Definition
Ice that remains on an aircraft's surfaces after a deicing system has been operated. Most deicing systems do not remove every trace of ice; small amounts can stay behind on leading edges, control surfaces, or other airframe areas even after the system has cycled.
Plain English
The leftover ice still stuck to the aircraft after the deicing system has done its job. It's the bit the system couldn't quite get off.
Context Anchor
Seen in winter operations, preflight inspections, deicing checks, and discussions of flight after icing conditions.
Derivation
From the Latin residuus, meaning 'remaining' or 'left over.' Residual ice is simply the ice left over after deicing.
Why Pilots Care
Even small amounts of residual ice can reduce lift, increase drag, and disrupt airflow over control surfaces, raising the risk of performance loss or loss of control.
Analogy
Like the thin layer of frost that stays on your car windows after you scrape most of it off.
Grounding Statement
Picture brushing snow and ice off a car window: the large pieces may be gone, but a thin hard patch left behind can still block your view.
Intuition Check
Do not assume “residual” means harmless or too small to matter. Here it means ice that remains, and even a small amount can matter on an aircraft surface.
Example Sentence 1
After cycling the deicing boots, the crew noted residual ice still clinging to the leading edges.
Example Sentence 2
During the climb through clouds, residual ice on the propeller blades caused a noticeable vibration until the anti-ice system cleared it.