altitude restrictions on the STAR.
Example Sentence 2
Crossing TOD early allowed for a shallower descent to meet the crossing restriction.