Icing

Three structural ice types

• Rime ice: milky, opaque, rough. Rapid freezing on impact traps air, giving it a porous, brittle appearance. Typically forms in stratus clouds and at colder temperatures (-10°C to -20°C) with small supercooled droplets. Easier to remove; lower density; less hazardous than clear.
• Clear ice: glossy, transparent, dense. Slower freezing — the droplet partially freezes on impact, then runs back along the airfoil before freezing fully. Forms horns at the leading-edge top and bottom that disrupt airflow far more than rime. Forms in cumulus clouds at warmer temperatures (0°C to -10°C) with large supercooled droplets. The most hazardous structural ice type.
• Mixed ice: Combination of clear and rime characteristics. Hardest to identify visually from the cockpit; behaves like a worse-than-rime accumulation. Typical range -8°C to -15°C.

Per AC 91-74B, droplet size and water content matter as much as temperature. Large-droplet icing (SLD) accretes aft of de-ice protection and can be catastrophic.

Freezing rain — the inversion mechanism

Freezing rain requires a temperature inversion: a warm layer aloft holds rain in liquid form, then drops freeze on contact with cold surfaces below freezing. The classic profile is along a warm front: warm air sliding over a cold air mass at the surface.

• Warmer-than-freezing layer aloft → liquid raindrops
• Sub-freezing surface layer → drops supercool but stay liquid
• Drops freeze on impact with the airframe

Severe icing potential. Climbing into the warm layer aloft is often the right escape (drops melt back to plain rain), but only if you can confirm the warm layer's depth and base. Without that confirmation, turn around immediately — freezing rain encounters in helicopters with no anti-ice are not survivable on extended exposure.

METAR codes: FZRA (freezing rain), FZDZ (freezing drizzle), PL (ice pellets — indicates an inversion above the surface).

Why helicopters are extra-vulnerable

• The rotor is the airfoil. Ice on the leading edge of a rotor blade changes lift, drag, and balance simultaneously. Asymmetric shedding from one blade produces immediate severe vibration.
• Most light helicopters (R-22, R-44, Cabri, Bell 206 baseline) are not certified for known icing. Operating in icing conditions is prohibited by the RFM.
• Tail-rotor icing is asymmetric and unpredictable; can degrade anti-torque authority before the pilot notices airframe ice.
• Engine inlet icing on turbines can occur at +5°C OAT in visible moisture — see your aircraft's RFM anti-ice procedures.
• Hover and slow flight provide minimal frontal airspeed to keep ice shedding; ice accumulates faster on a hovering helicopter than a transit aircraft.

Forecasting and avoidance

• AIRMET Zulu — moderate icing and freezing levels. Read every Z product on your route. aviationweather.gov/airmet.
• SIGMET for severe icing — affects all aircraft.
• Icing PIREPs — most accurate real-time data. File one if you encounter ice; read every one on your route.
• Graphical forecasts — GFA icing layer shows freezing level and probability of icing by altitude.
• Freezing level proximity — flying just below the freezing level in visible moisture (clouds, rain) is the recipe.
• Climb or descend out of the layer — if you encounter icing in cruise IFR, the fastest escape is often a 1,000 ft altitude change. Your fastest "out" is the option you've already briefed before launch.

If you encounter ice

1. Exit the icing condition — turn around, climb, or descend (whichever you've briefed). 180° turn is almost always available.
2. Declare an emergency if you cannot exit immediately. Don't wait for "real" trouble.
3. Maintain higher airspeed within RFM limits — degraded airfoils need extra margin over stall/RBS speeds. For the underlying physics, see RBS.
4. Anticipate vibration — asymmetric rotor ice will shed asymmetrically; brace for it.
5. Plan a longer-than-normal approach — avoid abrupt control inputs, plan a shallow approach, expect higher than normal landing speed.