1-1. PURPOSE. This advisory circular (AC) updates the previous version and contains essential information concerning safe flight in icing conditions, what conditions a pilot should avoid, and how to avoid or exit those conditions if encountered. The information provided is relevant to fixed-wing aircraft, including those operating under Title 14 of the Code of Federal Regulations (14 CFR) parts 91, 121, 125, and 135. The general guidance provided here in no way substitutes for aircraft-type-specific information in a particular Airplane Flight Manual (AFM) or pilot’s operating handbook (POH). This material is not regulatory, nor does it establish minimum standards. Where the term “must” is used in this AC, it reflects actual regulatory requirements; where the term “should” is used, it reflects recommendations from the Federal Aviation Administration (FAA).

1-2. CANCELLATION. AC 91-74A, Pilot Guide: Flight in Icing Conditions, dated December 31, 2007; and AC 91-51A, Effect of Icing on Aircraft Control and Airplane Deice and Anti-Ice Systems, dated July 19, 1996, are canceled.

 

1-3. DEFINITIONS.

a. Adiabatic Cooling. A process by which a parcel of air cools. When a parcel of air is lifted, pressure is reduced due to the elevation increase. This reduction in pressure causes the parcel of air to expand in volume and, in turn, the parcel cools to maintain an energy balance because no energy is added to the parcel.

b. Airmen’s Meteorological Information (AIRMET). In-flight weather advisories concerning weather phenomena of operational interest to all pilots and especially to pilots of aircraft not approved for flight in icing conditions. An AIRMET concerns weather of lesser severity than that covered by an advisory of significant meteorological information (SIGMET) or a convective SIGMET. AIRMETs may include advisories of moderate icing.

c. Automated Surface Observing System (ASOS). A suite of sensors that measure, collect, and disseminate weather data to help meteorologists, pilots, and flight dispatchers prepare and monitor weather forecasts, plan flight routes, and provide necessary information for correct takeoffs and landings. There are many differences between an ASOS and an All Weather Operations Specialist (AWOS) (see subparagraph 1-3d). It is important for pilots to understand the strengths and limitations of the various configurations. The ASOS is comprised of a standardized suite of weather sensors and is a product of a National Weather Service (NWS), Department of Defense (DOD), and FAA joint venture. One of ASOS’s most important features is its ability to detect precipitation, including intensity of rain, snow, and freezing rain. One current ASOS limitation is its inability to simultaneously detect and report freezing drizzle, ice pellets, or any other freezing precipitation without human augmentation when other forms of precipitation are present. A detailed description of ASOS’s capabilities can be found at the NWS ASOS homepage: http://www.nws.noaa.gov/asos/index.html.

d. Automated Weather Observation System (AWOS). A suite of weather sensors that are procured by the FAA or purchased by individuals, groups, airports, etc. It is important to note that the absence of reported precipitation does not mean that such conditions do not exist. The AWOS may not be configured to report this information or have precipitation reporting capability. A detailed description of AWOS’s capabilities can be found in the AIM.

e. Aviation Weather Service Program. Aviation weather service provided by the NWS and the FAA that collects and disseminates pertinent weather information for pilots, aircraft operators, and air traffic control (ATC).

 

f. Center Weather Advisory (CWA). An unscheduled weather advisory issued by NWS meteorologists for use by ATC in alerting pilots to existing or anticipated adverse weather conditions within the next 2 hours. A CWA may modify a SIGMET.

g. Clear Ice. A glossy, clear, or translucent ice formed by the relatively slow freezing of supercooled water drops. The terms “clear” and “glaze” have been used for essentially the same type of ice accretion, although some reserve “clear” for thinner accretions, which lack horns and conform to the airfoil. If the freezing becomes more rapid, clear ice will turn cloudy as small bubbles of air become trapped in the ice. If the conditions persist, the ice would be classified as mixed.

h. Convection. An atmospheric motion resulting in the transport and mixing of atmospheric properties.

i. Cumulus Clouds. Clouds in the form of detached domes or towers that are usually well defined. Cumulus clouds develop vertically in the form of rising mounds of which the bulging upper part often resembles a cauliflower; the sunlit parts of these clouds are mostly brilliant white. Their bases may be relatively dark and nearly horizontal.

j. Current Icing Product (CIP). A graphical planning product that combines sensor and numerical model data to provide a three-dimensional diagnosis of the probability and severity of icing, plus the potential for the presence of supercooled large drops (SLD). This product is automatically produced with no human modification. More information can be found on the Aviation Weather Center (AWC) Aviation Digital Data Service (ADDS) Web site.

k. Forecast Icing Conditions. Environmental conditions expected by an NWS or an FAA-approved weather provider to be conducive to the formation of in-flight icing on aircraft.

l. Forecast Icing Product (FIP). The FIP examines numerical weather prediction model output to calculate the probability and severity of icing conditions, plus SLD potential. This product is automatically produced with no human modification. More information can be found on the AWC ADDS Web site.

m. Freezing Drizzle. Drizzle is precipitation at ground level or aloft in the form of liquid water drops that have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle is water that remains in a liquid form at air temperatures less than 0 °C (supercooled) and can freeze upon contact with objects on the ground or in the air.

 

n. Freezing Rain. Rain is precipitation at ground level or aloft in the form of liquid water drops which have diameters greater than 0.5 mm. Freezing rain is rain that exists at air objects on the ground or in the air.

o. Front. The boundary between two air masses. A front can be classified as cold, warm, occluded, or stationary.

(1) Cold Front. Any nonoccluded front that moves in such a way that colder air replaces warmer air.

(2) Warm Front. Any nonoccluded front that moves in such a way that warmer air replaces colder air.

(3) Occluded Front. The front formed by a cold front overtaking a warm front and lifting the warm air above the Earth’s surface. An occlusion (or frontal occlusion) forms when an air mass is trapped between two colder air masses and is forced to higher and higher levels.

(4) Stationary Front. A front that has little or no movement because the opposing forces of the two air masses are relatively balanced.

p. Hazardous Weather Information. Summary of SIGMETs, Convective SIGMETs, urgent Pilot Weather Reports (PIREP), CWAs, AIRMETs, and any other significant weather provided to pilots that might not be routinely provided in a standard format or report.

q. Ice Crystals. Ice crystals, which are often in high concentrations near convective weather systems and lower concentrations in stratus or cirrus clouds, can accrete within turbine engines and cause power loss when in high concentrations. Ice crystals are not typically detected by either conventional ice detectors or airborne radar, and typically do not accrete on external airframe surfaces.

r. Icing Envelopes. Icing envelopes used for the certification of aircraft for flight in icing conditions specify atmospheric icing conditions in terms of altitude, temperature, Liquid Water Content (LWC), and drop size represented by the Median Volume Diameter (MVD). (The envelopes use the term mean effective diameter, but this equates to the MVD for the instrumentation and assumptions current at the time the envelopes were established.) There are two classes of icing envelopes: continuous maximum and intermittent maximum. The continuous maximum is for stratus-type clouds, and the intermittent maximum is for cumulus-type clouds.

s. Impingement. The striking and adherence of a water droplet on an aircraft surface. The impingement rate is the rate at which droplets of a given size collect on a particular surface. In general, impingement rates are higher for larger drops and smaller components, such as a very high frequency (VHF) or a Global Positioning System (GPS) antenna.

 

t. Known, Observed, or Detected Ice Accretion. Actual ice that is observed visually to be on the aircraft by the flightcrew or identified by onboard sensors.

u. Light Icing. The rate of ice accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Requires occasional cycling of manual deicing systems1 to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is ¼ inch to 1 inch (0.6 to 2.5 cm) per hour2 on the outer wing.3

v. Liquid Water Content (LWC). The total mass of water in all the liquid cloud drops within a unit volume of cloud. LWC is usually discussed in terms of grams of water per cubic meter of air (g/m3).

w. Median Volume Diameter (MVD). The diameter such that half the liquid water in a region of cloud is contained in drops of a smaller diameter, and half in drops of a larger diameter.

x. Mixed Ice. Simultaneous appearance of rime and clear ice or an ice formation that has the characteristics of both rime and clear ice.

y. Moderate Icing. The rate of ice accumulation requires frequent cycling of manual deicing systems1 to minimize ice accretions on the airframe. The rate of accumulation is such that anything more than a short encounter is potentially hazardous. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour4 on the outer wing.5

z. One-Minute Weather. The most recent 1-minute update weather broadcast based on ASOS/AWOS measurements and available to a pilot from an uncontrolled airport ASOS/AWOS.

aa. Orographic Cloud. A cloud that usually results from air flowing upslope from terrain and being cooled adiabatically.

bb. Outside Air Temperature (OAT). The measured or indicated air temperature outside the aircraft that is uncorrected.

 

cc. Pilot Briefing. A service provided by a Flight Service Station (FSS) or other FAA-approved provider that can assist pilots with flight planning. Briefing items may include weather information, Notices to Airmen (NOTAM), military activities, flow control information, and other items, as requested.

dd. Pilot Weather Report (PIREP). A report from a pilot of meteorological phenomena usually transmitted in a prescribed format. The letters “UA” identify the message as a routine PIREP while the letters “UUA” identify an urgent PIREP.

ee. Rime Ice. A rough, milky, opaque ice formed by the instantaneous freezing of small, supercooled water drops. It is generally rougher in appearance than clear ice.

ff. Runback Ice. Ice that forms from the freezing or refreezing of water leaving protected surfaces and running back to unprotected surfaces.

gg. Severe Icing. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and accumulation occurs in areas not normally prone to icing, such as aft of protected surfaces and other areas identified by the manufacturer. A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour6 on the outer wing. Immediate exit is required by many Airworthiness Directives (AD), flight manuals, and operations under part 91, §§ 91.13(a) and 91.527; part 121, § 121.341; part 125, § 125.221; and part 135, § 135.227.7

hh. Significant Meteorological Information (SIGMET). Information about in-flight weather of operational significance to the safety of all aircraft. SIGMETs may include severe icing. (See CWA and AIRMET.)

ii. Stagnation Point. The point on a surface where the local air velocity is zero. The region of maximum icing collection efficiency is near this point.

jj. Stratus Clouds. Clouds that form layers with a uniform base. Stratus clouds can appear in ragged patches and may produce drizzle, rain, or snow.

kk. Sublimation. A process in which ice turns directly into water vapor without passing through a liquid state.

ll. Supercooled Large Drops (SLD). Water drops with a diameter greater than 50 micrometers (0.05 mm) that exist in a liquid form at air temperatures below 0 °C. SLD conditions include freezing drizzle drops and freezing raindrops.

 

mm.Telephone Information Briefing Service (TIBS). A telephone recording of meteorological and/or aeronautical information obtained by calling an FSS.

nn. Trace Icing. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than ¼ inch (6 mm) per hour on the outer wing. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over 1 hour).

oo. Weather Advisory. In standard aviation weather forecast terminology, a warning of hazardous weather conditions not predicted in the forecast area that may affect air traffic operations. These reports are prepared by the NWS.

1-4. DISCUSSION. Aircraft icing remains a key aviation safety issue. Accident data has shown that pilots are (intentionally or inadvertently) flying aircraft not certificated for flight in icing conditions into such conditions, often with fatal results. Additionally, several accidents have involved aircraft that are certificated for flight in icing conditions. Such accidents are often the result of pilot complacency, lack of situational awareness (e.g., lack of awareness of loss of airspeed), poor technique, poor understanding of the airplane’s limitations and performance in icing conditions, misconceptions about certification of the airplane and systems for flight in icing, or a misunderstanding of icing terminology.

a. Certification. Pilots must determine if the aircraft to be flown is certificated for flight in icing conditions. An aircraft that is certificated for instrument flight rules (IFR) is not necessarily certificated for flight in icing conditions. To determine whether an aircraft is certificated for flight in icing conditions, the AFM or specific POH must be consulted. It is imperative that the pilot ensure the aircraft is certificated to fly in icing conditions and that the appropriate deicing/anti-icing equipment is installed and operational prior to operating in icing conditions. It is also critical that the pilot understand and comply with the applicable limitations and procedures when operating in icing conditions.

b. Flight Planning. If an aircraft is not certificated for flight in icing conditions, each flight should be planned carefully so that icing conditions are avoided. During a flight, the pilots should monitor available weather information (see Chapter 5, Icing Operations, on in-flight operations) and be aware of conditions that might require a change of flight plan to avoid icing conditions. In the event of an inadvertent icing encounter, the pilot should take appropriate action to exit the conditions immediately, coordinating with ATC as necessary, and declaring an emergency. In a recent study (American Institute of Aeronautics and Astronautics (AIAA) 200682, “A Study of U.S. Inflight Icing Accidents and Incidents, 1978 to 2002”), conflicts with ATC were common when pilots take action to exit icing conditions after an inadvertent icing encounter. Very often, this was because the pilot deviated from an IFR clearance and failed to declare an emergency or otherwise clarify the situation with the controller. In a subset of these cases, the controller actually offered to declare an emergency for the pilot, but the pilot declined. In another subset, the frequency was too busy for communications, often because the controller was overwhelmed with traffic. A number of pilots expected an immediate response from ATC when they reported difficulties after encountering ice and expected a blanket clearance to escape icing without first declaring a state of emergency. In many cases, such assumptions proved to be not only false, but fatal.

 

c. Engine Upsets. This AC also includes information about a recently identified icing threat, high-altitude ice crystal ingestion into turbine engines. Turbine engine upsets have occurred from ice accreting within the engine at altitudes up to 42,000 feet and temperatures colder than -45 °C (-50 °F). These high-altitude ice crystals in large concentrations, typically found near convective weather systems, do not accrete on external airframe surfaces and may not be visible on current-technology airborne radar systems.

d. Pilot Certification. Many pilots of aircraft certificated to operate in icing conditions have had numerous icing encounters in which the aircraft systems coped effectively with the icing conditions, despite a substantial ice buildup in some cases. However, a pilot should not relax his or her vigilance in icing conditions because of such experiences. A thin ice accretion on critical surfaces that develops in a matter of minutes can have dramatic effects on stall speeds, stability, and control. Wind tunnel testing indicates that if such accretions are particularly rough, they can have more adverse effects than larger accretions that are relatively smooth.


1 It is expected that deicing or anti-icing systems will be activated and operated continuously in the automatic mode, if available, at the first sign of ice accumulation, or as directed in the AFM.  Occasional and frequent cycling refers to manually activated systems.
2 These rates can be measured by a suitable icing rate meter.
3 It is assumed that the aircraft is approved to fly in the cited icing conditions. Otherwise, immediate exit from any of these intensity categories is required by regulations (14 CFR part 91 §§ 91.13(a) and 91.527, part 121 § 121.341, part 125 § 125.221, and part 135 § 135.227).
4 See footnote 2.
5 See footnote 3.
6 See footnote number 2.
7 Severe icing is aircraft dependent, as are the other categories of icing intensity. Severe icing may occur at any ice accumulation rate when the icing rate or ice accumulations exceed the tolerance of the aircraft.