Basics of Aircraft Icing Detection and Ice Protection

Physics of Clouds and Icing

Condensation of pure water vapor requires super-saturation of several hundred percent.

Since condensation nuclei are common in the atmosphere, supersaturation of more than 1% is rare.

The number of cloud condensation nuclei usually determines the number of droplets in a cloud.

Fixing the liquid water content (LWT), clouds forming in clean air have larger droplets than clouds forming in polluted air.

Pure liquid water can be cooled to about -40 C without freezing.

Since ice nuclei are rare in the atmosphere, supercooled liquid water (SLW) is ubiquitous.

SLW droplets freeze when contacting a solid such as aircraft airframe (contact nucleation), causing icing.

Certification of Ice Protection Systems

Ice protection systems are designed to mitigate problems when an airplane is exposed to icing conditions.

The design and test of ice protection systems involves consideration of:

  • The meteorological conditions of 14 CFR Part 25, Appendices C and O
  • The operational conditions which would affect the accumulation of ice on protected and unprotected surfaces of an airplane and its power system(s)
  • The ability of an airplane to either detect-and-exit SLD icing conditions safely, or operate safely in them (added to 14 CFR Part 25 in 2014)
Impingement rate is a function of droplet size and concentration.

A Mean Effective Diameter (MED) of 20 μm is usually used to determine the water catch rate, and an MED of 40-50 μm is used to determine the impingement limits (AC 20-73).

New certification requirements are described in 14 CFR Part 25 and AC 25-28.