Aircraft Icing Explained

Aircraft icing is dangerous not mainly because ice makes an airplane heavier, but because even relatively small ice accretions can change the shape and surface condition of wings, tails, propellers, engine inlets, sensors, and control surfaces.

Those changes can:

• Increase drag
• Reduce maximum lift
• Reduce the wing's critical angle of attack
• Increase stall speed
• Reduce climb performance
• Change control effectiveness
• Interfere with instruments or propulsion systems
• Produce handling characteristics the pilot may not expect

The central hazard is therefore aerodynamic degradation.

And there is an equally important operational lesson:

Ice protection does not make all icing conditions safe.

Some aircraft are approved for flight in specified icing environments. Others have equipment intended only to deal with limited or inadvertent encounters. In every case, the aircraft's AFM or POH, operating limitations, and approved procedures determine what the airplane can actually tolerate.

What Is Aircraft Icing?#

Aircraft icing is the accumulation of ice on or in parts of an aircraft.

It is not one single phenomenon.

Pilots may encounter several different icing hazards, including:

• Structural icing on wings, tails, fuselage, antennas, or other surfaces
• Propeller icing
• Engine-inlet icing
• Carburetor or induction icing
• Pitot-static icing
• Windshield icing
• Ice affecting flight-control surfaces or mechanisms

These hazards do not all form under exactly the same conditions and do not produce the same symptoms.

This guide focuses primarily on in-flight structural icing, then explains how the other forms differ.

How Structural Ice Forms#

Most in-flight structural icing begins with supercooled water droplets.

A supercooled droplet is liquid water at a temperature below 0°C.

It remains liquid in the atmosphere until something causes it to freeze.

When these droplets strike a sufficiently cold aircraft surface, some or all of the water can freeze and form ice.

The amount and shape of the accretion depend on several variables, especially:

• Liquid water content
• Droplet size
• Temperature
• Aircraft speed
• Airfoil shape
• Surface temperature
• Time spent in the icing environment

This is why two airplanes flying through apparently similar cloud can experience different amounts or types of ice.

Freezing Temperature Alone Does Not Mean Structural Icing#

An outside-air temperature below freezing is not enough by itself.

The air must contain liquid water capable of reaching and freezing on the aircraft.

A clear, dry atmosphere at −10°C may present no structural-icing accretion at all.

Conversely, a cloud or area of precipitation containing significant supercooled liquid water can produce rapid ice accumulation.

The better mental model is:

subfreezing conditions + supercooled liquid water + aircraft exposure = potential structural icing

The exact potential-icing limits for an aircraft may be defined more specifically in its AFM or operating manual.

What Temperature Is Most Icing Found In?#

There is no single universal "icing temperature."

FAA weather guidance says that almost all ordinary supercooled-liquid-water icing tends to occur between approximately:

0°C and −20°C

with especially frequent reports in portions of that range.

But that does not mean icing suddenly becomes impossible below −20°C.

Supercooled liquid droplets can exist at substantially colder temperatures, with about −40°C representing the physical lower limit at which liquid droplets can remain unfrozen without ice nuclei.

Temperature therefore changes the probability and type of icing rather than creating a simple on/off boundary.

Aircraft Surface Temperature Matters Too#

Outside-air temperature is not the only temperature that matters.

An airplane can be cold-soaked.

For example, after spending time at very low temperature aloft, its structure may remain below freezing for a period after descending into air that is slightly warmer than 0°C.

Liquid water contacting that cold structure can then freeze.

This is particularly relevant to aircraft structures near cold fuel tanks.

So even the rule: "Above 0°C means ice cannot form"

needs qualification.

Rime Ice#

Rime ice forms when supercooled droplets freeze rapidly after striking the aircraft.

Because freezing happens quickly, air can become trapped within the accretion.

Rime ice typically appears:

• Rough
• Milky
• Opaque
• Porous or brittle

It often forms when droplets are relatively small and temperatures are colder.

But temperature alone does not determine whether rime forms.

Liquid-water content, droplet size, aircraft geometry, and other conditions matter as well.

Clear or Glaze Ice#

Glaze ice, also commonly called clear ice, forms when supercooled droplets do not freeze completely the instant they hit the surface.

Some liquid water can flow or spread before freezing.

The resulting ice may be:

• Clear or translucent
• Dense
• Hard
• Irregular
• Horn-shaped on leading edges

Larger droplets and relatively warmer subfreezing conditions can favor this type of accretion.

The term clear ice can be misleading because the most aerodynamically important characteristic is often the shape of the accretion, not whether it actually looks transparent from the cockpit.

Mixed Ice#

Real icing encounters do not always fit neatly into rime or clear categories.

Mixed ice contains characteristics of both.

It can form when an aircraft encounters changing:

• Droplet sizes
• Liquid-water content
• Temperatures
• Cloud microphysics

The resulting ice may contain alternating opaque and clearer regions and can develop complex aerodynamic shapes.

This is why pilots should not assume that identifying an ice "type" tells them exactly how severe the aerodynamic problem is.

Ice Type Is Not the Same as Icing Severity#

This distinction matters.

Type describes the physical character of the ice:

• Rime
• Clear/glaze
• Mixed

Intensity or severity describes how rapidly ice is accumulating and how it affects the aircraft.

A visually dramatic rime accretion is not automatically worse than a less obvious clear accretion.

Nor is clear ice universally "the most dangerous" in every airplane and every encounter.

The hazard depends on:

• Accretion shape
• Accretion rate
• Location
• Aircraft sensitivity
• Protection-system capability
• Flight condition

The airplane's actual performance and handling response matters more than a simple color classification.

Supercooled Large Droplets#

Some of the most serious icing environments involve supercooled large droplets, commonly abbreviated SLD.

These are larger droplets found especially in:

• Freezing drizzle
• Freezing rain

Large droplets can strike the leading edge and continue flowing aft before freezing.

That creates an important problem:

ice may form behind the areas protected by conventional leading-edge ice-protection systems.

This is sometimes called runback or aft accretion.

Such ice can disrupt airflow over a much larger portion of the wing and can affect control surfaces.

SLD was a major factor in several important icing accidents and led to significant changes in aircraft certification requirements.

Freezing Drizzle and Freezing Rain#

Freezing drizzle and freezing rain consist of supercooled liquid precipitation.

Unlike ordinary cloud droplets, the drops can be large enough to produce extensive impingement and ice accretion.

These conditions can exist:

• Inside cloud
• Below cloud
• Over broad horizontal areas

This creates another important correction to the simplistic advice: "Just descend below the cloud and the icing will stop."

In freezing precipitation, descending may leave the airplane inside the same hazardous liquid-water environment.

An icing escape plan therefore cannot be based on altitude alone.

Frost Is a Different Contamination Problem#

Frost is not the same phenomenon as ordinary in-flight structural icing.

Frost can form when water vapor deposits or freezes on a sufficiently cold aircraft surface.

Even a thin rough contamination can disturb airflow over a wing.

For takeoff, the relevant principle is the clean-aircraft concept:

critical aerodynamic surfaces must meet the aircraft's approved contamination limitations before departure.

The fact that frost looks thinner or smoother than a dramatic in-flight ice accretion does not make it aerodynamically harmless.

Aircraft-specific procedures and applicable regulations determine what contamination, if any, is permissible.

Why Ice Hurts Aerodynamics So Much#

Ice does not need to cover the whole wing.

The leading edge is particularly sensitive because it helps establish the pressure distribution and airflow that allow the wing to produce lift efficiently.

An ice shape can alter:

• Airfoil geometry
• Surface roughness
• Pressure distribution
• Airflow separation
• Maximum lift coefficient
• Critical angle of attack

The result can be a major loss of performance.

Ice Usually Hurts More Through Aerodynamics Than Through Weight#

Ice has mass, so accumulating ice does increase aircraft weight.

But for most hazardous aircraft-icing encounters, the additional mass is not the primary problem.

The much larger concern is that the ice has changed the aerodynamic surfaces.

An ice accretion can produce substantial:

• Drag increase
• Maximum-lift reduction
• Stall-angle reduction

before the added ice mass represents a large fraction of aircraft weight.

Likewise, a blanket statement that icing "shifts the center of gravity" is not a useful general explanation.

The aerodynamic degradation should remain the central mental model.

Ice Increases Drag#

Ice creates roughness and shapes that disturb the airflow.

That can dramatically increase drag.

The consequences include:

• More thrust or power needed to maintain speed
• Reduced cruise speed
• Reduced rate of climb
• Lower service ceiling
• Increased fuel consumption
• Reduced range
• Worse missed-approach or go-around performance

If the aircraft cannot produce enough additional thrust to overcome the growing drag, airspeed may begin to decay.

Ice Reduces Maximum Lift#

Ice contamination can also reduce the maximum lift the wing can produce.

That is not the same as simply saying: "Ice destroys lift."

The wing may continue flying while contaminated.

But its aerodynamic capability has changed.

It may require:

• More airspeed
• Different angle of attack
• More power

to achieve the performance previously available when clean.

There Is No Universal "Quarter-Inch" Performance Penalty#

NASA icing research has demonstrated dramatic performance degradation from relatively short ice exposures.

In specific wind-tunnel tests, some clear-ice accretions substantially increased drag, reduced maximum lift, and reduced stall angle.

Those experiments demonstrate a crucial lesson:

small-looking ice accretions can have large aerodynamic effects.

They do not establish a universal rule that every quarter-inch of ice always reduces lift by exactly 25 percent or increases drag by exactly 40 percent.

The effect depends on:

• Airfoil
• Ice shape
• Accretion location
• Aircraft configuration
• Reynolds number
• Flight condition

That is why a fixed percentage is a poor cockpit rule.

Ice Changes Stall Behavior#

A clean wing stalls when it exceeds its critical angle of attack.

Ice contamination can lower that critical angle.

The contaminated aircraft can therefore stall:

• At a lower angle of attack
• At a higher airspeed than when clean
• With different warning cues
• With different roll behavior

There is no universal rule saying icing raises stall speed by exactly five or ten knots.

The amount is aircraft- and contamination-specific.

Stall Warning May Change#

A stall-warning system is designed around the aircraft configuration and certification basis.

Ice can alter the airflow in ways that change the relationship between the warning system and the actual aerodynamic stall.

On some aircraft, the normal clean-aircraft stall warning may provide less margin in icing.

Other aircraft incorporate systems or logic specifically designed to account for icing.

The pilot should therefore follow the aircraft's AFM or POH icing speeds and procedures rather than assuming the clean-aircraft stall warning will behave identically.

Ice Can Affect Roll Control#

Ice near the wingtip or ahead of an aileron can change the flow reaching the ailerons.

Possible effects include:

• Reduced aileron effectiveness
• Changes in control force
• Localized wing stall
• Unexpected rolling moments

In some severe icing or SLD situations, ice can accumulate aft of protected leading-edge surfaces and disrupt the flow ahead of the aileron.

This can produce a serious roll-control problem even before the whole wing stalls.

For how ailerons normally produce roll, see Control Surfaces Explained.

Tailplane Icing#

The horizontal tail deserves special attention.

A tailplane can collect ice too.

On conventional airplanes, the horizontal tail often produces an aerodynamic force that balances the airplane's pitching moments.

Ice changes the tailplane's aerodynamic characteristics just as it changes those of a wing.

Under certain conditions, the contaminated tail can approach or experience an aerodynamic stall.

This is called an ice-contaminated tailplane stall.

Wing Stall and Tailplane Stall Are Not the Same Event#

A wing stall and a tailplane stall can both involve separated airflow, but their aircraft responses and corrective procedures can differ significantly.

This is particularly important during configuration changes such as flap extension.

Flap deployment can alter the airflow and aerodynamic load seen by the tail.

A response appropriate for a wing stall may not be appropriate for a tailplane-stall problem.

For that reason, generic internet recovery instructions are inappropriate.

The AFM or POH procedure for the specific airplane takes precedence.

Flaps Can Matter in Icing#

Flap extension changes:

• Wing lift
• Pitching moment
• Downwash
• Tailplane loading

On aircraft susceptible to tailplane icing, those changes can become important.

Some aircraft procedures may therefore limit flap settings or specify different approach techniques when ice is present.

Other aircraft have been certificated and evaluated differently.

There is no universal "always use less flap in icing" rule.

Use the aircraft-specific procedure.

Autopilot Can Hide a Deteriorating Aircraft#

An autopilot can make an icing encounter appear deceptively calm.

Suppose ice increases drag and reduces climb performance.

The autopilot may continue adjusting pitch or control forces to maintain:

• Altitude
• Vertical speed
• Flight path

while the aircraft's airspeed or performance margin quietly deteriorates.

The pilot may not feel the increasing control force because the autopilot is supplying it.

This is why airspeed, power requirement, trim, and aircraft performance must still be monitored closely during an icing encounter.

The autopilot has not removed the aerodynamic degradation.

It may simply be masking some of its symptoms.

Pitot-Static Icing#

Ice can also affect the systems used to measure aircraft motion and altitude.

A blocked pitot tube can produce erroneous airspeed information.

Blocked or obstructed static-system components can affect instruments that depend on static pressure.

Aircraft may use:

• Heated pitot probes
• Heated static sources
• Alternative static systems
• Multiple redundant air-data sensors

depending on design.

Ice protection for those systems should be used according to aircraft procedures.

A sudden unusual airspeed indication in icing should not automatically be assumed to mean that the wing itself has suddenly changed speed.

Instrument icing is another possibility.

Propeller Icing#

Ice on a propeller blade changes its airfoil shape.

That can cause:

• Reduced propeller efficiency
• Reduced thrust
• Vibration
• Imbalance

Ice may also shed unevenly from different blades, producing severe vibration.

Propeller ice-protection systems vary by aircraft and may use:

• Electrical heating
• Fluid systems

The approved operating procedure determines when and how they are used.

Engine Inlet Icing#

Turbine-engine inlets can collect ice.

Ice can:

• Restrict airflow
• Distort the airflow reaching the compressor
• Reduce engine performance
• Shed into the engine

Many turbine aircraft therefore use bleed air, electrical systems, or other methods to protect engine-inlet areas.

The exact icing conditions that require engine anti-ice are engine- and aircraft-specific.

Carburetor Icing Is Different#

Carburetor icing demonstrates why the statement: "Aircraft icing requires subfreezing outside air" is not universally true.

Inside a carburetor, pressure reduction and fuel vaporization can cool the airflow substantially.

Ice can therefore form internally even when outside-air temperature is above freezing and even when the airplane is flying in clear air.

Carburetor icing is an induction-system problem, not the same process as supercooled cloud droplets freezing on a wing.

That is why structural icing and carburetor icing should not be taught as one weather condition.

Anti-Icing Versus De-Icing#

Aircraft ice-protection systems are commonly divided conceptually into two groups.

Anti-icing#

Anti-ice systems are intended to prevent or limit ice formation on protected areas.

Examples can include:

• Heated surfaces
• Engine-inlet heat
• Pitot heat
• Propeller heat
• Fluid systems

De-icing#

De-icing systems are intended to remove ice after some accretion has occurred.

The classic example is the pneumatic leading-edge boot.

The terminology is useful, but system operation is aircraft-specific.

A pilot should not decide system timing simply from whether a component is generally labeled "anti-ice" or "de-ice."

Pneumatic De-Icing Boots#

Pneumatic boots are flexible structures installed on protected leading edges.

During a de-icing cycle, the boots inflate.

That deformation cracks accumulated ice so that aerodynamic forces can carry it away.

The boots then deflate back into their normal shape.

Ice may remain:

• Between boot cycles
• After a cycle
• On unprotected portions of the aircraft

So a boot-equipped aircraft does not become aerodynamically identical to a completely clean airplane every time the system cycles.

Do You Have to Wait for Ice to Build Before Using Boots?#

One of aviation's most persistent icing myths concerns ice bridging.

Older training sometimes taught pilots to delay boot activation until a substantial layer of ice had accumulated, out of concern that operating the boot too early would stretch the ice into a shell that the boot could no longer break.

FAA testing found little evidence of ice bridging on modern pneumatic-boot designs.

The correct operating philosophy is therefore:

follow the AFM or POH for that specific aircraft.

Do not invent a delay based on the generic ice-bridging myth.

Some systems are designed for automatic cycling or early activation.

Aircraft procedures remain the authority.

Thermal Anti-Icing#

Some aircraft heat critical surfaces so that ice cannot form or cannot remain bonded.

Heat may come from:

• Engine bleed air
• Electrical heating

Protected areas can include:

• Wing leading edges
• Engine inlets
• Probes
• Windshields

Thermal protection consumes aircraft resources.

Bleed-air systems can affect engine performance, while electrical systems require significant power.

Protection capability therefore belongs to the overall aircraft design rather than being "free."

Fluid Ice-Protection Systems#

Some airplanes use fluid-based protection systems.

A porous or perforated leading-edge surface releases freezing-point-depressant fluid over the protected area.

Systems commonly associated with the TKS concept can protect areas such as:

• Wings
• Tail surfaces
• Propellers
• Windshields

depending on installation.

Fluid capacity is finite.

Coverage is aircraft-specific.

A fluid system should not be treated as unlimited icing protection.

Ice Detectors#

Some aircraft use automatic ice-detection systems.

These systems can:

• Alert the crew to ice accretion
• Trigger or assist ice-protection logic
• Provide another cue that icing conditions have been entered

But not every aircraft has one.

And the absence of an ice-detector warning does not justify ignoring obvious visual or performance evidence of icing.

What Does "FIKI" Mean?#

Pilots often use FIKI as shorthand for:

Flight Into Known Icing

The phrase generally refers to an aircraft whose certification and equipment permit operation in specified icing conditions.

That shorthand can create a dangerous misconception.

"FIKI" does not mean: "This airplane can safely remain in any icing condition."

Certification applies to defined atmospheric envelopes and demonstrated capabilities.

The aircraft still has:

• Limitations
• Required systems
• Required procedures
• Conditions that may exceed its certification

The AFM is the correct source for exactly what that aircraft is approved to do.

Icing Certification Has Changed Over Time#

Two airplanes that both have de-icing boots may have very different icing capabilities.

Certification standards have evolved.

Older aircraft may have been evaluated under requirements very different from those applied to newer designs.

Modern transport-category rules also include certification requirements addressing supercooled-large-droplet conditions that were not part of older icing envelopes.

So the presence of:

• Boots
• Heated propellers
• Pitot heat
• A windshield panel

does not by itself establish that the airplane is approved for intentional flight in icing.

"Known Icing" Is Not Simply "Cloud Below Freezing"#

FAA terminology distinguishes several related concepts.

Known or observed ice accretion refers to actual ice observed on the aircraft or detected by onboard systems.

The AIM describes known icing conditions in terms of atmospheric conditions where ice formation has been observed or detected.

Aircraft manufacturers may separately define potential icing conditions using combinations such as temperature and visible moisture.

FAA legal interpretations also require pilots to consider the reasonable likelihood of encountering icing based on the information available for the planned flight.

The practical lesson is not to reduce "known icing" to one weather-app checkbox.

The pilot must consider:

• Aircraft limitations
• Forecasts
• Reports
• Route
• Altitude
• Timing
• Actual observed conditions

An Icing Forecast Is Not a Guarantee#

Atmospheric icing can vary dramatically over short distances and time periods.

A PIREP reporting icing does not guarantee that the same intensity will exist when another aircraft arrives.

Likewise, a report of no icing does not prove the area will remain ice-free.

Forecast products therefore provide risk information—not certainty.

Current U.S. Icing Weather Products#

For operations in the contiguous United States, useful icing information includes:

• G-AIRMET icing products
• SIGMETs for severe icing
• Current Icing Product (CIP)
• Forecast Icing Product (FIP)
• PIREPs
• Freezing-level information
• Cloud and precipitation forecasts
• METARs
• TAFs

CONUS text AIRMETs have been replaced by graphical G-AIRMETs.

Alaska continues to use AIRMET products in a different structure.

Aviation Weather Explained covers how these products fit into a complete weather briefing.

G-AIRMETs and SIGMETs#

A G-AIRMET can identify broad areas where moderate icing is expected.

A SIGMET may be issued for severe icing.

But neither product tells you exactly what your aircraft will experience at every point inside the depicted area.

Icing severity is aircraft-dependent.

A condition reported as manageable by a large, well-protected transport aircraft may be much more serious for a small airplane with limited ice protection.

PIREPs Are Especially Valuable#

A PIREP provides actual pilot-observed conditions at a particular:

• Place
• Time
• Altitude
• Aircraft type

That aircraft type matters.

"Light icing" reported by one airplane is not necessarily light for another.

Different aircraft have different:

• Collection characteristics
• Speeds
• Ice-protection systems
• Aerodynamic sensitivity

PIREPs should therefore be interpreted in context rather than copied directly onto another aircraft.

CIP and FIP#

The Current Icing Product and Forecast Icing Product combine multiple meteorological data sources to estimate icing:

• Probability
• Severity
• SLD potential

These are valuable decision-support products.

They are not direct measurements of ice on your airplane.

They should be combined with:

• PIREPs
• Advisories
• Forecasts
• Aircraft limitations
• Pilot judgment

Freezing Level Is Useful—but Not an Escape Command#

The freezing level tells you where the atmosphere reaches approximately 0°C.

That helps identify where supercooled-liquid-water icing may become possible.

But it does not mean: "If icing occurs, always descend."

Why not?

Because below you may be:

• Terrain
• An unsafe minimum altitude
• Another subfreezing layer
• Freezing rain
• Freezing drizzle
• Worse weather
• An airport or approach environment with additional hazards

The freezing-level chart is planning information.

It is not an automatic emergency maneuver.

Build an Escape Plan Before Entering the Area#

A good icing plan answers this before departure:

If the conditions are worse than expected, where can I go?

Possible exits may include:

• Climbing
• Descending
• Turning around
• Deviating laterally
• Diverting to another airport

Which one is appropriate depends on:

• Aircraft capability
• Terrain
• Cloud tops
• Freezing levels
• Temperature profile
• Air traffic
• Available altitude
• Icing type and severity
• AFM procedures

FAA guidance specifically recommends knowing how you will exit icing conditions before the encounter.

The First Goal Is to Leave Conditions the Aircraft Cannot Handle#

If ice accumulation exceeds what the airplane or its protection systems can manage, the objective is not to prove that the equipment can eventually remove it.

The objective is to exit the hazardous environment.

That may require coordination with ATC.

If necessary for safety, communicate the urgency clearly.

But there is no universal instruction that every icing encounter requires:

• A descent
• A climb
• A 180° turn

The correct exit is situational and aircraft-specific.

Takeoff With Contamination#

Takeoff is especially sensitive to wing contamination because the aircraft is operating at relatively low speed and high lift coefficient.

Ice, frost, or snow on critical surfaces can:

• Reduce maximum lift
• Increase drag
• Change stall characteristics
• Increase required takeoff distance
• Reduce climb performance

The correct preflight standard is not: "That doesn't look like much ice."

It is compliance with the aircraft's approved clean-aircraft and contamination limitations.

Initial Climb in Icing#

Climb can become dangerous because the airplane is already:

• Relatively slow
• Producing high lift
• Operating at significant angle of attack
• Dependent on excess power

Ice increases drag and can reduce the aircraft's available climb margin.

If an autopilot is trying to maintain a selected vertical speed, it may raise the nose as performance deteriorates.

That can allow airspeed to fall toward an unsafe value.

Monitor the actual aircraft state rather than assuming the commanded climb rate proves the aircraft still has adequate performance.

Cruise in Icing#

During cruise, increasing ice may first appear as:

• Increasing power required
• Falling airspeed
• More trim
• Reduced acceleration
• Reduced climb capability

A seemingly stable cruise condition does not mean the ice has stopped affecting the airplane.

Residual and intercycle ice on protected surfaces can continue imposing aerodynamic penalties.

Approach and Landing#

The final phases of flight deserve special attention because several margins become smaller at once.

The aircraft is:

• Closer to stall
• Closer to the ground
• Changing configuration
• Often using lower power
• Subject to increased workload

Ice may affect:

• Wing stall margin
• Tailplane behavior
• Roll control
• Approach speed
• Flap selection
• Landing distance

The correct configuration and minimum speeds are aircraft-specific.

Go-Around With Ice#

A go-around requires immediate climb performance at a time when the aircraft may have:

• High drag
• Reduced lift capability
• Residual ice
• Landing configuration
• Limited excess power

An aircraft that could comfortably go around when clean may have substantially less margin when contaminated.

This is another reason an icing approach should be planned using the airplane's approved procedures rather than normal clean-aircraft habits.

Common Myths About Aircraft Icing#

Myth: Structural icing only occurs between −15°C and 0°C#

No.

Most common supercooled-liquid-water icing is concentrated roughly between 0°C and −20°C, but icing can occur colder than that.

Temperature is only one factor.

Myth: Any cloud below freezing contains dangerous ice#

No.

Structural icing requires supercooled liquid water capable of accreting on the airplane.

Many cold clouds contain mainly ice crystals rather than substantial supercooled liquid water.

Myth: Clear or glaze ice is always more dangerous than rime#

Not universally.

Clear ice can form extremely hazardous shapes and SLD environments are particularly serious, but actual danger depends on accretion, aircraft, severity, and system capability.

Myth: A quarter-inch of ice always reduces lift by 25 percent#

No.

NASA testing demonstrates that small accretions can produce very large performance penalties, but the exact percentage depends on airfoil and ice shape.

Myth: Ice mainly matters because it makes the airplane heavier#

No.

Added weight usually matters far less than the aerodynamic changes produced by ice roughness and shape.

Myth: Ice always increases stall speed by five or ten knots#

No.

The stall-speed increase is aircraft- and contamination-specific.

Myth: If the aircraft has boots, wait for a thick layer before using them#

Not as a generic rule.

FAA research found little evidence of ice bridging on modern boot designs.

Use the boots according to the AFM or POH.

Myth: FIKI means the airplane can stay in any ice#

No.

Icing certification applies to specified conditions and limitations.

Some environments may exceed the aircraft's demonstrated capability.

Myth: Descending below the freezing level is always the best escape#

No.

Freezing precipitation, terrain, minimum altitudes, inversions, or other conditions can make descent ineffective or unsafe.

Plan multiple exit options.

Myth: Above-freezing air means aircraft icing cannot happen#

Not in every sense.

A cold-soaked aircraft can collect frozen contamination after entering warmer air, and carburetor icing can occur at above-freezing outside-air temperatures.

Myth: The autopilot makes an icing encounter safer automatically#

An autopilot can reduce workload, but it can also mask increasing control forces or deteriorating performance.

The aircraft still needs to be monitored.

Myth: If the wings look clean, the airplane is clean#

Ice can exist on:

• Tail surfaces
• Propellers
• Engine inlets
• Probes
• Unseen portions of the wing
• Areas behind protected leading edges

The visible portion of one wing is not a complete ice detector.

Frequently Asked Questions#

Key Takeaways#

• Aircraft icing is primarily an aerodynamic hazard, not simply an added-weight problem.
• Structural icing usually forms when supercooled liquid droplets strike and freeze on sufficiently cold aircraft surfaces.
• Most ordinary supercooled-liquid-water icing occurs roughly between 0°C and −20°C, but icing can occur at colder temperatures.
• Rime, clear/glaze, and mixed describe physical ice types; they do not by themselves define severity.
• Freezing drizzle and freezing rain contain supercooled large droplets that can produce hazardous ice aft of conventional protected areas.
• Even visually small ice accretions can substantially increase drag and reduce maximum lift.
• There is no universal percentage loss of lift, drag increase, or fixed stall-speed penalty for a given thickness of ice.
• Ice contamination can reduce the wing's critical angle of attack and change stall-warning behavior.
• Ice can affect wings, tailplanes, control surfaces, propellers, engine inlets, pitot-static systems, and windshields.
• Tailplane icing is a different aerodynamic problem from a conventional wing stall, so aircraft-specific recovery procedures matter.
• Anti-icing systems prevent or limit accretion; de-icing systems remove accumulated ice.
• Modern pneumatic boots should be operated according to the AFM or POH rather than delayed because of a generic fear of ice bridging.
• An aircraft approved for flight in icing is approved only within defined certification and operational limitations.
• FIKI does not mean unlimited safe flight in every icing condition.
• Current weather products such as G-AIRMETs, SIGMETs, CIP, FIP, and PIREPs describe icing risk but cannot guarantee the exact condition one airplane will encounter.
• Descending is only one possible icing escape strategy; climb, lateral deviation, reversal, or diversion may be more appropriate.
• Autopilots can mask deteriorating performance, so airspeed and power margin still require close monitoring.
• Carburetor icing is an induction-system phenomenon and can occur under conditions that do not produce structural icing.
• Aircraft-specific AFM, POH, checklists, limitations, and approved procedures always take precedence over generic icing advice.

Sources & References#

• FAA Advisory Circular AC 91-74B, Pilot Guide: Flight in Icing Conditions.
• FAA Aviation Weather Handbook, FAA-H-8083-28B, Chapter 20: Icing.
• FAA Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C, Chapters 7, 8, 12, and 13 as applicable.
• FAA Aeronautical Information Manual, Chapter 7: Safety of Flight — current icing terminology, conditions, and reporting.
• FAA Aviation Weather Center — G-AIRMET, SIGMET, CIP, FIP, freezing-level, and PIREP products.
• NASA Glenn Research Center — aircraft icing aerodynamics, ice-accretion research, and Icing Research Tunnel data.
• 14 CFR Part 23 and Part 25 icing certification requirements, including applicable Appendix C and Appendix O icing envelopes.
• Aircraft-specific AFM, POH, approved icing supplements, checklists, and operating limitations.