Definition
Airflow that moves along the length of a wing — from root to tip on the lower surface, and from tip to root on the upper surface — rather than straight back from leading edge to trailing edge. It is caused by the pressure difference between the high-pressure air below the wing and the low-pressure air above it, with the air spilling around the wingtip from bottom to top.
Plain English
Air doesn't just flow straight back over a wing. Some of it slides sideways along the wing because the pressure underneath is higher than the pressure on top, and that high-pressure air leaks around the tip toward the low-pressure side.
Context Anchor
Seen in discussions of induced drag, wingtip vortices, and how a wing makes lift.
Derivation
Spanwise' means 'along the span' — the span being the distance from one wingtip to the other. So spanwise flow is simply air moving in the direction of the wingspan, sideways across the wing, instead of front-to-back.
Why Pilots Care
Spanwise flow rolls up at the wingtips into vortices that produce induced drag, lowering efficiency at low speeds and high angles of attack.
Grounding Statement
Picture air under the wing being pushed sideways toward the tip, then curling around the tip into a swirl behind the wing.
Intuition Check
Do not assume airflow over a wing always moves straight front to back. In induced drag discussions, some of the air also moves sideways along the wing.
Example Sentence 1
Spanwise flow on the lower surface of the wing moves outward toward the tip, where it spills upward into the lower-pressure region above the wing.
Example Sentence 2
Winglets reduce spanwise flow and therefore lower induced drag in cruise.