Definition
Ice that forms on an aircraft surface aft of a heated leading edge. Supercooled water droplets strike the protected, heated portion of the wing or other surface and melt, but the resulting liquid water then flows rearward along the surface into an unheated area, where it refreezes as ice.
Plain English
Ice that forms behind a heated section of the wing. The heat melts the incoming water, but as it runs back over the colder, unheated part of the wing, it freezes again.
Context Anchor
Seen in aircraft icing discussions, especially with heated or protected wing and tail leading edges.
Derivation
From 'run back' -- the water literally runs back along the surface before refreezing. The name describes exactly what is happening.
Why Pilots Care
Runback ice can create drag and disrupt airflow over a larger portion of the wing or control surfaces than leading-edge ice, reducing performance and altering handling characteristics.
Grounding Statement
Picture water hitting the front edge of a cold wing, staying liquid for a moment, then freezing after it has moved farther back.
Intuition Check
Runback ice is not usually the first ice that forms on the front edge. It is ice that forms after liquid water has moved back and then frozen.
Example Sentence 1
After extended flight in icing conditions, the crew suspected runback ice had formed behind the heated leading edge of the wing.
Example Sentence 2
Post-flight inspection revealed runback ice ridges on the wing after the aircraft had passed through freezing rain.