U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Pilot Guide: Flight In Icing Conditions Date: 12/31/07 Initiated by: AFS-800 AC No: 91-74A Change:
FOREWORD This advisory circular (AC) contains updated and additional information for the pilots of airplanes under parts 91, 121, 125, and 135 of Title 14 of the Code of Federal Regulations (14 CFR). The purpose of this AC is to provide pilots with a convenient reference on the principal factors related to flight in icing conditions and the location of additional information in related publications. This AC does not authorize deviations from established company procedures or regulatory requirements.
ORIGINAL SIGNED by Carol Giles for James J. Ballough Director, Flight Standards Service
CONTENTS Paragraph Page
CHAPTER 1. INTRODUCTION1
CHAPTER 2. TERMINOLOGY, ACRONYMS, AND ABBREVIATIONS3
2-1. Definitions3
CHAPTER 3. ATMOSPHERIC CONDITIONS ASSOCIATED WITH ICING9
3-1. Aircraft Icing Conditions9
3-2. Cloud Types and Aircraft Icing10
3-3. Fronts13
3-4. Convective Weather and Ice Crystals14
CHAPTER 4. ICING EFFECTS, PROTECTION, AND DETECTION15
4-1. Forms of Icing15
4-2. General Effects of Icing on Airfoils17
4-3. Effects of Icing on Wings18
4-4. Effects of Icing on Roll Control20
4-5. Tailplane Icing20
4-6. Propeller Icing21
4-7. Antenna Icing22
4-8. Cooling Inlet Icing22
4-9. Effects of Icing on Critical Systems22
4-10. Certification for Flight in Icing Conditions24
4-11. Significance of Icing Certification25
4-12. Ice Protection Equipment on Airplanes Not Certificated for Icing27
4-13. Anti-Icing Systems27
4-14. Deicing Systems28
4-15. Maintenance Considerations30
4-16. Icing Detection31
4-17. Visual Clues of SLD Conditions32
CHAPTER 5. FLIGHT PLANNING35
5-1. General Aviation Resources35
5-2. Air Carrier Dispatch Services35
5-3. Pireps35
5-4. Pirep Cautions36
5-5. Icing Forecasts37
5-6. Terminology Used in Icing Forecasts37
5-7. Current and Forecast Icing Products (CIP and FIP)38
5-8. Preflight Planning Information39
5-9. Airmet and Sigmet39
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iv
CHAPTER 6. IN-FLIGHT OPERATIONS41
6-1. Available In-Flight Information41
6-2. Pilot Strategies42
CHAPTER 7. ICING CONSIDERATIONS FOR THE PHASES OF FLIGHT45
7-1. Preflight45
7-2. Taxi47
7-3 Takeoff and Climbout47
7-4. Cruise47
7-5. Descent49
7-6. Holding49
7-7. Approach and Landing50
7-8. Wing Stall51
7-9. Ice-Contaminated Tailplane Stall (ICTS)51
7-10. Roll Upsets53
CHAPTER 8 SUMMARY55
APPENDIX 1, AIRCRAFT ACCIDENTS RELATED TO ICING (7 pages)1
APPENDIX 2, REGULATORY ISSUES RELATED TO ICING (7 pages)1
APPENDIX 3, ICING CHECKLISTS (8 pages)1
APPENDIX 4, RECOMMENDED READING (2 pages)1
CHAPTER 1. INTRODUCTION
1-1. This advisory circular (AC) updates the previous version and contains essential information concerning safe flight in icing conditions, what conditions should be avoided, and information on 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 specific Pilot’s Operating Handbook (POH).
1-2. This material is not regulatory, nor does it establish minimum standards. Where the term “must” is used in this AC, such use reflects actual regulatory requirements; where the term “should” is used, such reflects recommendations from the Federal Aviation Administration (FAA).
1-3. Aircraft icing remains a key aviation safety issue. Accident data has shown that pilots are (advertently and inadvertently) flying aircraft not certificated for flight in icing conditions into such conditions, often with fatal results. Even more disturbing are the numbers of accidents involving aircraft that are certificated for flight in icing conditions. Such accidents are often the result of pilot complacency, poor technique, poor understanding of the airplane’s limitations, and performance in icing conditions, misconceptions of airplane and system icing certification, and a misunderstanding of icing terminology. Education is the first step in reducing aircraft icing accidents.
1-4. Pilots must determine whether 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 ensures that the aircraft is certificated to fly in icing conditions and that the appropriate deicing/anti-icing equipment is installed and operational. It is also of critical importance that the pilot understands and complies with the aircraft manufacturer’s limitations and procedures when operating in icing conditions.
1-5. If an aircraft is not certificated for flight in icing conditions, each flight should be planned carefully so that icing conditions are avoided. Chapter 3 provides information on when and where icing conditions may occur and Chapter 5 discusses flight planning. The flight should be planned to avoid clouds or precipitation where temperatures are near or below freezing. During flight, the pilot should monitor the information available (see Chapter 6 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 air traffic control (ATC) as necessary, and declaring an emergency. In a recent study (AIAA 2006-82, “A Study of U. S. Inflight Icing Accidents and Incidents, 1978 to 2002”), conflicts with ATC are very prevalent. 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 offers to declare an emergency for the pilot, but the pilot declines. In another subset, the frequency is too busy for communications, often because the controller is overwhelmed with traffic. A number of Par 1-1 Page 1
Page 2 Par 1-5 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 also fatal.
1-6. This AC also includes information on 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.
1-7. 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 — in some cases, even with a substantial ice buildup. However, a pilot should not relax his/her vigilance in icing conditions because of such experiences. A thin ice accretion on critical surfaces, developing 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.
CHAPTER 2. TERMINOLOGY, ACRONYMS, AND ABBREVIATIONS
2-1. 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. Airman’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. AIRMETs concern weather of less severity than that covered by significant meteorological information (SIGMET) or Convective SIGMET.
AIRMETs may include moderate icing.
c. Automated Surface Observing System (ASOS)/Automated Weather Observation System (AWOS). 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 these two automated weather systems and 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 ASOS limitation that currently exists is its inability to detect and report freezing drizzle without human augmentation. Pilots should be aware of this limitation at airports deemed service level C or D, as described in the Aeronautical Information Manual (AIM). A detailed description of ASOS’s capabilities can be found at the NWS ASOS Homepage:
http://www.nws.noaa.gov/asos/index.html.
d. The AWOS is a suite of weather sensors that are procured by the FAA or purchased by individuals, groups, airports, etc. There are seven different configurations of AWOS: AWOS A, AWOS I, AWOS II, AWOS III, AWOS III-P, AWOS-T, and AWOS P/T. Precipitation identification is only available with AWOS III-P or higher. 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. A detailed description of AWOS’s capabilities can be found in FAA AC 150/5220-16C, Automated Weather Observing Systems (AWOS) for Non-Federal Applications. The AIM also contains useful information on both ASOS and AWOS.
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 ATC. Available aviation weather reports and forecasts are produced at NWS offices and displayed at FAA Flight Service Stations (FSS).
f. Center Weather Advisory (CWA). An unscheduled weather advisory issued by NWS meteorologists for use by ATC in alerting pilots of existing or anticipated adverse weather conditions within the next two hours. A CWA may modify a SIGMET.
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g. Clear Ice. A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets. 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.
h. Cold Front. Any nonoccluded front that moves in such a way that colder air replaces warmer air.
i. Convection. An atmospheric motion that is predominantly vertical, resulting in the transport and mixing of atmospheric properties.
j. Cumulus Clouds. Clouds in the form of detached domes or towers and 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.
k. 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). CIP is an unrestricted supplementary weather product.
l. Forecast Icing Product (FIP). The FIP examines numerical weather prediction model output to calculate the probability of encountering in-flight aircraft icing conditions. This icing product demonstrates the confidence that an atmospheric location, represented by a three-dimensional model grid box, will contain supercooled liquid water that is likely to form ice on an aircraft.
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 drizzle that exists at air temperatures less than 0°C (supercooled), remains in liquid form, and freezes upon contact with objects on the surface or airborne.
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 temperatures less than 0°C (supercooled), remains in liquid form, and freezes upon contact with 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.
p. Hazardous Weather Information. Summary of SIGMETs, Convective SIGMETs, urgent pilot weather reports, CWAs, AIRMETs, and any other weather, such as isolated thunderstorms rapidly developing and increasing in intensity or low ceilings and visibilities becoming widespread, that is considered significant and is not included in a current hazardous weather advisory.
q. Heavy Icing. The rate of ice accumulation requires maximum use of the ice protection systems to minimize ice accretions on the airframe. A representative accretion rate for Page 4 Par 2-1
reference purposes is more than 3 inches (7.5 cm) per hour1 on the outer wing. Consider immediate exit from the conditions.2
r. Icing Conditions.
(1) Potential Icing Conditions. Atmospheric icing conditions that are typically defined by airframe manufacturers relative to temperature and visible moisture that may result in aircraft ice accretion on the ground or in flight. The potential icing conditions are typically defined in the airplane flight manual or in the airplane operation manual.
(2) Forecast Icing Conditions. Environmental conditions expected by a NWS or an FAA-approved weather provider to be conducive to the formation of in-flight icing on aircraft.
s. Ice Crystals. Ice crystals, often found in high concentrations near convective weather systems, can accrete within turbine engines and cause power loss. Ice crystals are not typically detected by either conventional ice detectors or airborne radar, and typically do not accrete on external airframe surfaces. Ice crystals found in stratus and cirrus clouds are usually in relatively low concentrations, and do not represent a threat to turbine engines.
t. Icing Envelopes. These icing envelopes, found in 14 CFR part 25, appendix C, are used for the certification of aircraft for flight in icing conditions. They specify atmospheric icing conditions in terms of altitude, temperature, liquid water content (LWC), and droplet size represented by the median volume diameter (MVD). (The envelopes use the term mean effective diameter (MED), but this equates to the median volume diameter 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.
u. Impinge. The striking and adherence of a droplet on an aircraft surface. The impingement rate is the rate at which droplets of a given size impinge a particular surface. In general, impingement rates are higher for larger droplets and smaller components, such as antennae.
v. Known, Observed, or Detected Ice Accretion. Actual ice that is observed visually to be on the aircraft by the flight crew or identified by onboard sensors.
w. Light Icing. The rate of ice accumulation requires occasional cycling of manual deicing systems 3 to minimize ice accretions on the airframe. A representative accretion rate for
1 These rates can be measured by a suitable icing rate meter.
2 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.
3 See footnote number 2 Par 2-1 Page 5
reference purposes is 1⁄4 inch to one inch (0.6 to 2.5 cm) per hour 4 on the outer wing. The pilot should consider exiting the condition.5
x. Liquid Water Content (LWC). The total mass of water in all the liquid cloud droplets within a unit volume of cloud. LWC is usually discussed in terms of grams of water per cubic meter of air (g/m3).
y. Median Volume Diameter (MVD). The diameter is such that half the liquid water in a region of cloud is contained in smaller drops, and half in larger drops.
z. Mixed Ice. Simultaneous appearance of rime and clear ice or an ice formation that has the characteristics of both rime and clear ice.
aa. Moderate Icing. The rate of ice accumulation requires frequent cycling of manual deicing systems6 to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour 7 on the outer wing. The pilot should consider exiting the condition as soon as possible.8 bb. 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.
cc. 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.
dd. Orographic Cloud. A cloud that usually results from air flowing upslope from terrain and being cooled adiabatically.
ee. Outside Air Temperature (OAT). The measured or indicated air temperature (IAT) corrected for compression and friction heating. Also referred to as true air temperature.
ff. Pilot Briefing. A service provided by a Flight Service Station (FSS) to assist pilots with flight planning. Briefing items may include weather information, notices to airmen (NOTAMs), military activities, flow control information, and other items, as requested.
4 See footnote number 1.
5 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 §§ 91.13(a), 91.527, 121.341, 125.221, and
135.227).
6 See footnote number 1.
7 See footnote number 2.
8 See footnote number 3.
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gg. Pilot Report (PIREP). A report from a pilot of meteorological phenomena encountered by aircraft in flight, usually transmitted in a prescribed format. The letters “UA” identify the message as a routine PIREP while the letters “UUA” identify an urgent PIREP.
hh. Rime Ice. A rough, milky, opaque ice formed by the instantaneous freezing of small, supercooled water droplets.
ii. Runback Ice. Ice that forms from the freezing or refreezing of water leaving protected surfaces and running back to unprotected surfaces.
jj. Severe Icing. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer.
Immediate exit from the condition is necessary.9 kk. 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.)
ll. 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.
mm. Stationary Front. A front that has little or no movement because the opposing forces of the two air masses are relatively balanced.
nn. Stratus Clouds. Clouds that form layers with a uniform base. Stratus clouds can appear in ragged patches and may produce drizzle, rain, or snow.
oo. Sublimation. A process where ice turns directly into water vapor without passing through a liquid state.
pp. Supercooled Large Drop (SLD). A supercooled droplet with a diameter greater than 50 micrometers (0.05 mm). SLD conditions include freezing drizzle drops and freezing raindrops.
qq. Supplemental Weather Service Location. An airport facility staffed with contract personnel who take weather observations and provide current local weather to pilots via telephone or radio.
9 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. Icing certification implies an increased tolerance to icing intensities up through heavy.
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Page 8 Par 2-1 rr. Telephone Information Briefing Service (TIBS). A telephone recording of meteorological and/or aeronautical information obtained by calling an FSS.
ss. Total Air Temperature (TAT). Kinetic heating causes the Total (or Ram) Air Temperature (TAT) to be warmer than the Static Air Temperature (SAT). TAT is close to the temperature of the wing leading edge, which also experiences ram rise.
tt. 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 1/4 inch (6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they become worse. Pilots should be aware that any ice, even in trace amounts, could be potentially hazardous.
uu. Warm Front. Any nonoccluded front that moves in such a way that warmer air replaces colder air.
vv. 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.
CHAPTER 3. ATMOSPHERIC CONDITIONS ASSOCIATED WITH ICING
3-1. AIRCRAFT ICING CONDITIONS.
a. Nearly all aircraft icing occurs in supercooled clouds. Liquid droplets are present at outside air temperatures below 0°C (32°F) in these clouds. At outside air temperatures close to 0°C (32°F), the cloud may consist entirely of such droplets, with few or no ice particles present.
At decreasing temperatures, the probability increases that ice particles will be found in significant numbers along with the liquid droplets. In fact, as the ice water content increases, the liquid water content tends to decrease since the ice particles grow at the expense of the water particles. At temperatures below about -20°C (-4°F), most clouds are made up entirely of ice particles.
b. The general rule is that the more ice particles and the fewer liquid droplets that are present, the less ice accumulation on the airframe. This is because the ice particles tend to bounce off an aircraft surface, while the supercooled droplets freeze and adhere. As a result, ice accumulation is often greatest at temperatures not too far below 0 °C (32°F), where liquid water content can be abundant, and is usually negligible at temperatures below about -20°C (-4°F).
c. An exception to the general rule just stated may be made for surfaces heated by a thermal ice protection system (or by aerodynamic heating near the stagnation point of an aircraft component at speeds in excess of perhaps 250 knots). For such surfaces, ice particles may melt upon impact and then run back to colder aft regions and refreeze.
d. The higher liquid water content associated with temperatures near freezing is not the only concern. Tests have shown that at outside air temperatures near freezing, the total air temperature may be above freezing. The result is no ice accretion near the stagnation, but the refreezing of the water running back on the airfoil, causing runback ice accretions, possibly behind the protected areas. The formation of a ridge is possible. Pilots should be vigilant at total temperatures (TAT) between -5°C (23°F) and +2°C (35°F). Pilots should know whether the temperature instrument they are reading in the cockpit is outside air temperature (OAT) or TAT.
e. The greater the liquid water content of the cloud, the more rapidly ice accumulates on aircraft surfaces. The size of the droplets also is important. Larger droplets have greater inertia and are less influenced by the airflow around the aircraft than smaller droplets. The result is that larger droplets will impinge on more of the aircraft surface than smaller droplets.
f. Every supercooled cloud contains a broad range of droplets, starting from between 1 and 10 micrometers (millionth of a meter) and usually not exceeding 50 micrometers (by comparison, the thickness of the average human hair is approximately 100 micrometers). A single droplet size must be chosen as representative, and in icing terminology this is the MVD, the diameter such that half the liquid water is in smaller drops, and half in larger drops. An icing-certificated aircraft is certificated for flight in stratus-type clouds with MVDs up to 40 micrometers and for cumulus-type clouds with MVDs up to 50 micrometers. The MVD distribution is such that aircraft that undergo icing certification are not evaluated for operations in clouds with a significant amount of liquid water in droplets with diameters larger than 100 micrometers. Such conditions are sometimes encountered, and accidents and incidents in Par 3-1 Page 9