Appendix 1
- AC 61-72B, Flight Instructor - Airplane - Written Test Guide.
-- AC 61-73A, Private & Commercial Pilot Rotorcraft Helicopter Written Test Guide.
- AC 61-74A, Flight Instructor - Rotorcraft, Helicopter Written Test Guide.
-- AC 61-75A, Flight Instructor - Glider Written Test Guide.
- AC 61-81A, Private & Commercial Pilot - Glider - Written Test Guide.
- Exam-O-Gram Number 21, "Flying Into Unfavorable Weather."
- Exam-O-Gram Number 28, "Factors Affecting Stall Speed."
-- DOD (USN) Film, "Ice Formation On Aircraft."
- FAA Film, "Some Thoughts on Winter Flying."
- FAA Slide Presentation, "Fog, Stratus, and Icing."
-- FAA Slide Presentation, "Thunderstorms and Turbulence."
- DOD Training Circular, Number 1-12, "Cold Weather Flying Sense," January 1978.
3. OTHER RELATED INFORMATION.
-- SAE, AMS 1425A, Deicing/Anti-Icing Fluid, Aircraft, Ethylene Glycol Base.
- SAE, AIR-1335, Ramp Deicing.
- FAA Film, "The Cold Front."
- FAA Film, "The Warm Front."
-- AOPA Article, "The Icing Options," APA Pilot, September 1981.
-- Wing Surface Roughness, Cause, and Effect by Ralph E. Brumby, Principal Engineer Aerodynamics published in Issue No. 32 of DC Flight Approach, January
- Methods of Prediction of the Influence of Ice on Aircraft Flying Characteristics by M. Ingelman Sundberg, O. K. Trunov, and A. Ivaniko; a joint report from the Swedish-Soviet Working Group on Scientific-Technical Cooperation in the Field of Flight Safety, Report No. JR-1, 1977.
-- Wind Tunnel Investigations of the Hazardous Tail Stall Due to Icing by M. Ingelman-Sunberg and O.K. Trunov; a joint report from the Swedish-Soviet Working Group on Scientific-Technical Cooperation in the Field of Flight Safety, Report No. JR-2, 1979.
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Appendix 1
- A Study of Some Methods and Means for Protecting Aircraft Against Ground Icing by O. K. Trunov and I. Aaro; a joint report from the Swedish-Soviet Working Group on Scientific-Technical Cooperation in the Field of Flight Safety, Report No. JR-4, 1980.
-- Roughness Penalties tor Flight Simulators, P. Haines and J. K. Luers, University of Dayton Research Institute, Dayton, Ohio, presented to the AIA 20th Aerospace Sciences Meeting, January 11-14, 1982, Orlando, Florida.
-- 737 Wing Leading Edge Condition published in July-September 1981 issue of Boeing Airliner Magazine.
-- 737 Wing Leading Edge Condition - Part II, published in the October-December 1981 issue of Boeing Airliner Magazine.
-- Mirabel Deicing Project 1978-1979, Airports and Construction Services Directorate, Airport Facilities Branch, Transport Canada, Air, Report No. IR 2159, May 1979.
- Cold Weather Operations, Frank J. Billand, published in the October-December 1982 issue of Boeing Airliner Magazine.
-- Recommendations for DE-ICING/ANTI-ICING of Aircraft on Ground, Association of European Airlines, October 1982.
-- Air Dynamic Effects of Deicing/Anti-icing Fluids, Service Letter, Boeing Commerical Airplane Company, December 2, 1982.
-- Ground Deicing and Anti-icing of Aircraft, Douglas Service, September/October 1982.
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Appendix 2 APPENDIX 2. METHODS OF ESTIMATING FREEZING POINT DEPRESSANT (FPD) FLUID EFFECTIVENESS.
1. GENERAL. Many variables can influence the time of the effectiveness of freezing point depressant (FPD) fluids, as discussed in appendix 3 of this advisory circular (AC), that make it impractical, if not impossible, to estimate that time following deicing or anti-icing processes. However, during eiforts to ascertain whether or not accurate estimates could be made, mathematical relationships were developed to allow estimates to be made using only a few known or assumed parameters. This appendix contains the rationale for these mathematical relationships only for the purpose of providing the basis of estimates presented in the body and appendix 3 of this AC. It is emphasized that the mathematical relationships derived herein are an oversimplification of the problem. Extreme caution is emphasized that these relationships not be usec for estimating time available between deicing and takeoff or as a substitute for RATIONALE. To estimate the time of effectiveness of FPD fluids in conditions of precipitation, several parameters must be known or assumed. To simplify this rationalization, figure 2-1 depicts a segment of a surface containing a film of FPD fluid of a certain depth (ff), and of a certain mixture M1 (% by weight of FPD in water). This fluid will freeze (begin to crystallize) at a given fluid temperature (tf) when its mixture (M2) reaches the freeze point (onset of crystallization). That is to say that at a given tf an additional amount of water must be mixed with the FPD fluid to turther dilute the mixture to M2. Precipitation in the form of snow, sleet, hail, freezing rain or drizzle or any other source of water such as dew, frost, spraying, and/or splashing can dilute the FPD mixture. If the rate of addition of water is known (Rw) the time to reach M2 can be estimated.
Precipitation Aircraft Surface FIGURE 2-1 Par 1
12/17/82 AC 20-117 Appendix 2 To further simplify this rationalization, figure 2-2 depicts schematically the relationships of the various parameters.
of FIGURE 2-2 of = Original fluid film thickness 0g = Quantity of FDP fluid (glycol) in the FPD/water mixture ow* Quantity of water in the original FPD/water mixture
o. = The quantity of water needed to further dilute the mixture M1 and M2 can then be calculated as follows:
M1 = Is the percentage of FPD in the mixture
1. -5,76. 72+8. +6.
2 Par 2
Appendix 2
a. From this relationship the amount of water needed to be added to the FPD fluid mixture can be determined since:
b. If a rate of addition of water (Rw) is known, the time (I) required to add the amount of water can be estimated:
T = Rw Rw M2
c. For example calculation assume:
фf = 0.1 mm Rg = 1.0 in/hour Rw = 1R = 0.1 in/hour M1 = 90% ethylene glycol Ta = + 20°F Under these assumptions M2 can be determined by referring to the phase diagram (figure 3-1, appendix 3) and at 20°F it can be seen that the mixture of ethylene glycol that will begin to freeze at 20°F is 168.
Therefore, M2 = 0.16 and under the above assumptions, time can be estimated as follows:
0.1 mm T -
0.1 in/hr = 2.38 Par 2 3
Appendix 3 APPENDIX 3. GENERAL INFORMATION RELATING TO GROUND AND FLIGHI OPERATIONS IN CONDITIONS CONDUCIVE TO AIRCRAFT ICING.
1. INTRODUCTION. This advisory circular (AC) deals with the hazards following ground deicing and ground operations in conditions conducive to aircraft icing.
This appendix provides general information necessary for the overall understanding of these hazards and includes causes and effects of ice formations (induced on the ground or in flight) as well as ground related issues such as:
methods of ground deicing, capabilities and limitations of freezing point depressant (FPD) ground deicing fluids, and discussions of variables that can influence the effectiveness of ground deicing fluids.
2. CONDITIONS CONDUCIVE TO AIRCRAFT ICING.
a. Aircraft on the ground or in flight are susceptible to accumulation of ice formations under various atmospheric and operational conditions. Aircraft in-flight can encounter a variety of atmospheric conditions that will individually or in combination produce ice formations on various components of the aircraft. These conditions include:
(1) Supercooled Clouds. Clouds containing water droplets (below 32°F) that have remained in the liquid state. Supercooled water droplets will freeze upon impact with another object. water droplets can remain in the liquid state at ambient temperatures as low as -40°F. The rate of ice accretion on an aircraft component is dependent upon many factors such as droplet size cloud liquid water content, ambient temperature, and component size, shape, and velocity.
(2) Ice Crystal Clouds. Clouds existing usually at very cold temperatures where moisture has frozen to the solid or crystal state.
(3) Mixed Conditions. Clouds at ambient temperatures below 32°F containing a mixture of ice crystals and supercooled water droplets.
(4) Freezing Rain and Drizzle. Precipitation existing within clouds or below clouds at ambient temperatures below 32°F where rain droplets remain in the supercooled liquid state.
(5) Frozen precipitation such as snow, sleet, or hail.
D. Aircrait on the ground, during ground storage or ground operations, are susceptible to many of the conditions that can be encountered in flight in addition to conditions peculiar to ground operations. These include:
(1) Supercooled ground fog and ice clouds.
(2) Operation on ramps, taxiways, and runways containing moisture, slush, or snow.
(3) Blown snow from snow drifts, other aircraft, buildings, or other ground structures.
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12/17/82 AC 20-117 Appendix 3
(4) Snow blown by ambient winds, other aircraft, or ground support equipment.
(5) Recirculated snow made airborne by engine, propeller, or rotor Operation of jet engines in reverse thrust, reverse pitch propellers, and helicopter rotor blades are common causes of snow recirculation.
(6) Conditions of high relative humidity that may produce frost formations on aircraft surfaces having a temperature at or below the frost point. Frost accumulations are common during overnight ground storage and after landing where aircraft surface temperatures remain cold following descent from higher altitudes. This is a common occurrence on lower wing surfaces in the vicinity of fuel cells. Frost formations can also occur on upper wing surfaces in contact with cold fuel.
3. THE EFFECTS OF ICE, SNOW, AND FROST FORMATIONS ON AIRCRAFT PERFORMANCE AND
FLIGHT CHARACTERISTICS General. During flight operations ice will form on leading edges of various components, within forward facing air intakes (e.g., engine inlets) and frontal areas of the airframe. During ground storage or operations, ice will form on other portions of aircraft components such as upper surfaces of wings, fuselages, engine nacelles, horizontal stabilizer surfaces, and control surfaces. The effects that inflight or ground accreted ice formations will have on aircraft performance and flight characteristics are many, are varied and are highly dependent upon aircraft design, ice surface roughness, ice shape, and areas covered. These effects will generally be reflected in the form of decreased thrust, decreased lift, increased drag, increased stall speed, trim changes, and altered stall characteristics and handling qualities. siight weight increases will also occur, however, the effect of weight increase (with the exception of heavy snow and freezing rain deposits during ground operations) is usually insignificant in comparision to aerodynamic degradation.
b. Aircraft Certification for Flight in Icing Conditions.
(1) Most commercial transport aircraft and many other aircraft types are designed for safe flight in most atmospheric icing conditions that can be encountered in conventional ilight; i.e., from takeoff to landing. They are not certificated for takeoff or flight with ice formed as a result of ground storage or operations. Such formations must be removed and the aircraft sustained in a clean configuration prior to initiation of takeoff and throughout the takeoff Although several helicopters are now in the initial phases of icing certification, no commercial helicopters are currently certificated for flight in icing conditions. Only a small number of helicopters have been certificated for flight in falling or blowing snow. Before an aircraft is certificated to fly in atmospheric conditions conducive to icing, that capability must be demonstrated to the FAA. This is accomplished through extensive analyses and tlight testing. It an aircraft is not so certificated, it should not be intentionally flown in atmospheric icing conditions. Aircraft that are certificated for flight in icing conditions are equipped with ice protection systems to reduce the adverse effects of ice formations, either by preventing the formation of ice (anti-icing) or by periodically removing ice (deicing).
Some components of some aircraft certificated for flight in icing conditions do 2 Par 2
Appendix 3 not require ice protection equipment. Aircraft so certificated have been demonstrated to be capable of safe flight with ice of certain shapes adhering to critical areas. Aircraft certificated for flight in icing conditions are capable of sustained operations in supercooled cloud conditions. Their engines and engine inlets are capable of operation without serious performance degradation in supercooled clouds. Some aircraft may have limited capability for flight in freezing rain and drizzle, in mixed conditions or pure ice crystal clouds, however, ice protection systems are certificated only for operation under the supercooled cloud conditions noted in paragraph za(l) above. Small aircraft are generally less tolerant to freezing rain conditions than large aircraft.
(2) Many aircraft in service today (generally small aircraft) have ice protection equipment installed, but are not certificated for flight in icing conditions. Aircraft of this type have only been demonstrated to show the equipment is nonhazardous for flight in nonicing conditions with the equipment installed. Aircrews should be aware of these limitations and be cautious because this type of equipment may not provide safe flight during icing encounters.
(3) Many aircraft in service today (generally large aircraft) are permitted by maintenance manuals to be dispatched for flight with slight amounts of frost adhering to fuel tank areas of wing undersurfaces. Maintenance manuals of such aircraft specify limits of frost thickness (generally between 1/8 and 3/8") depending upon the aircraft characteristics. It is emphasized that this practice is based upon operational experience only and no FAA certification or other test data has verified the accuracy of these limits. Operational experience as well as research experiments indicate that underwing frost formations do not generally influence aircraft performance and flight characteristics as severely as leading edge and upper wing frost; however, some wing designs may be more sensitive to underwing frost than others.
c. Effects of Ice Formations
(1) Wind tunnel and flight testing conducted in the past under research, development, and certification efforts, as well as operational experience, have shown that ice formations on various aircraft components can have very significant and sometimes devastating effects on aircraft equipment operation, aircraft performance, and flight characteristics. Components of an aircraft normally affected by ice formations are highly dependent upon aircraft design, however, they generally fall in the following categories: liftinc wies winant ta sad other transparen devices; stabilizing devices; control surtaces; engine inlets; engines;
propellers; rotor blades; anti-torque devices; windshields and other transparent structures; cooling air inlets; fuselage sections; antennas; undercarriage devices; fuel cap vents; and fuel tank vents.
(2) The effects of ice formations on some of these components and the contribution to degradation of aircraft performance and flight characteristics are itemized in the following list:
(1) Slight surface roughness can have significant effects on stall speed and power required to achieve or to sustain flight.
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Appendix 3
(ii) Surface roughness on the afterbody of a wing can have an effect approximately equal to the effect of similar surface roughness on the leading edge on some airfoils. Leading edge surface roughness is more significant on most airfoils.
(iii) Increased surface roughness, due to ice formation, on wing leading edges and afterbodies will produce additional drag and further reduce lift.
(iv) Due to increased stall speed, maneuvers should be more gentle and airspeed margin during approach should be increased.
(V) Stall angle-of-attack will decrease and in some aircraft stall will occur prior to activation of stall warning devices.
(vi) aircraft design, the nature of ice formations can cause either violent stall or a slower progression of stall. In some aircraft pitch-up tendencies may be greater and roll-off tendencies can be exaggerated.
(vii) Controllability may be reduced requiring more stick deflection for maneuvers or stall recovery.
(viii) Power available may be reduced due to ice formations on propellers or jet engine inlets.
(ix) Ice or excessive quantities of FPD fluid have been known to cause control surface flutter.
(x) Trim effectiveness can deteriorate with the accumulation of ice on unprotected surfaces.
(xi) Power failures may occur due to carburetor icing or ingestion of ice particles into jet engines or clogging of fuel tank vents and fuel caps.
(xii) Severe vibrations may occur due to asymmetric shedding of ice from propellers or rotor blades. Helicopter autorotation capability can be significantly changed or lost.
(xiii) Control surfaces such as ailerons, elevators, and wing spoilers can freeze in place if water deposits, snow, and FPD fluids are not properly cleaned or drained from critical areas.
(xiv) Wing flaps may be damaged if retracted with ice formations adhering to critical areas.
(xv) Landing gear mechanisms may be damaged or frozen in place if not properly cleared of ice formations.
(xvi) Forward visibility may be lost or significantly reduced if windshield anti-icing systems are not available or are not properly utilized.
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Appendix 3
(xvii) Radio, radar, and other communication and navigation antennas may be damaged or efficiency reduced due to ice formations.
(xviii) Ventilation, air conditioning, and other air inlets can be blocked or flow restricted.
(xiv) Ice dislodged from fuselage sections, antennas and other components forward of engine inlets and other critical components can produce damage.
(xx) Ice formations, under certain conditions, may not have noticeable effects on aircraft performance and flight characteristics, however, these effects may become quite apparent in the event of engine failure or other emergencies.
(xxi) Flight, engine, and other instruments are subject to error if ice formations exist on external probes, in pressure lines, or on areas forward of or adjacent to external probes. Operational experience indicates that typical sources of error are icing of pitot-static probes used for airspeed, altitude, and engine pressure ratio measurements.
(xxii) Automatic systems that utilize external signal sources such as AFCS, auto-pilots, autothrottle speed command systems, or stability augmentation systems may be adversely affected by ice formations on or in the vicinity of external sensors.
(xxiii) Residual moisture on door and cargo hatch seals may freeze under certain conditions causing leaks or seal damage.
4. METHODS OF DEICING OR CLEANING ICE FROM AIRCRAFT SURFACES.
a. Ground deicing procedures have been under development, practically speaking, since the time of the invention of the aircraft. Early methods employed the use of hangars to avoid exposure to the elements or use of wing covers and covers for other critical components such as windshields, engine air intakes, pitot probes, etc. But these devices were useful only to lessen the extent of work required to remove frost, snow, or other ice formations from the aircraft. Various devices such as brooms, brushes, ropes, squeegees, fire hoses, or other devices were used to remove dry snow accumulations but caution had to be exercised to preclude damage to aircraft skins and other critical components. Common sense prevailed. Many of these manual methods are still used today for both small and large aircraft. As larger aircraft were introduced and the numbers of air carrier fleets and scheduled flights increased, more expeditious and less costly procedures were developed. Thus, freezing point depressant (FPD) fluids were introduced to prevent or retard the formation of frost during overnight storage, to assist in melting and removal of frost, snow, or other ice formations such as would develop as a result of freezing rain or drizzle or for assisting in the removal of ice or frost formations accumulated during a previous flight.
b. Various methods of applying FPD fluids were utilized, such as mopping the fluid on the surface requiring treatment from a bucket, use of hand pumps Par 3 5
12/17/82 AC 20-117 Appendix 3 attached to a supply tank and spreading the solution with a mop, brush or other suitable devices to, in time, melt the ice to the extent that it could be removed by using manual means.
c. These manual methods of deicing provided a capability, in clear weather, to clean an aircraft adequately to allow a safe takeoff and flight.
inclement, cold weather conditions, however, the only alternative was to place the aircraft in a protected area such as a hangar to perform the cleaning process by whatever means were available. In freezing precipitation conditions, takeoff had to be initiated almost immediately following removal from the protected area. Common practice developed was to clean the aircraft in the hangar and provide a protective coating of FPD fluid (anti-icing) to protect the aircraft from ice or snow accumulation prior to takeoff.
d. Many of these techniques remain in use today, depending upon the local facilities and services that exist. However, most modern airports have traffic conditions and limitations of hangar space that, for the most part, preclude indoor ground deicing. These airports usually have one or more fixed base operators who have the equipment, capability, and experience to clean the aircraft and provide brief protection to allow safe takeoff to be performed.
Many airlines have prepositioned ground deicing equipment for ramp deicing at major airports where icing conditions are prevalent in the United States, Canada, and European countries. Several manufacturers of various types of aircraft ground deicing equipment exist today to meet the ground support equipment demands of the aviation community. These ground support equipments vary in types from simple trailers hauling a 55 gallon drum of FPD fluid with a wobble pump and mop to exotic equipment capable of heating and dispensing large quantities of water and deicing fluid and capable of elevating deicing personnel to neights necessary to have access to any area of the largest of today's aircraft. This technology exists and it is believed that any demand for aircraft ground deicing can be readily met by the ground support equipment industry.
Although modern and sophisticated ground support equipment exists, cost considerations sometimes dictate combination of ice removal methods. For example, heavy accumulations of snow may be more cost effectively removed using brooms, brushes, ropes, fire hoses, and other techniques followed by final cleaning with aqueous solutions of FPD fluid. Again, common sense, experience, and planning prevail to make the aircraft ground deicing process a cost etiective and sate operation. With the rising cost of petroleum products, the primary base of most commonly used aircraft deicing fluids, the expense of FPD fluid deicing has become a very significant parameter in the final decision process. An answer to this problem has been developed in recent years employing use of very hot water. Some manufacturers of such equipment have conducted extensive testing and evaluation to develop and perfect the procedures, precautions, and recommended aqueous FPD fluid mixtures necessary for cost effective, but safe, deicing. However, an understanding of the equipment, the characteristics, and limitations regarding use of FPD fluids is essential to assure safe winter operations.
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Appendix 3
5. CHARACTERISTICS OF FREEZING POINT DEPRESSANT (FPD) AIRCRAFT GROUND DEICING FLUIDS.
a. Deicing fluid manufactured in accordance with MIL-A-8243C is by specification 3 parts ethylene giycol and l part propylene glycol mixed with small quantities of water, corrosion inhibitors and wetting agents to allow smooth, even coverage of a surface. MIL-A-8243C fluid is specified in two Type I, used by the U.S. Air Force contains the ingredients described above while Type II, used by the U.S. Navy contains a small amount of additive to inhibit a chemical reaction between ethylene glycol and silver oxide. This additive is used to preclude a possible fire hazard relating to electrical relays and switches that incorporate silver oxide electrical contacts.
b. All known commercially available FRD fluids for aircraft deicing use are of the ethylene glycol or propylene glycol family. The exact formulae of various manufacturers' fluids are proprietary. It is very important to understand that some commercially available PPD fluids contain either ethylene glycol or derivatives of ethylene glycol such as diethylene glycol with small quantities of additives and water; that various FPD manufacturers, upon request, will premix aqueous solutions of FPD for specific customer reasons; and that before using a solution of FPD it is imperative that the ingredients be checked by close examination of the stock number and by a quality control examination to ascertain that the fluid supply conforms to the need. FPD fluid manufacturers can supply methodology and suggest equipment needed for quality control examinations.
phase diagram or freeze chart as illustrated in figure 3-1. It is emphasized that this diagram is not representative of any commercially available FPD fluids. FPD fluid manufacturers can furnish phase diagrams for their product.
This diagram illustrates the variation of the freezing point of various mixtures with water. FPD fluids are very soluble in water. The addition of glycol to water will lower the freezing point of the water mixture. The freeze point of glycol or a glycol/water mixture is not a sharp point as is the case with water alone. The freeze point of glycol or a glycol water mixture can be said to occur when crystals begin to form. As the temperature is lowered, glycol or water begins to crystallize and the mixture will assume a slushy consistency.
As cooling proceeds the mixture will thicken and may no longer flow and eventually if cooled to very low temperatures can solidify to a hard granular solid.
It can be seen from figure 3-1 that minimum freeze point (onset of crystallization) occurs when the mixture consists of approximately 60 percent glycol and 40 percent water. This is commonly referred to as the eutectic point. It can also be seen that beyond the eutectic point the freezing temperature of ethylene glycol is higher. This means that pure ethylene glycol will freeze (onset of crystallization) at warmer temperatures than aqueous solutions of ethylene glycol. Propylene glycol does not exhibit this characteristic as shown in figure 3-1. For this reason, all currently available commercial and military deicing fluids contain small quantities of water.
Military deicing fluids contain propylene glycol. This facilitates outside storage at cold temperatures. Generally, sufficient water is added to allow the Par 5