Definition
The point at which ice forming on an aircraft surface first becomes visible to the pilot. At this stage, the rate of accumulation is only slightly greater than the rate at which ice sublimates away, so buildup is minimal. Trace icing is not considered hazardous unless the aircraft is exposed to it for more than one hour, and anti-icing or de-icing equipment is generally not required for short exposure.
Plain English
It's the moment a thin layer of ice first shows up on the aircraft. There's so little of it that it's barely growing, and short exposure isn't dangerous on its own.
Context Anchor
Seen in icing reports, weather briefings, and pilot reports that describe how quickly ice is forming on an aircraft.
Derivation
Trace' comes from the Old French 'trace' meaning a mark or track left behind — the smallest visible sign of something. In icing terminology, it carries the same idea: the first faint sign that ice is forming.
Why Pilots Care
It marks the earliest moment a pilot should consider activating anti-ice systems or changing altitude to prevent further buildup.
Grounding Statement
Picture a thin first film of ice appearing on an exposed part of the airplane, with little change from minute to minute.
Intuition Check
Trace does not mean unimportant here. It means the smallest reporting level of aircraft icing: noticeable, slow-forming ice that still needs to be monitored.
Example Sentence 1
After climbing through the cloud layer, the pilot reported that trace ice had become noticeable on the wing leading edges.
Example Sentence 2
When trace ice becomes noticeable at night, the crew turned on the wing inspection light to monitor accumulation.