Weather - Icing
What
Structural icing is the accumulation of ice on the exterior of the aircraft.
How
2 conditions for structural icing to form:
- the aircraft must be flying through visible moisture.
- close to freezing or sub freezing temperatures.
Aerodynamic cooling can lower temperature of an airfoil to 0° C even though the ambient temperature is a few degrees warmer.
Where
It can happen on any surface of the aircraft, but small and narrow objects will typically ice up the fastest.
The tail being usually much thinner than the wings it’s more susceptible to icing. If ice builds up on the tail, it may stall.
- Instrument Icing: Pitot tube, static port, antennas
- Induction Icing: air filter, carburetor
- Structural icing: clear / rime / mixed, on wing, propeller, etc.
Types
3 types:
- clear ice: usually smooth, transparent and hard; formed between 0° C and -10° C. Favorable condition: freezing rain below a frontal surface. Rain forms above the frontal surface at temperatures warmer than freezing. Subsequently, it falls through air at temperatures below freezing and becomes supercooled. This type forms when drops are large as in rain or in cumuliform clouds.
- rime ice: rough, milky and opaque; formed between -10° C and -20° C. Usually brittle and can usually be broken off easily by anti-icing equipment. Rime ice forms when drops are small, such as those in stratified clouds or light drizzle. The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread over the aircraft surface; lighter in weight than clear ice.
- mixed ice: a mix of clear and rime ice. drops vary in size or when liquid drops are intermingled with snow or ice particles.
When to expect clear ice?
Between 0° C and -10°; when flying through freezing rain or cumuliform clouds.
How to deal with Icing
Right after you extend your flaps for the landing, you feel a sudden nose down pitch. If you suspect that ice is causing a tail stall, you should retract flaps to the previous setting immediately.
Intensity
- Trace
- Light
- Moderate
- Severe
Anti-ice vs De-ice
- Anti-ice: prevents the accumulation of ice on the aircraft.
- De-ice: removes ice that has accumulated on the aircraft.
AC 91-74
AC 91-74B - Pilot Guide: Flight In Icing Conditions: https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentid/1028388
Freezing Rain vs Freezing Drizzle
- Freezing raindrops are defined as drops of 500 micrometers (0.5 mm) diameter or larger.
- Drops of freezing drizzle consist of supercooled liquid water drops that have diameters smaller than 500 micrometers (0.5 mm) and greater than 50 micrometers (0.05 mm).