Airframe Icing
Airframe icing is ice accretion on wings, fuselage, control surfaces, and probes of the aircraft. One of the biggest hazards for PPL VFR in winter and in wet clouds.
Source: FAA AC 91-74B Pilot Guide: Flight in Icing Conditions; ICAO Annex 6 Part II; EASA SIB 2017-09.
Dangers of icing
Icing on aircraft poses significant dangers including increased stall speed, centre of gravity shifts, control surface blocking, wing shape deterioration leading to stalls, and potential engine failure:
| Effect | Mechanism |
|---|---|
| Lift loss | Wing-profile shape changed by ice layer |
| Stall speed rise | Reduced lift, increased drag |
| CG shift | Ice concentrates on leading edges, possibly asymmetric |
| Control-surface stiffening | Thick ice → ailerons/elevator sluggish |
| Pitot/static blocked | False IAS/altimeter/VSI |
| Engine failure | Icing in carburettor or inlet → no combustion air |
Icing types
| Type | Formation | Properties |
|---|---|---|
| Rime ice | At colder T (-10 to -20 °C), small supercooled drops | Milky, rough, light. Changes profile shape (lift loss). |
| Clear ice | At warmer T (0 to -10 °C), large supercooled drops (FZRA) | Glass-clear, hard, very heavy. Hard to see. |
| Mixed ice | Transition | Combines rime and clear properties |
| Hoar frost | Sublimation on supercooled surfaces | Feather-like, thin, can prevent take-off |
Freezing rain — extremely dangerous
The most dangerous icing conditions are found in freezing rain:
- Liquid rain that freezes on contact.
- Forms clear ice in minimal time on all surfaces.
- Profile deformation extremely fast.
- Consequence: leave the icing zone immediately (see Subject 070 lesson on low visibility — icing zone).
Icing due to freezing rain has to be expected in front of warm fronts or warm-front occlusion after winterly cold spells (see precipitation lesson).
Freezing level — key to avoidance
The height of the freezing level is one of the most important prerequisites to avoiding aircraft icing (see precipitation lesson).
Icing on or near the ground
Formation of hoar frost, icing of spray water or slush, a snow cover, sleet or freezing rain can cause icing at or close to the ground:
- Pilot inspects aircraft surface before take-off.
- Even a thin ice layer is a no-go for take-off.
Sublimation in cloud-free air
Sublimation of water vapour in cloud-free air can cause formation of hoar frost on an aircraft (see humidity lesson) — on a strongly supercooled aircraft.
Supercooled aircraft + high-humidity air
When a significantly supercooled aircraft flies into a layer of air with high humidity, airframe icing can also occur in cloud-free air at temperatures above 0 °C (see humidity lesson).
Mountain top winter vs summer
It is possible for an aircraft to fly below a mountain top in winter and above it in summer at the same atmospheric pressure and indicated altitude because when true air temperature is colder than the ICAO standard atmosphere:
- Winter (cold): indicated altitude > true altitude → aircraft actually lower.
- Summer (warm): indicated ≈ true altitude.
Icing per cloud type
In stratiform clouds: In stratiform clouds, pilots can normally expect no flight visibility, drizzle, and icing at temperatures around or below freezing.
In Cb: Cumulonimbus (Cb) clouds with great vertical extent often feature no flight visibility, severe turbulence, severe icing at temperatures around or below freezing, and electrical discharge due to intense convective activity and supercooled water droplets.
In shallow Cu: In shallow cumulus clouds often, the following conditions are encountered: no flight visibility, light to moderate turbulence, light icing at temperatures around or below freezing.
Cold weather operations*; FAA-H-8083-25B PHAK Ch. 13.*