Detecting severe icing (SLD) in flight
Large super-cooled droplets (SLD): big drops behind freezing rain and freezing drizzle. Icing severe enough to need visible clues, not just a temperature-and-cloud check (TC AIM AIR 2.12.3.6).
- Clues vary by aircraft type (consult manufacturer). TC AIM lists general ones.
Horns (AWH 20.3.2, 20.3.2.1, 20.3.3): big drops run back before freezing, so clear and mixed ice can build "horns" near top and bottom of leading edge.
- Disrupt airflow over a much larger area than rime.
- Reach past where de-ice boots or heated edges protect.
- Clear ice is hard to see.
Clues, by temperature validity ("Near freezing" = significant specifically near freezing):
| Validity | Where | Sign |
|---|---|---|
| Any temperature | Wing aft of area protected by de-icing equipment | Irregular or jagged ice lines, or self-shedding pieces |
| Any temperature | Unheated propeller spinner | Ice farther aft than normal |
| Any temperature | Unheated window portions | Granular or dispersed ice crystals, or total translucent or opaque coverage; ridge patterns can form within seconds to half a minute of exposure |
| Any temperature | Airframe parts where ice does not normally appear | Unusually extensive ice, or ice fingers or feathers |
| Any temperature | Flight performance | Airspeed or climb rate noticeably below what power setting should give |
| Near freezing | Rain | Very large droplets visible; landing or taxi lights, or audible fuselage impact, help catch it |
| Near freezing | Windscreen | Droplets splashing or splattering: freezing-drizzle droplets (0.2 to 0.5 mm) are big enough to see; 40 to 50 micron droplets in normal icing-certification envelope (Airworthiness Manual Chapter 525, Appendix C) are not |
| Near freezing | Windows, heated or unheated | Water droplets or rivulets streaming: high-liquid-water-content sign |
| Near freezing | Weather radar | Precipitation returns while outside air temperature is near freezing |
Trap: these clues mean IN an SLD environment, not that an icing upset is under way (sloppy aileron forces = roll upset; elevator force or trim change = tailplane stall, both separate lessons).
References
- TC AIM AIR 2.12.3.6 (Detecting Large Super-Cooled Droplets Conditions in Flight)
- AWH 20.3.2 (Clear Icing: horns), 20.3.2.1, 20.3.3 (Mixed Icing)