How ice forms — and why it matters
Clouds and precipitation often contain supercooled water: liquid droplets well below 0 °C that freeze the moment they hit something. Icing is most likely between about 0 °C and −20 °C, can occur down to −40 °C, and the worst cases involve supercooled large droplets (freezing drizzle and freezing rain) that run back beyond the protected areas before freezing. Thin and sharp objects collect droplets most efficiently, so probes, antennas, the tailplane and wing tips ice up before the thick wing root.
- Aerodynamics — even a thin, rough layer reduces CL max and the stalling angle of attack (the stall can come before the stall warning), increases drag and changes pitching moments; tail icing can cause a nose-down pitch upset when flaps are lowered.
- Weight and control — ice adds weight, can jam control surfaces and unbalance them.
- Systems — blocked pitot and static ports, frozen AoA vanes, ice on windscreens, ice shedding into engines (fan and compressor damage), reduced propeller efficiency and antenna performance.
Classification of ice
| Type | Formation | Appearance and hazard |
|---|---|---|
| Rime | Small droplets freeze instantly on impact, trapping air; colder temperatures (about −10 to −40 °C) | Milky, opaque, rough and brittle; follows the leading edge shape; easier to remove |
| Clear (glaze) | Large droplets spread and flow back before freezing; near 0 to −10 °C | Transparent, hard, heavy and tenacious; can form horns; the most dangerous |
| Mixed | A combination as droplet sizes vary | Rough and hard |
| Frost (hoar frost) | Water vapour turns directly to ice on a surface below 0 °C — overnight on the ground, or when a cold-soaked aircraft descends into moist air | Thin and feathery, but rough enough to raise the stalling speed — remove before take-off |
Pilot reports grade intensity as trace, light, moderate or severe. On the ground, the clean-aircraft concept applies: no ice, snow or frost may be on critical surfaces at take-off. Ground de-icing uses heated Type I fluid (orange, low viscosity, short holdover time) to remove contamination; Type II and Type IV thickened fluids (Type IV is green) then protect the aircraft for a holdover time that depends on the weather. Fluid must not be sprayed into engines, APU inlets, pitot-static ports or brakes.
Ice detection
- Visual — the crew watch the wiper bosses, windscreen frames, an ice-evidence probe in view of the cockpit, and at night the leading edges lit by wing inspection (ice) lights (Chapter 17).
- Vibrating-probe (magnetostrictive) detectors — as above, the most common electronic type.
- Obstruction detectors — a slowly rotating cylinder beside a knife-edge scraper: ice build-up increases the drive torque and operates a switch.
- Optical and differential-pressure detectors — ice interrupts a light beam, or blocks small holes in a pitot-type probe.
- A primary ice detection system turns the protection on automatically; an advisory system only alerts the crew, who remain responsible for switching protection on using visual cues and temperature/moisture criteria.
De-icing versus anti-icing
Anti-icing prevents ice from forming — it must be switched on before or as soon as icing conditions are met and runs continuously (hot air, continuous electric heat, fluid). De-icing removes ice after it has formed, periodically (boots, cyclic electric heater mats).
De-icing systems
Pneumatic boots
- Rubber boots bonded to wing, tail and strut leading edges, with spanwise or chordwise inflatable tubes. A vacuum (from an ejector driven by the same air source) holds them flat in normal flight to keep their aerodynamic shape.
- When selected, a timer and distributor (control) valves inflate the tubes in sequence with regulated engine bleed or pneumatic pump air for a few seconds each, cracking the ice. Modern guidance is to operate them as soon as icing begins.
- Maintenance: inspect for cuts, abrasion and debonding; repair with approved patches and cement; keep the surface conductive coating intact (static discharge); clean with approved materials only and never stand or lean ladders on them.
Electrical de-icing
Heater mats bonded into or onto a surface are energised cyclically: a continuously heated parting strip along the leading edge splits the ice, and zones either side are heated in turn so the ice loosens and is blown off. Used for propellers (power reaches the rotating blades through slip rings and brushes), helicopter rotor blades and, on the B787, the wing leading edges.
Hot air and chemical de-icing
Hot-air systems are normally run as anti-icing, but some designs cycle the air as a de-icer. Chemical systems spread a freezing-point depressant (glycol-based fluid, or alcohol on older propellers and windscreens): the TKS "weeping wing" pumps fluid out through laser-drilled porous panels on the leading edges, both preventing and loosening ice; propeller slinger rings spread fluid along the blades.
Anti-icing systems
- Thermal (hot air) — engine bleed air heats the wing leading edges (usually the outboard slats) through a wing anti-ice valve and piccolo tubes, and each engine's inlet lip through a cowl anti-ice valve fed from its own compressor. Systems may be "evaporative" (enough heat to evaporate the water) or "running wet" (water runs back and may refreeze behind the protected area). Leak and overheat detection protects the structure; ground operation is usually inhibited or time-limited.
- Electrical — windscreen heating (Chapter 4), air-data probe heaters, propeller spinners and inlet lips on some turboprops, tail leading edges on some business jets.
- Chemical — TKS fluid systems, alcohol windscreen sprays on older aircraft, and ground anti-icing fluids with holdover times.
Probe and drain heating
- Pitot probes, static ports (on some types), AoA vanes, total-air-temperature probes and ice detectors have electric heaters; they operate at a reduced level on the ground and full power in flight, and on many aircraft switch on automatically when the engines run.
- Heater current is monitored: a failed heater gives a caution, because a cold probe can ice up and corrupt air data.
- Drain masts (which discharge galley and basin waste water overboard) and potable-water lines are heated so they do not freeze (Chapter 18).
- Heated probes are hot enough to cause burns — check them only as the AMM describes, and fit covers only with the heat off.
Rain removal: wipers and rain repellent
- Wipers — electric motors (hydraulic on some older aircraft) drive a converter that oscillates each arm; speeds are selectable (slow, fast, sometimes intermittent) and the blades return to a park position. Arm pressure (blade force) is set with a spring scale per the AMM. Never run wipers on a dry windscreen — it scratches the surface and damages the blades.
- Rain repellent — a chemical sprayed onto the wet windscreen makes water form beads that the airflow blows away. It must not be applied to a dry windscreen (it smears and impairs vision), and older fluids are being withdrawn for environmental reasons. Modern windscreens carry a permanent hydrophobic coating that does the same job and must be protected from abrasive cleaning.
- Air-blast systems on some aircraft blow a sheet of bleed air across the windscreen to sweep rain away.