top of page

Aircraft Icing Types: Clear, Rime, Mixed, and SLD — Recognition, Hazards, and Escape Strategies

Updated: 4 days ago

Aircraft icing is the silent killer. Unlike thunderstorms — visible from miles away, audible on the radio, dramatic in their hazards — icing accumulates quietly. You can be in the clouds at 6,000 feet feeling nothing dramatic, yet within minutes have ice transforming the aerodynamics of your wings into something the airplane isn't certified to fly with. Pilots have spiraled out of control from icing. Others have run out of altitude trying to climb out of it. Many have died not understanding what was happening until it was too late.


This post covers icing in the practical depth that matters for staying alive: the four icing types pilots need to recognize (clear, rime, mixed, and SLD), the temperature and droplet conditions that produce each, the FAA intensity categories, escape strategies that actually work, and what "known icing" really means in practice.



Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >


The Physics: Why Aircraft Icing Happens

The fundamental requirement: supercooled liquid water in the atmosphere. This is water that exists as a liquid below the freezing point — possible because the water needs a "nucleus" to begin freezing around, and pure water in the atmosphere can remain liquid down to about -40°C without freezing.


When supercooled water encounters an aircraft, the impact provides the disturbance needed to trigger freezing. The water freezes on contact, forming ice on whatever surface it touched — wings, tail, propellers, antennas, pitot tubes, and so on.


The temperature range where icing occurs:

  • Most common: 0°C to -20°C

  • Possible: 0°C to -40°C (supercooled water can exist this cold in some clouds)

  • Below -40°C: Water typically exists as ice crystals, which generally don't accumulate on warm aircraft surfaces

T

he four key variables determining icing type:

  1. Droplet size — Large droplets produce different ice than small droplets

  2. Outside air temperature (OAT) — Affects how quickly droplets freeze

  3. Aircraft speed — Affects droplet impact dynamics

  4. Liquid water content (LWC) — More liquid water means more ice


These variables combined produce the different icing types.


Type 1: Rime Ice

The most common icing type encountered in routine flight.


Formation:

  • Small supercooled water droplets

  • Cold temperatures (typically -10°C to -20°C, but possible across the icing range)

  • Droplets freeze quickly on impact, trapping air bubbles

  • Common in stratiform clouds (stratus, altostratus, nimbostratus)


Appearance:

  • Rough, opaque, milky white or grey color

  • Forms primarily on leading edges of wings, tail, antennas, and propeller

  • Typically follows the leading edge contour

  • Often described as "frosty" or "crystalline"


Hazards:

  • The rough texture severely disrupts airflow

  • Significant lift reduction even with relatively thin accumulation

  • Stall speed increases noticeably

  • Drag increases

  • Less weight than clear ice (because of trapped air), but more aerodynamic disruption per unit weight


Where you'll encounter it:

  • Most common icing type in stratiform clouds

  • Common in the cool, layered cloud systems behind cold fronts

  • Frequent companion of stratus and altostratus

  • Often found at moderate altitudes (5,000-15,000 feet)


Pilot recognition:

  • Most easily detected — the rough white texture is visible from the cockpit

  • Wing inspection lights at night make rime ice particularly visible

  • Performance degradation noticeable: airspeed declining, increased power needed for level flight


Type 2: Clear Ice

The most dangerous common icing type.


Formation:

  • Large supercooled water droplets

  • Temperatures typically just below freezing (0°C to -10°C)

  • Droplets are big enough that they don't freeze instantaneously on impact — they spread out across the wing surface before completely freezing

  • Common in cumulus clouds where strong updrafts produce larger droplets

  • Also forms in freezing rain and freezing drizzle


Appearance:

  • Smooth, glossy, transparent — looks like glass or wet paint

  • Spreads beyond the leading edge, sometimes back along the wing surface

  • Hard to see — particularly difficult at night or in low light

  • Can run back and refreeze (called "runback icing")


Hazards:

  • Heavy — significantly more weight per unit area than rime ice

  • Hard to detect — you might not see it accumulating

  • Spreads beyond protected leading edges — can build up behind anti-ice boots, where they can't break it off

  • Asymmetric accumulation can cause control problems

  • Particularly dangerous in freezing rain — accumulation rates can be extreme


Where you'll encounter it:

  • Cumulus and cumulonimbus clouds

  • Freezing rain and freezing drizzle conditions

  • Areas with warm rain falling through cold surface air (warm front scenarios)

  • Below the freezing level in convective clouds where supercooled water is abundant


Pilot recognition:

  • May not be visible from the cockpit at all

  • Performance degradation may be the first indication

  • Watch for ice on the windshield wipers, struts, antennas — visible accumulation there means clear ice on the wings as well

  • Strut icing is a particularly good indicator because struts are easier to see than the actual wing leading edge


Type 3: Mixed Ice

A combination of clear and rime characteristics.


Formation:

  • Combination of small and large supercooled droplets

  • Temperatures typically -8°C to -15°C

  • Some droplets freeze immediately (rime) while others spread before freezing (clear)

  • Common in cumuliform clouds with mixed droplet sizes


Appearance:

  • Combination of rough and smooth surfaces

  • Often forms in irregular shapes

  • May be patchy in distribution

  • Color varies from white to translucent


Hazards:

  • Combines the worst features of both types

  • Heavy AND aerodynamically disruptive

  • Difficult to remove with anti-ice systems (boots may not break it off as effectively as pure rime)

  • Often encountered in convective clouds with strong vertical motion


Where you'll encounter it:

  • Cumulus clouds (above the freezing level)

  • Convective frontal systems

  • Mixed-phase clouds in the central part of the icing temperature range


Type 4: SLD (Supercooled Large Droplets) — The Killer

The most severe icing type, requiring its own category.


What SLD is:

  • Supercooled water droplets larger than 50 micrometers in diameter

  • Includes freezing drizzle (50-500 micrometers) and freezing rain (500+ micrometers)

  • Special category in FAA regulations because of its extreme hazard


Why SLD is so dangerous:

  • Droplets are large enough to spread far back on the wing before freezing

  • Ice accumulates BEHIND the protected leading edges

  • Anti-ice boots cannot remove ice that has formed behind them

  • TKS systems can be overwhelmed

  • Even FIKI-certified aircraft are not always certified for SLD encounters

  • Accumulation rates can be extreme — pilots have reported severe ice buildup in just minutes


Conditions favoring SLD:

  • Active warm fronts with cold surface air (warm rain falling through cold air)

  • Specific temperature profiles (warm layer above cold air at surface)

  • Freezing drizzle conditions

  • Warm front passages over cold air masses


FAA SLD certification:

  • Aircraft must be specifically tested and certified for SLD operations

  • Many FIKI aircraft are NOT certified for SLD

  • Check your POH carefully

  • If your aircraft is not SLD-certified, exit SLD conditions immediately


The classic SLD accident: The 1994 ATR-72 accident at Roselawn, Indiana — an SLD event caused ice ridges to form behind the deicing boots, leading to aileron malfunction and loss of control. This accident led to the SLD certification requirements still in effect today.


FAA Icing Intensity Categories

Pilots must report icing using standardized intensity terms:

  • Trace — Ice becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. Anti-ice/de-ice equipment is not utilized unless encountered for an extended period of time.

  • Light — Rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of anti-ice/de-ice equipment removes/prevents accumulation.

  • Moderate — Rate of accumulation is such that even short encounters become potentially hazardous, and use of anti-ice/de-ice equipment or diversion is necessary.

  • Severe — Rate of accumulation is such that anti-ice/de-ice equipment fails to reduce or control the hazard. Immediate diversion is necessary.

  • SLD — Specific category requiring immediate exit from icing conditions, regardless of equipment. Even certified aircraft should exit SLD as soon as possible.


For PIREPs: Pilots should report icing as accurately as possible: "Cessna 12345, 5,000 feet, 30 miles east of Roanoke, light rime, OAT minus 5." This information helps following pilots and ATC understand conditions ahead.


Where Icing Forms: The Aviation Hot Zones

Cumulus and cumulonimbus clouds:

  • Highest icing intensity due to supercooled water content

  • Large droplets common from convective updrafts

  • Rapid accumulation possible

  • AVOID — these are also thunderstorm clouds


Stratus, altostratus, nimbostratus:

  • More moderate icing typically

  • Persistent layers can produce sustained icing

  • Common in warm front conditions

  • Most "routine" icing encounters happen here


Freezing precipitation:

  • Most severe icing type (SLD)

  • Common in warm front situations

  • Visual identification: rain falling at OAT below freezing

  • AVOID at all costs


Mountain wave and orographic clouds:

  • Strong vertical motion can produce supercooled droplets

  • Lenticular clouds may contain icing

  • Often combined with severe turbulence


Induction Icing: Don't Forget About It

Structural icing (the topic above) gets most of the attention, but induction icing is equally important and often overlooked.


Carburetor icing:

  • Most common form of induction icing

  • Forms from the venturi effect cooling air in the carburetor below freezing

  • Most dangerous range: 20°F to 70°F with high humidity

  • Indicators: power loss, rough running, RPM/MP drop

  • Solution: Carburetor heat (covered in detail in the induction systems post)


Air filter icing:

  • Ice accumulating on the engine air filter (in fuel-injected aircraft)

  • Causes power loss

  • Solution: Alternate air source


Pitot tube icing:

  • Ice covers pitot tube, blocking ram air

  • Airspeed indicator becomes unreliable or fails

  • Solution: Pitot heat (covered in detail in the anti-ice systems post)


Static port icing:

  • Less common, but ice on static ports affects multiple instruments

  • Some aircraft have alternate static sources


Escape Strategies That Actually Work

When you encounter icing, the most important pilot skill is exiting the conditions. No anti-ice or de-ice system is a substitute for getting out of ice.


Strategy 1: Climb above the clouds

If you have the performance and the cloud tops are reachable:

  • Climbing into clear air typically eliminates icing

  • However, performance is degraded by ice accumulation, making climb harder

  • Time matters — climb sooner rather than later

  • Don't try to climb through more cloud layers if it means extended exposure


Strategy 2: Descend to warmer air

If above-freezing temperatures exist at lower altitudes:

  • Descend to where OAT is above 0°C

  • Existing ice will begin to melt and shed

  • Often the best option in warm front scenarios where the surface is above freezing

  • Confirm warm air exists below before committing


Strategy 3: Reverse course

If you knew the conditions where you came from:

  • Turn around and exit the icing area

  • Often the safest option when above and below options are poor

  • Don't continue into worsening conditions


Strategy 4: Land as soon as practicable

When other options aren't available:

  • Find the nearest suitable airport

  • Declare an emergency if needed

  • Approach with extra airspeed

  • AVOID FULL FLAPS — tailplane stall risk with icing

  • Land with minimum flap setting per POH guidance


Strategy 5: Declare an emergency

If conditions deteriorate beyond your ability to manage:

  • Declare to ATC

  • Request priority handling

  • Get vectors to nearest suitable airport

  • Don't hesitate — pride has killed pilots in icing

  • ATC can provide significant workload relief in emergencies


The "Known Icing" Question

Pilots often ask whether they can fly into "known icing." The answer is more nuanced than yes or no.


What "known icing" means:

  • Reported icing in PIREPs along your route

  • AIRMET or SIGMET for icing covering your area

  • Forecast icing conditions in the area

  • Visible accumulation on aircraft on the ground


What it doesn't mean:

  • Just "potential" icing conditions (clouds at freezing temperatures)

  • Possible icing in forecasts without specific reports


Aircraft certification matters:

  • Aircraft NOT certified for FIKI: Cannot legally fly into known icing conditions

  • Aircraft certified for FIKI: Can fly into known icing within the certification limitations

  • No aircraft is certified for SLD unless specifically tested and approved (rare)


Practical implications:

  • Most GA aircraft are not FIKI-certified

  • The presence of anti-ice equipment doesn't automatically grant FIKI certification

  • Check the POH/AFM for specific certification status

  • "Inadvertent icing encounter" capability ≠ FIKI


The legal and practical reality:

  • FIKI certification doesn't make icing safe — it just means the aircraft has been tested to exit icing safely

  • Even FIKI aircraft should avoid known icing when possible

  • The strategy is always: avoid, exit, divert — not penetrate


Pre-Flight Decision-Making

Check forecasts:

  • AIRMETs for icing (Zulu icing or AIRMET ZULU)

  • SIGMETs for severe icing

  • Forecast temperatures aloft

  • Cloud bases and tops along route


Check PIREPs:

  • Real-world reports from pilots in your area

  • Note altitude, location, intensity, type

  • Trust PIREPs over forecasts (real conditions often differ)


Assess your aircraft:

  • Is it FIKI-certified?

  • What anti-ice/de-ice equipment do you have?

  • What's the operational status of your equipment?


Plan alternates:

  • Always have an alternate plan if conditions deteriorate

  • Identify warmer air below if available

  • Identify clear-of-clouds altitudes if available

  • Identify nearest suitable airports along route


The decision test:

  • Would you fly this trip if conditions were just slightly worse?

  • If marginal weather makes the trip questionable, the trip can wait

  • "Get-there-itis" kills pilots in icing season


On the Written Test and Checkride

Icing appears consistently on weather knowledge tests. The most commonly tested topics:

  • Definition and characteristics of clear, rime, and mixed ice

  • Temperature ranges for each type

  • Cloud types associated with each

  • Effect of icing on aircraft performance (lift loss, drag increase, weight)

  • FAA icing intensity categories

  • SLD recognition and hazards

  • Anti-ice vs. de-ice equipment


Escape Strategies:

  1. Climb above clouds (if possible)

  2. Descend to warmer air (if possible)

  3. Reverse course

  4. Land as soon as practicable

  5. Declare emergency if needed


Critical principles:

  • Avoid icing conditions in non-FIKI aircraft

  • Even FIKI aircraft should exit icing when practical

  • No flaps or minimum flaps on landing with airframe ice

  • Approach with extra airspeed when carrying ice

  • SLD requires immediate exit regardless of equipment



Study Full Aviation Courses:

wifiCFI's full suite of aviation courses has everything you need to go from brand new to flight instructor and airline pilot! Check out any of the courses below for free:


Study Courses:


Checkride Lesson Plans:


Teaching Courses:



Author: Nathan Hodell

CFI, CFII, MEI, ATP, Creator and CEO

Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.



 
 
bottom of page