Types of Aircraft Icing: Clear, Rime, Mixed and Induction Ice

Where and How Aircraft Ice Forms

Ice forms on aircraft structures when supercooled droplets adhere to them and freeze. AC 91-74B explains that nearly all aircraft icing occurs in supercooled clouds. At temperatures close to 0 degrees Celsius (32 degrees Fahrenheit) the cloud may be entirely liquid drops. As the temperature falls, the chance of ice particles rises, and below about minus 20 degrees Celsius most clouds are made up entirely of ice particles. Ice particles tend to bounce off an aircraft, while supercooled drops freeze and stick, so accumulation is often greatest at temperatures not too far below freezing, where liquid water can be abundant.

Two more points from the circular. The more liquid water in the cloud, the more rapidly ice accumulates, and larger drops adhere to more of the aircraft surface than smaller ones. And near freezing, runback ice can form behind protected areas as water refreezes on the airfoil, so the FAA says pilots should be vigilant at outside air temperatures between minus 5 degrees Celsius (23 degrees Fahrenheit) and plus 2 degrees Celsius (35 degrees Fahrenheit).

Fronts matter because lifting creates the cloud. Along a warm front the warm air slides over cold air and forms stratus clouds conducive to icing. In a cold front the cold air plows under the warm air and lifts it quickly, forming cumulus clouds with high liquid water if the lifted air is moist. The FAA's advice for crossing a front is to take the shortest route through it, not fly along it, to reduce time in potential icing.

The FAA also warns about supercooled large drops, called SLD, which are freezing drizzle and freezing rain. They can be found aloft in cloud or in freezing precipitation below a cloud deck, and they can cause very rapid stall speed and drag increases for some aircraft and roll control anomalies for others.

The Three Types of Structural Ice

Small and narrow parts collect ice fastest. That's why a small protuberance within sight of the pilot can be used as an ice evidence probe, and why a tailplane, being thinner than the wing, collects ice better than the wing does.

The FAA says it's the location, size, shape and roughness of the ice that's most important aerodynamically, not its color.

Induction Icing

Ice in the engine intake reduces the air available for combustion. The most familiar case is carburetor ice, which forms when moist air cools as it passes through the carburetor venturi. The circular says it can occur at temperatures between 20 degrees and 70 degrees Fahrenheit (minus 7 to 21 degrees Celsius), and the pilot handbook adds that the sudden cooling in the carburetor means icing can occur even in outside air as warm as 100 degrees Fahrenheit (38 degrees Celsius) with humidity as low as 50 percent. Carburetor heat is the remedy. Fuel-injected engines are usually less vulnerable but can still be affected if the air source blocks with ice, and manufacturers provide an alternate air source for that case.

Turbine engines have their own problems. In marginal icing conditions the pressure drop at the inlet can cool the air enough to ice the inlet, and chunks that break off can damage fan blades or cause compressor stall or flameout. Ice crystals in high concentration tend to form ice deeper in the compressor. The circular reports that ice has accreted within engines at altitudes up to 42,000 feet. Above flight level 250, clouds are mostly ice particles with little liquid water, and the FAA says such clouds return little to weather radar, so they can be hard to detect.

How Icing Intensity Is Reported

Icing intensity is reported in terms of how fast ice accumulates and how the pilot has to respond. The FAA's representative rates are for the unprotected part of the outer wing.

Treat PIREPs with care. The circular says icing is extremely variable in space and time, and a slight change in altitude or a few minutes can mean the difference between icing and none. Reports also depend on the aircraft. It gives the example that a Boeing 747 may report light icing in conditions that would cause a Mooney to experience severe icing.

What Ice Does to an Airplane

Ice changes how an airfoil works, and the circular puts numbers on it.

• It raises stall speed. Ice sharply reduces maximum lift and the angle of attack at which a stall occurs. Ice that had little effect in cruise can cause a stall at the higher angle of attack of an approach. Even a thin layer of rough ice on the leading edge can have a significant effect, and the effect may be larger if ice forms behind areas normally protected.
• It adds drag. The pilot sees it as a loss of airspeed or more power needed to hold speed. The longer the encounter, the bigger the drag. In an airplane with limited power, the FAA says, it may soon approach stall speed.
• It can disturb roll control. The tips are thinner than the rest of the wing and collect ice most efficiently, which can cause a partial stall at the tips that affects the ailerons. A ridge of ice aft of the boots but forward of the ailerons, possibly from SLD, can interfere with the airflow even without a tip stall. The circular links this to the 1994 ATR-72 accident at Roselawn, Indiana.
• It can stall the tail. Most airplanes need downward lift from the tailplane. Actions that move the wing away from a stall, such as extending flaps, can increase the tail's negative angle of attack and stall it if it's iced. The FAA says a no-flap landing should be considered, consistent with the flight manual.
• It cuts thrust. Ice on propeller blades reduces thrust for the same reasons wings lose lift. In one suspected SLD flight test, ice along the whole span of the blades caused a 50-knot airspeed loss in 1 minute 25 seconds. The governor of a constant-speed propeller flattens the blade pitch to hold rpm, so the pilot sees no change in rpm or torque.
• It affects instruments and equipment. The pitot tube is vulnerable because even light icing can block the entry hole. Antennas that don't lie flush ice quickly, can vibrate with a "howl", and can break off. A stall warning vane or switch can freeze, and ice on the wing can make it stall before the normal warning activates, particularly on older designs. The circular says airplanes certificated to Amendment 23-43 or later have stall warning that works in their certified icing conditions.

Anti-Ice and De-Ice Equipment

The FAA draws a clear line between two jobs. Anti-icing equipment prevents ice from forming, and de-icing equipment removes it once it has formed. The circular adds that anti-icing is designed to be turned on before the aircraft enters icing conditions, and operating it as de-icing equipment may result in system failure or damage. As a rule of thumb, it says to activate anti-icing at the first signs of visible moisture in conditions conducive to icing, while the aircraft's flight manual is the final authority.

How early to use the boots has changed. The pilot handbook says "bridging", where ice expands over a boot instead of breaking off, doesn't occur with any modern boots, so pilots can cycle the boots as soon as ice appears. AC 91-74B agrees: studies since the late 1990s found few if any documented cases on modern boots, and the FAA recommends activating the deicing system at the first indication of icing, because activating the boots early and often never results in more ice on the wing than waiting. The circular also suggests continuing to cycle for a while after leaving the ice, so the wing is as clean as possible for the approach.

Most light aircraft have only a heated pitot tube and aren't certificated for flight in icing. The pilot handbook says those aircraft have limited cross-country capability in cooler climates from late fall through early spring. Ice protection isn't intended to sustain long-term flight in icing conditions. Our electrical systems guide covers the heated-element circuits, and our aircraft lights guide covers the wing inspection lights used to see ice at night.

Certified for Icing or Not

The FAA's pilot guide stresses that an aircraft certificated for instrument flight rules isn't necessarily certificated for flight in icing conditions. Pilots must check the flight manual or pilot's operating handbook. The circular says aircraft certificated for icing under Amendment 25-121 or higher go through an extensive process that includes analysis, wind-tunnel and dry-air testing, tests behind an icing tanker and flight in natural icing, and that certification includes testing and analysis showing the aircraft can hold in significant icing for up to 45 minutes. Even so, the circular says pilots of certificated aircraft shouldn't be casual about extended operations, because an unusual condition outside the tested envelopes is always possible.

Two limits are worth knowing. The circular cautions that certification for flight into known icing conditions doesn't include freezing drizzle and freezing rain, and that some airplanes are prohibited from flying into them regardless of intensity, so check the flight manual. SLD requirements for new transport category airplanes only became effective on November 4, 2014, and the circular says SLD hadn't been incorporated into certification of other aircraft types.

For small airplanes, the circular gives ways to tell whether yours is certificated for icing: the flight manual references "part 25 appendix C" icing conditions or 14 CFR 23.1419 at Amendment 23-14 or later, or the certification basis on the type certificate data sheet says the same. If the manual has only a minimum equipment list for icing conditions, the certification basis is earlier than 1973.

If your airplane isn't certificated for icing, the FAA says to avoid icing and to exit immediately if you meet it inadvertently. It adds three warnings. These airplanes have been involved in more icing accidents than icing-certified airplanes in recent years. The idea that thicker general aviation airfoils tolerate ice is a myth, because effects not apparent mid-envelope show up at the edge of it. And some of these airplanes have ice protection on the wings and tail as a margin for emergencies only: performance is unknown, stall warning most likely won't activate before the stall, and the controls may jam.

What the rules say

14 CFR 91.527 sets the operating rules, with a catch. The circular says Part 91 subpart F applies only to large and turbojet-powered multiengine airplanes and fractional ownership aircraft, and 91.527 falls within it, so for Part 91 operators outside subpart F it doesn't apply, although 91.9 still requires following the operating limitations in the flight manual. Parts 121, 125 and 135 have their own icing rules.

Flying With Ice: Approach and Landing

The circular is blunt that the dangerous part is the end of the flight. Accident statistics show the majority of icing-related accidents occur in the final phases, with configuration changes, low altitude, higher workload and reduced power all contributing. Ice picked up in cruise often seemed minor at the angle of attack of cruise. It matters at approach speeds. Its advice, always subject to the flight manual:

• Fly faster. Unless the manual or a placard specifies an airspeed for flight in icing, increase approach airspeed by at least 25 percent above the non-icing airspeed for the flap setting.
• Plan the runway. The FAA says to estimate that landing distance will increase 20 percent for each 10 percent increase in airspeed, and more on a contaminated runway.
• Be careful with flaps. Deploy flaps and slats in stages and watch how the aircraft behaves at each. Ice-contaminated tailplane stall rarely occurs before flaps are fully extended. Airplanes certificated for icing after 1994 were tested for it, so follow their flight manual limits. For an airplane certificated before 1994 and under 19,000 pounds, the FAA says to consider a partial- or no-flap landing.
• Carry power in the flare. With ice on the leading edges, expect a loss and unpredictable response of lift. Carry higher-than-normal power and limit the flare, since many icing accidents have been attributed to an induced stall during the flare.
• Cycle the boots before the approach. Boots may increase stall speed, and it's preferable not to use them while landing.

For a roll upset, the circular's steps are to reduce the angle of attack by reducing pitch, roll the wings level, set power and monitor airspeed and angle of attack, and not retract flaps unless the upper surface of the wing is known to be clear of ice. It warns that these steps resemble wing stall recovery and are in some respects opposite to recovery from tailplane stall, so identifying the right problem matters.

Before You Go

The circular's preflight advice starts on the ground. Remove all frost, snow and ice from the aircraft surfaces, because even very small amounts may adversely affect the wing's aerodynamics, and if you use a heated hangar, make sure the aircraft is dry before taking it out so moisture doesn't refreeze. Plan an exit strategy, such as whether a climb or descent is viable on the planned route, and note alternates. Expect worse climb performance and extra fuel burn in icing, and a higher approach speed at the destination. Consider a flashlight for checking for ice at night.

Pilots see cloud, temperature and visibility in a METAR before they fly. Our guide shows how to read and decode METARs, and our JFK Airport weather by month page shows how often a busy airport sits in low cloud and fog.

Frequently Asked Questions

What are the types of aircraft icing?

Structural icing forms on the airframe and comes in three types: clear, rime and mixed. Induction icing forms in the engine intake, and carburetor ice is the most common example.

What's the difference between clear ice and rime ice?

Clear ice is glossy, transparent and dense, and forms in conditions with large drops and temperatures close to freezing. Rime ice is rough, milky and brittle, and forms in low temperatures with small drops and less liquid water.

What temperatures does aircraft icing occur at?

Nearly all icing occurs in supercooled clouds below 0 degrees Celsius. The FAA says accumulation is often greatest at temperatures not too far below 0, and below about minus 20 degrees Celsius most clouds are made entirely of ice particles. Runback ice is possible between minus 5 and plus 2 degrees Celsius.

Why does ice make an airplane stall at a higher speed?

Ice sharply reduces the maximum lift a wing can produce and lowers the angle of attack at which it stalls. The FAA says a 30 percent reduction in maximum lift isn't unusual, and a thin layer of rough ice on the leading edge can have a significant effect.

What's the difference between anti-icing and de-icing?

Anti-icing equipment prevents ice from forming and should be turned on before entering icing conditions. De-icing equipment removes ice after it has formed.

Is an IFR-certified aircraft approved for flight in icing?

Not necessarily. The FAA says the flight manual or pilot's operating handbook must be checked to see whether the aircraft is certificated for flight in icing conditions.

What's freezing rain and why is it dangerous?

Rain that falls through a subfreezing layer and becomes supercooled, with drops 500 micrometers or larger. Ice forms far aft of where it would normally form, and the FAA cautions that certification for flight into known icing conditions doesn't include freezing rain or freezing drizzle.

What's tailplane icing?

Ice on the horizontal tail. On most airplanes the tail makes downward lift, and extending flaps can increase its negative angle of attack, so an iced tail can stall. The FAA says ice-contaminated tailplane stall rarely occurs until flaps are fully extended.

How much faster should you land with ice on the airplane?

Unless the flight manual says otherwise, the FAA says to increase approach airspeed by at least 25 percent above the non-icing speed for the flap setting, and to expect about 20 percent more landing distance for each 10 percent of extra speed.

What temperature does carburetor ice form at?

The FAA's icing circular says between 20 and 70 degrees Fahrenheit (minus 7 to 21 degrees Celsius), when moist air cools in the carburetor venturi, and the pilot handbook says it can happen even at outside air temperatures as high as 100 degrees Fahrenheit (38 degrees Celsius). Carburetor heat is the remedy.