Two lessons supplied where ice forms and what kind grows; this one finishes the icing trilogy with the questions that matter in the cockpit. What does the ice actually do to the aircraft? Which deposit is worst? Where exactly do the dangerous zones sit (in a front, in a cloud, in the precipitation below it), and how does a crew plan and fly around them? And then the modern twist the syllabus added after a series of high-altitude engine events: the icing that builds inside the engines while the wings stay perfectly clean.
How icing is reported: light, moderate, severe
Icing in reports, forecasts and SIGMETs comes graded, and the ICAO ladder has three rungs:
Light — accumulation slow enough that no change of course or altitude is considered necessary, though prolonged exposure still adds up;
Moderate — even short encounters are potentially hazardous: use of de-icing or anti-icing equipment, or a diversion, may be necessary;
Severe — the de-icing and anti-icing equipment can no longer reduce or control the hazard: an immediate change of level or route is necessary.
Notice what the ladder actually measures: not centimetres of ice, but the contest between accumulation rate and the aircraft's protection. The same cloud can be moderate icing for one type and severe for another — which is why the terms travel with the report rather than with the weather.
What ice does to an aircraft
The general hazards read like a tax on every system at once:
lift down, drag up, weight up, stall speed up — the distorted profile flies worse in every respect at once;
controls stiffen or jam as ice bridges hinges and gaps;
propellers shed unbalanced ice — vibration, thrust loss, and chunks flung against the fuselage;
engine intakes and carburettors choke (the conditions lesson's warm-day trap included);
pitot and static ports block — unreliable airspeed and altitude;
antennas ice over or snap, and windshields glaze blind — communications and vision go the same way as the aerodynamics.
Ranking the deposits
Set the previous lesson's four deposits against that list and the ranking writes itself. Clear ice is the most dangerous: heavy, hard and tenacious, it spreads aft beyond the protected strips, distorts the profile most, and sheds, when it finally sheds, in large, unbalancing chunks. Mixed ice runs close behind, combining clear ice's bulk and grip with rime's roughness, often growing fastest of all where snow freezes into it. Rime is the least dangerous of the three (light and brittle), but the least dangerous accretion is not harmless: its roughness costs lift, it narrows intakes, and stratiform cloud serves it for hours at a time. And hoar frost weighs almost nothing yet endangers the takeoff, which is why the previous lesson ended by insisting it be removed.
Where the dangerous zones are
The syllabus wants the danger zones placed — in fronts, in the two cloud families, and in precipitation:
Warm front — the freezing-rain wedge ahead of the surface front, where rain from the warm air aloft falls through the sub-zero cold air below: the worst icing zone of all; above it, deep nimbostratus on the frontal slope gives prolonged moderate to severe icing;
Cold front — the cumulonimbus line, with severe clear and mixed icing in the towering cloud;
Stratiform cloud — light to moderate rime or mixed, but with the exposure measured in hours; the liquid water (and the icing) is greatest near the cloud top, and the conditions lesson's orographic stratus is worse again;
Cumuliform cloud — severe icing, worst just above the freezing level where the updraught carries the biggest supercooled drops, through the whole 0 to −20 °C band;
Precipitation — freezing rain the heaviest icing there is, freezing drizzle next, wet snow packing near 0 °C, and ice pellets on the ground announcing that freezing rain is close.