Collection Efficiency, often referred to as "Catch Rate", is a product of two factors:
- Collision efficiency - determined by aerodynamics, and
- Adhesion efficiency - determined by the adhesive qualities of the component surface.
Specifically, it is the fraction of the liquid water in the direct path of an aircraft component (aerofoil, antenna, windscreen etc) which is deposited as ice on that component whilst flying in icing conditions. Collection efficiency varies directly with both droplet size and aircraft speed and inversely with the geometric size of the collecting surface.
To a large degree, the rate at which ice accumulates on an aircraft depends upon the collection efficiency of the aircraft component involved. The size of the collecting surface of an aircraft component is described in terms of the curvature radius of its leading edge. Those components which have large curvature radii (canopies, windscreens, thick wings, etc.) collect only a small percentage of the cloud droplets, especially the smaller droplets, and, therefore, have a low collection efficiency. Conversely, components which have a small curvature radii (antenna masts, thin wings, etc.) deform the airflow less and permit a high proportion of droplets of all sizes to be caught. These components are said to have a high collection efficiency. Once ice begins to form, the shape of the collecting surface is modified by the ice itself, resulting in the curvature radius nearly always becoming smaller. Therefore, as ice accumulates, the collection efficiency increases leading to further and more rapid ice accumulation. On most aircraft, the curvature radius of the horizontal tail surface is smaller than that of the wing. This can lead to tail plane icing before any ice accumulation on the wing and, in some cases, could lead to Ice Contaminated Tailplane Stall.
Droplet size has an effect on where ice will form. If the droplets are small, ice formation is limited to the leading edge radius. As droplet size increases, ice formation will extend aft of the leading edge radius but with medium size droplets will not normally extend aft of surfaces normally protected by aircraft ice protection systems. Ice formation from large droplets can extend aft of the protected surfaces. Freezing rain or freezing drizzle can result in ice formation extending aft to the point of maximum component projection into the air stream.
The faster the speed of the aircraft, the less chance there is for the droplets to be carried around the airfoil in the air stream. As a consequence, collection efficiency increases with aircraft speed.