SLD (supercooled large droplets) / freezing rain / freezing drizzle

SLD — Supercooled Large Droplets (Freezing Rain / Drizzle)

SLD = Supercooled Large Droplets are the most dangerous form of atmospheric icing. They form clear ice on the aircraft.

Source: FAA AC 91-74B; EASA AMC1 CAT.IDE.A.115; ICAO Annex 3 / Annex 6.

Definition

SLD are water droplets still liquid at T < 0 °C with diameter > 50 µm (ordinary cloud droplets: 10-20 µm).

Main forms

1. Freezing rain (FZRA) — drops ≥ 0.5 mm.
2. Freezing drizzle (FZDZ) — drops 0.1-0.5 mm.

Formation

Classic scenario: warm layer above a surface inversion:

• Aloft: warm air (> 0 °C) with rain.
• Below: cold layer (< 0 °C).
• Rain falls through cold layer → becomes supercooled → freezes on contact with the aircraft.

Danger

The most dangerous icing conditions are found in freezing rain:

• Clear ice with high density — very heavy (up to 1 g/cm³).
• Accumulation rate up to 10-25 mm/h.
• Forms behind ice-protection devices too (boots, heat strips).
• Aerodynamic deformation in minutes — stall possible.

Link to warm fronts

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) — typical constellation: cold surface air from pre-front high, warm air aloft from the warm front.

Pilot response (PPL without anti-ice)

1. Leave the icing zone immediately (see Subject 070 lesson on low visibility — icing zone).
2. Change altitude: down or up into a layer with T > 0 °C or drier air.
3. Heading change to the shortest path out of the precipitation zone.
4. Mayday/PAN-PAN if severe.

Recognition

• Visual: glass-clear ice on wing leading edge (often noticed late).
• Sensory: airspeed drop, stall warner.
• Forecast: SIGWX chart symbol for SLD.

Meteorological Service*; NTSB AAR-96/01 (American Eagle ATR-72 Roselawn 1994 — an SLD accident).*