AC 20-117

117 Advisory U.S. Department of Transportation Circular Federal Aviation Administration AC - 64D DATE: March 29, 1988 NOTICE

RECENT ACCIDENTS INVOLVING LARGE TRANSPORT

AND SMALL GENERAL AVIATION AIRCRAFT HAS

PROMPTED THE FEDERAL AVIATION ADMINISTRATION TO RE-DISTRIBUTE AC 20-117, HAZARDS FOLLOWING

GROUND DEICING AND GROUND OPERATIONS IN

CONDITIONS CONDUCIVE TO AIRCRAFT ICING DATED 12/17/82.

0014139 Advisory U.S. Department of Transportation Circular Federal Aviation Administration

CONSOLIDATED REPRINT This consolidated reprint incorporates Change 1 Subject: HAZARDS FOLLOWING GROUND Date: 12/17/82 AC No: 20-117 DEICING AND GROUND OPERATIONS Initiated by: AWS-100 Change:

IN CONDITIONS CONDUCIVE TO

AIRCRAFT ICING

1. PURPOSE. To emphasize the "Clean Aircraft Concept" following ground operations in conditions conducive to aircraft icing and to provide information to assist in compliance.

RELATED FEDERAL AVIATION REGULATIONS (FAR) SECTIONS. Sections 121.629,

91.209, and 135.227.

3. BACKGROUND. Recent accidents involving large transport and small general aviation aircraft indicate that misconceptions exist regarding the effect of slight surface roughness caused by ice accumulations on aircraft performance and flight characteristics and the effectiveness of Freezing Point Depressant (FPD) ground deicing and anti-icing fluids. During development of information contained herein it was recognized that guidance information should be directed to all segments of aviation to include aircraft manufacturers; airline engineering, maintenance, service and operations organizations; aircraft maintenance and service personnel; and aircrews of all aircraft types and categories. Information contained herein therefore is general in nature for basic understanding purposes to facilitate development of standardized procedures and guidance by various segments of the aviation industry. The FAA will assist in development of specific industry standards and will publish additional advisory information as necessary.

4. DISCUSSION.

a. Regulations were established by the Civil Aeronautics Board (CAB) in 1950 prohibiting takeoft of aircrait when frost, snow, or ice is adhering to wings, propellers, or control surfaces of the aircraft. These regulations remain in effect as cited under FAR 121.629, 135.227, and 91.209. The basis of these regulations, which are commonly referred to as the clean aircraft concept, is known degradation of aircraft performance and changes of aircraft flight characteristics when ice formations of any type are present. These effects are wide ranging, unpredictable, and dependent upon individual aircraft design. The magnitude of these changes is dependent upon many variables and is thus unpredictable, but these changes can be significant. wind tunnel and flight tests indicate that ice, frost, or snow formations on the leading edge and upper suriace of a wing, having a thickness and surface roughness similar to medium or coarse sandpaper, can reduce wing lift by as much as 30 percent and increase drag by 40 percent. These changes in lift and drag will significantly increase stall speed, reduce controllability and alter aircraft flight characteristics.

Thicker or rougher ice accumulations in the form of frost, snow, or ice deposits can have increasing effects on lift, drag, stall speed, stability, and control, but the primary influence is surface roughness relative to critical portions of an aerodynamic surface. It is therefore imperative that takeoff not be attempted unless it has been ascertained, as required by regulation, that all critical components of the aircraft are free of adhering snow, frost, or other ice formations.

b. Most transport aircraft used in commercial transportation as well as some other aircraft types are certificated for flight in icing conditions. It is emphasized that to date rotorcraft and most small, general aviation fixed wing aircraft have not been certificated by the FAA for flight in icing conditions. Aircraft so certificated have been designed and demonstrated to have the capability of penetrating supercooled cloud icing conditions in the forward flight regime. This capability is provided either by ice protection equipment installed on critical surfaces (usually the leading edge) or demonstration that ice formed, under supercooled cloud icing conditions, on certain unprotected components will not significantly affect aircraft performance, stability and control. Ice, frost, or snow formed on these surfaces on the ground can have a totally different effect on aircraft flight characteristics than ice formed in flight. Exposure to weather conditions on the ground that are conducive to ice formation can also cause accumulation of frost, snow, or ice on ice protected areas of the aircraft that are designed for inflight use only and that are not designed for use during ground operation. In addition, aircraft are considered airworthy and are certificated by the FAA only after extensive analyses and testing have been accomplished. With the exception of analyses and testing to ascertain the flight characteristics of an aircraft during flight in icing conditions, all analyses and certification testing are conducted with a clean aircraft flying in a clean environment. If ice formations are present, other than those considered in the certification process, the airworthiness of the aircraft may be invalid and no attempt should be made to fly the aircraft until it has been restored to the clean configuration. The ultimate responsibility for this determination rests with the pilot in command of the aircraft.

Common practice developed by the North American and European aviation community over many years of operational experience is to deice an aircraft prior to takeoff. Various techniques of ground deicing were also developed.

The most modern of these techniques is use of FPD fluids to aid the ground deicing process and to provide a protective film of FPD (anti-icing) to delay formations of frost, snow, or other ice.

In scheduled airline operations, where large numbers of aircraft are dispatched, the process of assuring airworthiness must be a team effort where each member of the team has specific duties and responsibilities. In the case of private aircraft operations, all functions may be performed by only one person, the pilot. In all cases, the pilot has the ultimate responsibility of ascertaining that the aircraft is in a condition for safe flight.

The only method currently known of positively ascertaining that an aircraft is clean prior to takeoff is by close inspection. Under conditions of precipitation or where moisture can be splashed, blown, or sublimated onto 2 Par 4

critical surfaces in subfreezing weather, many factors influence whether ice, frost, or snow may accumulate and result in surface roughness.

These variables are described in appendix 3 of this advisory circular (AC) but for convenience are listed as follows:

- Ambient Temperature

- Aircraft Surface Temperature

- Presence of Deicing Fluid

- Deicing Fluid Type

- Deicing Fluid Aqueous Solution (Strength)

- Precipitation Type and Rate

- Deicing Fluid Application Procedure

- Relative Humidity

- Solar Radiation

- Operation in Close Proximity to other Aircraft, Equipment, and Structures

- Operation on Snow, Slush, or Wet Surfaces

- Wind Velocity and Direction

- Aircraft Component Inclination Angle, Contour, and Surface Roughness Aircraft maintenance and operations personnel neither have the capability to quantify the occurrence or the effects of the many variables that can influence whether ice, frost, or snow may form prior to takeoff, the surface roughness of ice formations, nor the effect that surface roughness may have upon aircraft performance and handling characteristics. Therefore, the time that may be considered a safe interval between ground deicing and takeoff cannot be estimated. Calculations of time incorporating the effects of only a few of these variables (e.g., ambient temperature of 20°F, fluid strength of 50 percent, precipitation rate of 1/2 inch/hour, assumed water content of snow of

0.1, and assumed surface film thickness of FPD fluid of 0.1 mm) reveals that aircraft surfaces may remain free of ice formations (onset of FPD fluid crystallization) for approximately 10 minutes. Other variables listed above could reduce this time. Since neither the pilot in command nor ground support staffs have even these limited facts on hand, quantitative judgements of time available between the ground deicing or anti-icing process and takeoff cannot be conducive to icing is the clean aircraft concept. To understand the need for Par 4 3

qualities; (2) the various procedures that are available for aircraft ground deicing and anti-icing; (3) the capabilities and limitations of these procedures; (4) the variables that will influence the effectiveness of these procedures; (5) the critical areas of the particular aircraft; and (6) recognition that final assurance for a safe takeoff rests in pretakeoff inspection. Additional information to assist in development of this understanding and knowledge may be found in the appendices of this A.

success of the aviation community to date is attributed to many years of experience on the part of many companies where this knowledge has been gained, through experience, and passed on in the form of policy, procedures, quality assurance programs, and training programs.

5. ACCEPTABLE PRACTICES.

a . General. The clean aircraft concept is essential. The FAR makes the clean aircraft concept law. This law exists for flight safety reasons. The FAR states a general requirement but allows operators to comply with the requirement in an appropriate manner, depending upon local circumstances. The clean aircraft concept has been in effect since 1950. Many techniques of complying with the clean aircraft concept have been developed over the years by the aviation industry. Many of these techniques were developed prior to 1950 because of the need recognized by the aviation community. The consensus of the aviation community and the conclusion reached by the FAA is that the only method of assuring flight safety following ground operations in conditions conducive to aircraft icing, is by either close inspection prior to takeoff to ascertain that or a determination that any formations are not adhering to critical surfaces and will blow off in the early stages of takeoff roll. ihis consensus is valid regardless of the use of currently available FPD deicing fluids or the use of manual techniques of deicing. FPD fluids commonly used today should not be considered to have anti-icing qualities for a finite period of time because a multitude of variables make it impractical to estimate that time. However, under certain condition FPD fluids are known to be effective in retarding the formation of frost, snow, or ice and in this sense may be considered to have anti-icing qualities (to prevent the formation of ice) for a period of time during ground storage (overnight or during brief layover) thus making the process of deicing (removing ice formations) simpler and in many cases negating further deicing or treatment. It is emphasized, however, that the need for close inspection prior to takeoff remains. The following paragraphs are intended to provide suggested methods of assuring the clean aircraft concept.

(1) Aircraft Deicing and Anti-Icing.

(i) An airplane may be cleaned of ice formations (deiced) by any suitable manual method, by use of water, by use of FPD fluids, or mixtures of of FPD fluids for application to rotorcraft. Heated water, FPD fluids or aqueous solutions of FPD fluids are more effective in the deicing process. The deicing and anti-icing process may be performed in one stage or multiple stage processes as desired depending upon prevailing conditions, concentration of FPD utilized, facilities available and deicing methods. In any case the freeze point of residual fluids (water, FPD fluids or mixtures) should not be greater than 20°F below ambient or surface temperature whichever is less. Unheated FPD 4 Par 4

12/17/82 AC 20-117 fluids or aqueous solutions are more effective in the anti-icing process than heated fluids.

(ii) In conditions of freezing precipitation or high humidity when aircraft surface temperatures are near or below freezing and when it cannot be determined that snow or other ice crystal accumulations are not adhering and will blow off during initial stages of takeoff, surfaces should be anti-iced to retard the formation of ice prior to takeoff.

(iii) FPD freeze point can be determined using refractive index techniques. FPD fluid manufacturers can suggest or supply suitable equipment.

(iv) Critical surface temperatures under many circumstances are found in the vicinity of integral wing fuel tanks. When fuel temperatures are higher than ambient, critical surface temperatures will occur at other locations. These temperatures can be determined by direct measurement or by estimating fuel temperature. If surface temperature is not measured or estimated then the freeze point of residual fluids should be the lowest possible with available fluids.

(v) In conditions of nonprecipitation an anti-iced aircraft should be closely inspected to assure the freeze point of residual fluids remain 20°F below ambient or surface temperature whichever is lower. This is especially important when relative humidity is high.

(vi) Underwing frost should be removed and, where practical, the surface anti-iced to delay re-formation of frost. See appendix 3 for additional information on this subject.

(2) Preflight Inspection. Preflight inspection should be performed immediately following or during the ground deicing and anti-icing process.

Areas to be inspected depend upon the aircraft design and should be identified in an inspection checklist. The inspection checklist should include all items recommended by the aircraft manufacturer and may be supplemented, as necessary, to include special operational considerations, but this checklist should include the following general items:

- Wing leading edges, upper surfaces, and lower surfaces

- Stabilizing device leading edges, upper surfaces, lower surfaces, and side panels

- High lift devices such as leading edge slats and leading or trailing edge flaps

- Wing lift spoilers

- All control surfaces and control balance bays

- Propellers

- Rotor Blades, rotor heads and controls Par 5 5

12/17/82 AC 20-117

- Critical rotor system devices such as droop stops

- Engine inlets, particle separators and screens

- Windshields and other transparencies necessary for vasibility

- Antennas

- Fuselage sections forward of stabilizing, control and lifting surfaces, propellers, rotors, or engine air inlets

- Exposed instrumentation devices such as angle-of-attack vanes, pitot-static pressure probes, and static ports

- Fuel tank and fuel cap vents

- Cooling and APU air intakes/inlets/exhausts

- Undercarriage

(3) Once it has been determined through pre-flight inspection that the aircraft is clean and adequately protected, the aircraft should be released for takeoff as soon as possible. This is especially important in conditions of precipitation or high relative humidity.

(4) Pretakeoff Inspection.

(1) Fixed Wing Aircraft (A) Just prior to taking the active runway for takeoff or just prior to initiating takeoff roll, a visual pretakeoff inspection should be made.

The components to be inspected depend upon aircraft design. In some aircraft, the entire wing and portions of the empennage are visible from the cockpit or the cabin. In other aircrait, these suriaces are so remote that only portions of the upper surface of the wings are in view. Undersurfaces of wings and undercarriage are not viewable in any but high-wing type aircraft. A practice in use by some operators is to perform close visual inspection of wing surfaces, leading edges, engine inlets, and other components of the aircraft that are in view either from the cockpit or cabin (whichever provides maximum view). If surfaces have not been treated with FPD fluid, evidence of melting snow and possible freezing is sought. Also evidence of any ice formation that may have been induced by taxi operations is sought. If the aircraft has been treated with FPD fluids, evidence of a glossy smooth and wet surface is sought. If, as a result of these inspections, evidence of ice, snow, or frost formations is observed, the aircraft should be returned to a maintenance area for additional deicing.

(B) The fact that it is impractical for an aircraft crewmember to disembark at the end of a runway and perform pretakeoff inspections, means that the crewmember should perform that inspection from the best vantage point available from within the aircraft. The crewmember may elect to open windows, doors, or hatches to improve the view, but in many aircraft even this is impractical. In the darkness of night the crewmember must rely upon wing and 6 Par 5

other aircraft illumination lights that may not provide sufficient reflection to make appropriate visual observations. The crewmember may, where practical, call upon the assistance of qualified ground personnel. If under any circumstance, the pilot in command cannot ascertain that the aircraft is clean, takeoff should not be attempted.

(C) Conducting pretakeoff inspection in the manner described relies upon the pilot in command to be knowledgeable of ground deicing procedures, that the ground deicing process was conducted in a thorough and uniform manner, and that critical surfaces or components not in view during pretakeoff inspection will also be clean. The decision to takeoff, following pretakeoff inspection remains the responsibility of the pilot in command.

(ii) Rotorcraft.

(A) Only rotorcraft that have been certificated for flight in icing conditions should be operated in conditions conducive to icing such as freezing fog. To date none have been so certificated by the FAA.

(B) Rotorcraft certificated for flight in falling and blowing snow may operate in such conditions. In this case pretakeoff inspection of rotor systems should be conducted just prior to starting rotors turning. Rotor systems should not be started unless blade surfaces and other critical components are free of ice, frost or adhering snow.

b. Common practices or suggested practices necessary to assure the pilot has every advantage for his judgements:

(1) Establish training programs to continually update pilots on the hazards of winter operations, adverse effects of ice formations on aircraft performance and flight characteristics, proper use of ice protection equipment, ground deicing and anti-icing procedures, and preflight and pretakeofi inspectlon procedures following ground deicing or anti-icing and operatlons ir

(2) Establish training programs for maintenance or other personnel who perform aircraft deicing to assure thorough knowledge of the adverse effects of ice formations on aircraft performance and flight characteristics, critical components and specific ground deicing and anti-icing procedures for each aircraft type, and the use of ground deicing and anti-icing equipment including detection of abnormal operational conditions.

(3) Establish quality assurance programs to assure that FPD fluids being purchased and used are of the proper characteristics, that proper ground deicing and anti-icing procedures are utilized, that all critical areas are inspected, and that all critical components of the aircraft are clean prior to departure.

(4) Perform thorough planning of ground deicing activities to assure that proper supplies and equipment are available for forecast weather conditions and that responsibilities are specifically assigned and understood. This is to include maintenance service contracts.

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(5) Monitor weather conditions very closely to assure that planning information remains valid during the ground deicing or anti-icing process and subsequent aircraft operations. FPD fluids, deicing or anti-icing procedures and departure plans should be altered accordingly.

(6) Use FPD concentrations that will delay ice formations for as long a period as possible under the prevailing conditions.

(7) Deice or anti-ice areas that may be viewed by the pilot (from inside the aircraft) first so that during pretakeoff inspection he may have assurance that other areas of the aircraft are clean since areas deiced or anti-iced first will generally freeze first.

(8) Use the two-stage deicing process where ice deposits are first removed, and then all critical components of the aircratt are coated with an appropriate mixture of FPD fluid (anti-icing) to prolong effectiveness.

(9) Assure thorough coordination of the ground deicing and anti-icing process so that final treatments are provided just prior to takeoff.

(10) Use remote sites near the take-off position, where feasible, for deicing or anti-icing to reduce the time between deicing and takeoff or to provide additional FPD fluid to prolong anti-icing effectiveness.

(11) Use multiple aircraft deicing or anti-icing units for faster and more uniform treatment during precipitation.

(12) Use FPD fluids that are approved for use by the aircraft flight characteristics or cause control surface instabilities.

(13) Do not use substances that are approved for use on pneumatic boots (to improve deicing performance) for other purposes unless such uses are approved by the aircraft manufacturer.

c. Suggested practices for pilots to assure the clean aircraft concept.

(1) Be knowledgeable of the adverse effects of surface roughness on aircraft performance and flight characteristics.

(2) Be knowledgeable of ground deicing and anti-icing practices and procedures being used on your aircraft whether this service is being performed by your own company, a service contractor, or a fixed-base operator.

Do not allow deicing or anti-icing until you are familiar with the ground deicing practices and quality control procedures of the service organization.

(4) Be knowledgeable of critical areas of your aircraft and assure these areas are properly deiced and anti-iced, proper precautions are being taken during the deicing process to avoid damage to aircraft components, and 8 Par 5

12/17/82 AC 20-117 proper preflight inspections are performed even though this is also the responsibility of other organizations or personnel.

(5) Be knowledgeable of ice protection system function, capabilities, limitations, and operation.

(6) Perform additional preflight inspections related to deicing or anti-icing as necessary or required.

(7) Be aware that no one can accurately determine the time of effectiveness of an FPD deicing or anti-icing treatment because of the many variables that can influence this time.

(8) Be knowledgeable of the variables that can reduce time of effectiveness and their general effects.

(9) Assure that deicing or the anti-icing treatment is performed at the last possible time prior to taxi to the takeoff position.

(10) Do not start engines, propellers, or rotor blades until it has been ascertained that all ice deposits are removed. Ice particles shed from rotating components under centrifugal and aerodynamic forces can be lethal.

(11) Be aware that certain operations may produce recirculation of ice crystals, snow or moisture.

(12) Be aware that operations in close proximity to other aircraft can induce snow, other ice particles, or moisture to be blown onto critical aircraft components, or allow dry snow to melt and refreeze.

(13) Do not takeoff if snow or slush is observed splashing onto critical areas of the aircraft, such as wing leading edges, during taxi.

(14) Always perform pretakeoff inspections just prior to takeoff.

(15) Do not takeoff if positive evidence of a clean aircraft cannot be ascertained.

M. C. BEARD Director of Airworthiness, AWS-1 Par 5

Appendix 1 APPENDIX 1. RELATED FAA PUBLICATIONS, TRAINING MATERIAL, AND OTHER READING

MATERIAL

1. RELATED FAA PUBLICATIONS.

-- Air Carrier Operations Bulletin (ACOB), No. 7-81-1, Aircraft Deicing and Anti-Icing Procedures, dated April 10, 1981, DOT/FAA Order 8430.17, Change 21.

- ACOB, No. 7-76-9, Aircraft Control and Lifting Surfaces-Cold Weather Operations, October 20, 1976.

-- ACOB, No. 7-76-11, Engine Ice Accumulation - Ground Idle (Formerly Air Carrier Operations Alert, No. 70-9), October 20, 1976.

-- ACOB, No. 7-76-12, Turbojet Aircraft Engine Icing During Prolonged Ground Operations in Icing Conditions (Formerly Air Carrier Operations Alert No. 69-3), October 20, 1976.

-- ACOB, No. 7-76-13, Takeoff Warning Systems During Cold Weather Operations, October 20, 1976.

-- ACOB, No. 7-76-2, Water/Snow Entering Parked Boeing 727 Aircraft Causing Electrical Power Loss, October 20, 1976.

-- ACOB, No. 7-76-1, Winter Operations Under FAR 121 and 127 (Formerly Air Carrier Operations Bulletin No. 68-15), October 20, 1976.

- Air Carrier Operations Alert (ACOA), No. 67-3, Aircraft Control and Lift Surfaces - Cold Weather Operations.

-- ACOA No. 69-3, Turbojet Aircraft Engine Icing During Prolonged Ground Operations in Icing Conditions.

-- ACA No. 70-9, Engine Ice Accumulation - Ground Idle.

--Air Carrier Maintenance Bulletin (ACMB) No. 115, Deicing of Aircraft with Engines Operating.

- ACMB 127, Aircraft Deicer Fluids.

- ACMB 128, Responsibility for Aircraft Servicing.

- ACMB 155, Winter Operation of Aircraft.

- AC 00-6A, Aviation Weather, March 3, 1975.

-- AC 00-45B, Aviation Weather Services, 1979.

- AC 20-73, Aircraft Ice Protection, April 21, 1971.

- AC 20-93, Flutter Due to Ice or Foreign Substance On Or In Aircraft Control Surfaces, January 29, 1976.

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Appendix 1

- AC 20-113, Pilot Precautions and procedures to be taken in Preventing Aircraft Reciprocating Engine Induction System & Fuel System Icing Problems, Oct. 22, 1981.

- AC 20-106, Aircraft Inspection for the General Aviation Aircraft Owner, April 1978.

- AC 61-84A, The Role of Preflight Preparation, December 1, 1980.

- AC 91-6A, Water, Slush, and Snow on the Runway, May 24, 1978.

-- AC 91-13C, Cold Weather Operation of Aircraft, July 24, 1979.

- AC 91-51, Airplane Deice and Anti-Ice Systems, September 15, 1971.

- AC 135-9, FAR Part 135 Icing Limitations, May 30, 1981.

-- AC 121-12, Wet or Slippery Runways, August 17, 1967.

-- AC 150/5380-4, Ramp Operations During Periods of Ice and Snow Accumulations, September 11, 1968.

- FAA RD-80-50, Engine Inlet Anti-Icing System Evaluation Procedure (Report).

- ASF 140-75-3, Report of Propulsion Conference and Environmental Workshop.

- Aircraft Ice Protection, Report of Symposium, April 28-30, 1969.

- FAA General Aviation News Article "Rime Without Reason."

-- FAA Pamphlet "Weather (Pilot's) How It Is Forecast."

-- FAA Pamphlet "The Weather Decision."

2. TRAINING MATERIAL.

-- AC 61-8D, Instrument Rating, Written Test Guide.

-- AC 61-18B, Air Transport Pilot (Airplane, written test guide).

- AC 61-21A, Flight Training Handbook.

-- AC 61-23B, Pilots Handbook of Aeronautical Knowledge.

-- AC 61-31B, Gyroplane Pilot Written Test Guide, Private and Commercial.

- AC 61-70A, Flight Instructor, Instrument Airplane, Written Test Guide.

- AC 61-71B, Commercial Pilot - Airplane - Written Test Guide.

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