Definition
The point during a ground reference maneuver or traffic pattern leg where the airplane is flying with the wind directly behind it, producing the highest groundspeed of the maneuver.
Plain English
It's the spot where the wind is pushing you from behind, so you're moving across the ground faster than at any other point.
Context Anchor
Seen in ground-reference maneuvers and constant-radius turning flight, where the pilot must adjust the turn to keep the airplane on the intended path over the ground.
Derivation
Downwind' simply means going in the same direction as the wind is blowing -- the wind is at your back, pushing you along. Knowing this makes the higher groundspeed at this position obvious: airspeed plus tailwind.
Why Pilots Care
At this position the pilot must use the steepest bank angle of the entire maneuver to keep the turn radius constant despite the increased groundspeed.
Grounding Statement
Picture the airplane moving around a circle on the ground map; the downwind position is the part of the circle where the wind is helping push it along.
Intuition Check
Do not confuse downwind position with the downwind leg of an airport traffic pattern. Here, it means the part of a turn or ground path where the airplane is moving in the same direction as the wind.
Example Sentence 1
As the airplane reached the downwind position during the turn around a point, the instructor reminded the student to steepen the bank to maintain a constant radius.
Example Sentence 2
The instructor asked the student to call out when the airplane reached the downwind position so they could check the bank angle adjustment.