Plain English Meaning

Water from melted or impacted droplets that runs back along the wing or engine inlet from a warm area into a cold area, where it freezes again.

Full Definition

Liquid water that flows aft along an aircraft surface after striking a heated leading edge or anti-iced area, then refreezes on unheated surfaces downstream where it can form ridges or sheets of ice beyond the protected zone.

Context Anchor

Seen in icing discussions, especially when explaining how ice can form inside the engine air intake instead of only at the front opening.

Derivation

From 'run back' — the water literally runs backward (aft) along the surface in the airflow before it refreezes. The mechanical description became the term.