In-flight aircraft icing is the accumulation of super cooled liquid water on the airframe. Icing can occur from cloud droplets, freezing rain or drizzle.
The severity of in flight icing is largely a factor of super cooled liquid water prevalence, air temperature and droplet size. Most structural icing occurs at temperatures between 0 and -20 degrees celsius. In general, cloud ice and snow do not adhere to the airframe.
Icing can adversely affect the flight characteristics of an aircraft and create pilot control problems. Accumulated ice can increase drag, increase weight, decrease lift, and decrease power. Structural ice can block the pitot tube, static ports and cause breakage of navigation and communication antennas.
Aircraft Icing is classified into four categories of severity based on a pilot’s professional judgment of the aircraft’s capability to deal with ice accretion.
| TRACE | Ice is perceptible. Accumulation rate is slightly greater than rate of sublimation. Not hazardous unless encountered for extended periods of time – over one hour. Deicing/anti-icing equipment might not be utilized. |
| LIGHT | Accumulation rate may create problems if flight is prolonged in these conditions – over one hour. Environment does not present a problem if deicing/anti-icing equipment is used. Occasional use of deicing/anti-icing equipment removes or prevents accumulation. |
| MODERATE | Rate of accumulation for even short encounters becomes potentially hazardous. Use of deicing/anti-icing equipment or diversion is necessary. |
| SEVERE | Rate of accumulation exceeds deicing/anti-icing capability to reduce or control the hazard. Immediate diversion is necessary. |
There are several types of in-flight aircraft icing:
| Rime | Rough, milky, opaque ice formed by small instant freezing super cooled water droplets |
| Clear | Glossy, clear or translucent ice formed from slow freezing super cooled water droplets |
| Mixed | Mixture of rime and clear ice |
| Induction | i.e. Piston carburetor icing or turbine inlet icing. Induction icing always reduces engine performance. |
In general, the type of structural ice is most dependent on the air temperature; however, the likelihood of clear ice increases with droplet size.
| Type | Outside Air Temperature Range |
| Clear | 2 ° C to -10° C |
| Mixed | -10° C to -15° C |
| Rime | -15° C to -20 ° C |
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