U.S. Department AC 120-58 of Transportation DATE: 9/30/92 Federal Aviation Administration
PILOT GUIDE
LARGE AIRCRAFT GROUND
DEICING INITIATED BY: AFS-400
PREFACE This advisory circular (AC) contains recommendations for ensuring the safe operation of large airplanes during icing conditions and guidelines for the development of adequate procedures for the deicing of large airplanes. It is designed for the use of flight crewmembers, maintenance and servicing personnel, and other aviation personnel responsible for ground deicing and aviation safety in general. The guidelines and procedures offered in this AC are advisory in nature and do not carry the force of a regulatory requirement. However, prudent operators will find that this information can further enhance safe operations and procedures.
In addition to a brief summary of the information contained in AC 20-117, Hazards Following Ground Deicing and Ground Operations in Conditions Conducive to Aircraft Icing, this AC contains recent information and guidance materials regarding advanced deicing and anti-icing fluids and procedures for their use.
It recommends adherence to the clean aircraft concept which proposes "get it clean and keep it clean" during operations in adverse weather conditions.
This AC has been reproduced in a size and format that makes it easily added to flight manuals and checklists, or carried in flight cases. This AC does not change or interpret agency regulations and does not authorize deviations from regulatory requirements.
Attemunity David R. Harrington Acting Director, Flight Standards Service i (and ii)
CONTENTS Page Preface Introduction Clean aircraft concept Practices for pilots to ensure a clean aircraft Post-deicing/anti-icing check Pretakeoff check 8 Need for a clean aircraft 9 Frozen contaminants 10 Deicing and anti-icing fluids 11 Traditional North American fluids 14 International Standards Organization (ISO) commercial fluids 14 U.S. military aircraft deicing fluids 16 Characteristics of Freezing Point Depressant (FPD) fluids 16 Temperature buffer 18 Current Federal Aviation Administration (FAA) recommendations 19 Variables that can influence holdover time 20 Health effects 21 Deicing and anti-icing procedures 21 Recommended use of deicing/anti-icing codes 23 Deicing of aircraft surfaces 24 Deicing the engine area 28 Clear ice phenomena 28 Central and remote deicing 29 Techniques for implementing the clean aircraft concept 29 Appendix 1, Application guidelines tables (5 pages) iii
INTRODUCTION Federal Aviation Regulations (FAR) prohibit takeoff when snow, ice, or frost is adhering to wings, propellers, control surfaces, engine inlets, and other critical surfaces of the aircraft. This rule is the basis for the clean aircraft concept. It is imperative that takeoff not be attempted in any aircraft unless the pilot-in-command (PIC) has ascertained that all critical components of the aircraft are free of frozen contaminants.
The clean aircraft concept is essential to safe flight operations. The PIC has the ultimate responsibility to determine if the aircraft is clean and that the aircraft is in a condition for safe flight. This requirement may be met if the PIC obtains verification from properly trained and qualified ground personnel that the aircraft is ready for flight. The general consensus of the aviation community is that a critical ingredient in ensuring a safe takeoff in conditions conducive to aircraft icing is visual and/or physical inspection of critical aircraft surfaces and components shortly before takeoff.
Common practice developed by the North American and European aviation communities is to deice and, if necessary, to anti-ice an aircraft before takeoff. This is accomplished most commonly by the use of heated aqueous solutions of Freezing Point Depressant (FPD) fluids for deicing, followed by anti-icing using cold, rich solutions that are thicker and have a lower freeze point. Several different types of FPD fluids have been developed during the past 40 years, and many are in common use today. Each of these various fluids has unique characteristics and requires handling unique to that particular fluid. More recently developed fluids, such as those identified as International Standards Organization (ISO) Type Il and Society of Automotive Engineers (SAE) Type II, will last longer in conditions of precipitation and afford greater margins of safety if they are used in accordance with aircraft manufacturers' recommendations.
If improperly used, these fluids can cause undesirable and potentially dangerous changes in aircraft performance, stability, and control.
Ground deicing and anti-icing procedures vary depending primarily on aircraft type, type of ice accumulations on the aircraft, and FPD fluid type. All pilots should become familiar with the procedures recommended by the aircraft manufacturer in the Aircraft Flight Manual (AFM) or the maintenance manual and, where appropriate, the aircraft service manual.
FAA AC 20-117 provides a basic understanding of frozen contaminants and how they can affect aircraft performance and flight characteristics. Most aircraft manufacturers provide recommended procedures for deicing and anti-icing the aircraft. The information contained herein is intended for basic understanding purposes and as a quick-reference guide for pilots and others. The pilot must refer to the specific procedures developed for the aircraft.
The following list provides key points regarding aircraft deicing and anti-icing procedures.
• Most icing-related accidents have occurred when the aircraft was not deiced before takeoff attempt.
• The deicing process is intended to restore the aircraft to a clean configuration so that neither degradation of aerodynamic characteristics nor mechanical interference from contaminants will occur.
• The decision of whether or not to deice an aircraft is an integral part of the deicing process.
• The ultimate responsibility for the safety of the flight rests with the PIC of the aircraft.
• It is essential that the PIC have a thorough understanding of the deicing and anti-icing process and the approved procedures necessary to ensure that the aircraft is clean for takeoff.
• Heated solutions of FPD, water, or both are more effective in the deicing process than unheated solutions because thermal energy is used to melt the ice, snow, or frost formations.
• Unheated FPD fluids or aqueous solutions, especially SAE and ISO Type Il, are more effective in the anti-icing process because the thickness of the final residue is greater.
The freezing point of the final anti-icing coating should be as low as possible. The recommended minimum ambient temperature vs. freeze point buffers are shown below:
Fluid Type OAT** Range Buffer SAE and ISO Type I All 18°F SAE and ISO Type II above 19°F 5°F SAE and ISO Type lI below 19°F 13°F
• Undiluted SAE and ISO Type Il fluids contain no less than 50 percent glycols and have a freeze point of -32°C minimum (-25.6°F).
• SAE and ISO Type Il fluids have a longer time of effectiveness (up to 45 minutes in light precipitation) than conventional North American or SAE and ISO Type I fluids.
• A post-deicing/anti-icing check should be performed during or immediately following the ground deicing and anti-icing process.
• A pretakeoff check may be required before takeoff roll is initiated. The pilot may need the assistance of qualified ground crews to perform pretakeoff checks.
• Ice, frost, or snow on top of deicing or anti-icing fluids must be considered as adhering to the aircraft. Takeoff should not be attempted.
** Outside Air Temperature (OAT) 3
• FPD fluids used during ground deicing are not intended for, and do not provide, ice protection during flight.
• Flight tests performed by manufacturers of transport category aircraft have shown that most SAE and ISO Type Il fluid flows off lifting surfaces by rotation speeds (Vp). Some large aircraft experience performance degradation and may require weight or other takeoff compensation. Degradation is significant on small airplanes.
• Some fluid residue may remain throughout the flight. The aircraft manufacturer should have determined that this residue will have little or no effect on aircraft performance or handling qualities in aerodynamically quiet areas. However, this residue should be cleaned periodically.
CLEAN AIRCRAFT CONCEPT Test data indicate that ice, snow, or frost formations having a thickness and surface roughness similar to medium or coarse sandpaper on the leading edge and upper surface of a wing can reduce wing lift by as much as 30 percent and increase drag by 40 percent.
These changes in lift and drag significantly increase stall speed, reduce controllability, and alter aircraft flight characteristics. Thicker or rougher frozen contaminants can have increasing effects on lift, drag, stall speed, stability and control, with the primary influence being surface roughness located on critical portions of an aerodynamic surface. These adverse effects on the aerodynamic properties of the airfoil may result in sudden departure from the commanded flight path and may not be preceded by any indications or aerodynamic warning to the pilot. Therefore, it is imperative that takeoff not be attempted unless the PIC has ascertained, as required by regulation, that all critical surfaces of the aircraft are free of adhering ice, snow, or frost formations.
More than 30 factors have been identified that can influence whether ice, snow, or trost may accumulate and cause surface roughness on an aircraft and affect the anti-icing abilities of FPD fluids. These factors include ambient temperature; aircraft surface (skin) temperature; deicing concentration; relative humidity; and wind velocity and direction.
Because many factors affect the accumulation of frozen contaminants on the aircraft surface, FPD fluids used for deicing, anti-icing, or both should not be considered to have anti-icing qualities for a finite period. There is always a need for close inspection before takeoff.
Numerous techniques for complying with the clean aircraft concept have been developed by the aviation industry. The consensus of the aviation community is that the primary method of ensuring safe flight operations in conditions conducive to aircraft icing is through visual or physical inspection of critical aircraft surfaces to ascertain that they are clean before takeoff. This consensus is valid regardless of the deicing and anti-icing techniques used.
PRACTICES FOR PILOTS TO ENSURE A CLEAN AIRCRAFT
• Be knowledgeable of the adverse effects of surface roughness on aircraft performance and flight characteristics.
• Be knowledgeable of ground deicing and anti-icing practices and procedures being used on your aircraft, whether this service is being performed by your company, a service contractor, a fixed-base operator, or others.
• Do not allow deicing and anti-icing until you are familiar with the ground deicing practices and quality control procedures of the service organization.
• Be knowledgeable of critical areas of your aircraft and ensure that these areas are properly deiced and anti-iced.
• Ensure that proper precautions are taken during the deicing process to avoid damage to aircraft components and surfaces.
• Ensure that a thorough post-deicing/anti-icing check is performed prior to takeoff even though this may also be the responsibility of other organizations or personnel.
• Be knowledgeable of the function, capabilities, limitations, and operations of the ice protection systems installed on the aircratt.
• Perform additional post-deicing checks related to deicing or anti-icing as necessary or as required.
• Be aware that the time of effectiveness of FPD deicing or antiicing treatments can only be estimated because of the many variables that influence this time (holdover time).
• Be knowledgeable of the variables that can reduce time of effectiveness (holdover time) and the general effects of these variables. See list on page 20 of this AC and AC 20-117.
• Ensure that deicing and anti-icing are performed at the latest possible time before taxi to the takeoff position.
• Do not start engines or engage rotor blades until it has been ascertained that all ice deposits have been removed. Ice particles shed from rotating components may damage the aircratt or injure ground personnel.
• Be aware that certain operations may produce recirculation of ice crystals, snow, or moisture.
• 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 can cause dry snow to melt and refreeze.
• Do not take off if snow or slush is observed splashing onto critical areas of the aircraft, such as wing leading edges, during taxı.
• Do not take off if positive evidence of a clean aircraft cannot be ascertained.
POST-DEICING/ANTI-ICING CHECK Post-deicing/anti-icing checks should be performed during or immediately following the ground deicing and anti-icing process.
Areas to be inspected depend on the aircraft design and should be identified in a post-deicing checklist. The checklist should include, at a minimum, all items recommended by the aircraft manufacturer.
Generally, a checklist of this type includes the following items:
• Wing leading edges, upper surfaces, and lower surfaces;
• Vertical and horizontal stabilizing devices, leading edges, upper surfaces, lower surfaces, and side panels;
• High-lift devices such as leading-edge slats and leading or trailing-edge flaps;
• Spoilers and speed brakes;
• All control surfaces and control balance bays;
• Propellers;
• Engine inlets, particle separators, and screens;
• Windshields and other windows necessary for visibility;
• Antennas;
Fuselage;
• Exposed instrumentation devices such as angle-of-attack vanes, Pitot-static pressure probes, and static ports;
• Fuel tank and fuel cap vents;
• Cooling and auxiliary power unit (APU) air intakes, inlets, and exhausts; and
• Landing gear.
Once it has been determined through the post-deicing check that the aircraft is clean and adequately protected, the aircraft should be released for takeoff as soon as possible. This procedure is especially important in conditions of precipitation or high relative humidity (small temperature/dew point spread).
PRETAKEOFF CHECK Shortly before the aircraft takes the active runway for takeoff or initiates takeoff roll, a visual pretakeoff check is strongly recommended. The components that can be inspected vary by aircraft design. In some aircraft, the entire wing and portions of the empennage are visible from the cockpit or the cabin. In other aircraft, these surfaces are so remote that only portions of the upper surface of the wings are in view. Undersurface of wings and the undercarriage are viewable only in high-wing-type aircraft. A practice in use by some operators is to perform a visual inspection of wing surfaces, leading edges, engine inlets, and other components of the aircraft that are in view from either the cockpit or cabin, whichever provides the maximum visibility. The PIC may require the assistance of trained and qualified ground personnel to assist in the pretakeoff check.
If any aircraft surfaces have not been treated with FPD fluid, the PIC or another crewmember should look for, and examine any evidence of, melting snow and possible freezing. In addition, any evidence of ice formation that may have been induced by taxi operations should be removed. If the aircraft has been treated with FPD fluids, aircraft
surfaces should appear glossy, smooth, and wet. If these checks indicate accumulations of ice, snow, or frost, the aircraft should be returned for additional deicing and, where appropriate, additional anti-icing.
Conducting a pretakeoff check in the manner described requires the PIC and other crewmembers to be knowledgeable of ground deicing procedures and danger signs. The post-deicing check should ensure that ground deicing and anti-icing were conducted in a thorough and uniform manner and that critical surfaces or components not in view from the cockpit or cabin are also clean. The pretakeoff check provides final confirmation for the pilot that the aircraft is free of frozen contaminants.
The decision to take off following pretakeoff check remains the responsibility of the PIC.
NEED FOR A CLEAN AIRCRAFT FAR §§ 121.629, 125.221, 135.227, and 91.527 prohibit takeoff when snow, ice, or frost is adhering to wings, propellers, or control surfaces of an aircraft. This is commonly referred to as the clean aircraft concept. The degradation in aircraft performance and changes in flight characteristics when frozen contaminants are present are wide ranging, unpredictable, and highly dependent upon individual aircraft design. The magnitude of these effects can be significant. It is imperative that takeoff not be attempted unless the PIC has ascertained, as required by the FAR, that all critical components of the aircraft are free of ice, snow, or frost formations.
Flight safety following ground operations in conditions conducive to icing encompasses the clean aircraft concept. Understanding the need for a clean aircraft requires a knowledge of:
• Adverse effects of ice, snow, or frost on aircraft performance and flight characteristics, which are generally reflected in the 9