Definition
Irregular, horn-shaped protrusions of clear ice that grow forward and to the sides of an aircraft's leading edges when supercooled water droplets strike the surface, run back, and freeze. These shapes disrupt the smooth airflow over the wing or tail and significantly degrade aerodynamic performance.
Plain English
Pointed, curved chunks of ice that build up and stick out from the front of the wings or tail when freezing water hits the aircraft and freezes in place. They look a bit like animal horns and badly disturb the air flowing over the surface.
Context Anchor
Seen in discussions of clear ice and the way ice changes the shape of wings and tail surfaces.
Derivation
Named for their resemblance to animal horns -- curved, pointed shapes projecting forward from the leading edge. The visual likeness gives pilots an immediate mental picture of the hazard.
Why Pilots Care
Horn ice changes the wing shape enough to reduce lift, raise stall speed, and increase drag far more than other ice forms.
Grounding Statement
Picture a smooth wing front edge becoming rough and pointed with ice; the wing no longer has the clean shape it needs to work properly.
Intuition Check
Do not read horns here as a cockpit warning sound or an animal part. In icing, horns are horn-shaped ice projections on the aircraft.
Example Sentence 1
After flying through freezing rain, the crew noticed horns of clear ice forming on the leading edges and immediately requested a lower altitude with warmer temperatures.
Example Sentence 2
The pilot recognized the horns as a sign that the airfoil was now aerodynamically compromised.