Aircraft Icing for the INRAT: Types, Hazards, and Canadian Rules

Icing is one of the more heavily weighted meteorology topics on the INRAT — and one where candidates who studied American materials run into trouble, because the weather products and regulatory references are different in Canada. Here's what the exam actually tests.

The three types of structural icing

Structural icing forms when supercooled water droplets — liquid water below 0°C — hit an aircraft surface and freeze. The type of ice that forms depends on droplet size and temperature.

The INRAT will ask you to identify ice types from descriptions of conditions. The key distinguisher: large droplets and warmer sub-zero temperatures produce clear ice; small droplets and colder temperatures produce rime. Freezing rain — where rain falls from above-freezing air into below-freezing air near the surface — is one of the most dangerous icing scenarios and almost always produces clear ice.

Where icing occurs and how to find the freezing level

Structural icing requires: visible moisture (cloud, precipitation) AND temperatures at or below 0°C. Above the freezing level, temperatures are below zero — but there's no guarantee of visible moisture. In cloud between 0°C and approximately -40°C, supercooled water droplets can exist. Below -40°C, nearly all water has frozen to ice crystals, which don't accrete on airframes the same way.

In Canada, freezing level information comes from several sources:

• GFA (Graphical Forecast for Aviation) — icing and freezing level forecasts are depicted on the icing/turbulence chart. This is the primary Canadian pre-flight weather product. See our article on reading GFA charts for the full breakdown.
• PIREPs — pilot weather reports provide real-time icing reports including intensity and altitude. The INRAT expects you to know PIREP icing intensity codes: trace, light, moderate, severe.
• SIGMETs — significant meteorological advisories issued by NavCanada when icing conditions are severe or widespread.

Canadian SIGMETs for icing

In Canada, SIGMETs are issued by the Meteorological Service of Canada (Environment and Climate Change Canada) and distributed through NavCanada. A SIGMET for icing indicates severe icing not associated with thunderstorms, or icing that constitutes a significant hazard to aircraft operations.

An AIRMET (Airmen's Meteorological Information) covers less severe icing — moderate icing that may affect lighter aircraft. The INRAT distinguishes between the two: SIGMET = severe/significant conditions; AIRMET = moderate conditions that may affect VFR or lighter IFR traffic.

The Canadian regulatory rule: CARS 602.11

CARS 602.11 prohibits flight in known icing conditions unless the aircraft is certified and equipped for flight in icing conditions. "Known icing conditions" means conditions where ice formation on the airframe is actually occurring — not just forecast, but actually present.

This is an important distinction for the exam. Forecast icing is a risk factor that affects flight planning. Known icing — actual ice accretion observed or reported — triggers the regulatory prohibition for aircraft not certified for icing. If your aircraft has no ice protection equipment and you encounter actual icing, you must exit those conditions.

Carburettor icing: a separate hazard

Structural icing and carburettor icing are different problems. Carb ice forms in the venturi of carburetted engines and can occur at temperatures well above freezing — typically between +10°C and +21°C with high humidity. It's not a cloud-related phenomenon. The INRAT tests both, but don't confuse them. Structural icing = cloud/precipitation, cold temperatures. Carb icing = warm, humid air, engine venturi effect.

For more on how weather products integrate with approach planning and flight decision-making, see the IFR weather minimums article and the GFA weather chart guide.