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.
Plain English
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.
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 plain mechanical description became the standard term.
Why Pilots Care
Unmanaged runback water can cause additional ice formation that reduces engine power or leads to complete induction blockage.
Grounding Statement
Picture rain hitting a heated windshield: the drops melt or stay liquid on the warm glass, then trickle to the cold edges and freeze there. The same thing happens to a wing leading edge in icing conditions.
Intuition Check
Runback water does not mean any water on the airplane. It means water that moves rearward after it collects or melts, and may freeze somewhere downstream.
Example Sentence 1
After climbing through the icing layer, the crew noticed a ridge of runback water had refrozen just aft of the wing's heated leading edge.
Example Sentence 2
In icing conditions, runback water from melted ice can move farther into the induction system and cause power loss.