U.S. Department of Transportation Federal Aviation Administration Advisory Circular AC 135-17 DATE: 12/14/1994 AC No: 135-17 Date: 12/14/94 Initiated by: AFS-200 Subject: PILOT GUIDE Small Aircraft Ground Deicing
PREFACE This advisory circular (AC) contains information and recommendations to assist pilots in conducting ground operations during weather conditions conducive to aircraft icing. It also contains information that can be used by other flight crewmembers, maintenance, servicing, and other aviation personnel responsible for ground deicing and aviation safety in general.
Prudent operators will find that this information can further enhance safe operations and procedures.
This AC contains recent information and guidance regarding deicing and anti-icing fluids and procedures for their use. It provides information and guidance on how to comply with the clean aircraft concept, which requires the aircraft critical surfaces be free of contamination prior to beginning takeoff.
This AC has been reproduced in a size and format intended to make it easily added to flight manuals and checklists, or carried in flight cases. The guidelines and procedures included in this AC are advisory. This AC does not change, or authorize any deviations from the Federal Aviation Regulations (FAR).
/s/ William J. White Deputy Director, Flight Standards Service
CONTENTS Page Preface i Introduction 1 Practices for pilots to ensure clean aircraft 2 Frozen contaminants and their causes 4 Effects of contamination 8 Figure 1. Typical effect of contamination on lift and drag 9 Figure 2. Typical effect of wing surface contamination on airplane stall speed 10 Key points 12 Cold weather preflight procedures 15 Post-deicing/anti-icing checks 17 Pretakeoff contamination checks 18 Types of deicing and anti-icing equipment and facilities 19 Deicing/anti-icing equipment commonly used for small airplanes 20 Deicing and anti-icing fluids 21 Characteristics of FPD fluids 25 Temperature buffer 27 Holdover times 28 Deicing and anti-icing procedures 29 Health effects 36 Appendix A A-1 Table 1. Guideline for Application of SAE Type I Fluid Mixture A-3 Table 2. Guideline for Holdover Times Anticipated for SAE Type I Fluid Mixture as a Function of Weather Conditions and OAT A-4 Table 3. Guideline for Application of SAE Type II Fluid Mixtures A-5 Table 4. Guideline for Holdover Times Anticipated for SAE Type II Fluid Mixtures as a Function of Weather Conditions and OAT A-7
INTRODUCTION FAR Sections 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 made certain, as required by the FAR, that all critical areas of the aircraft are free of ice, snow, and frost formations.
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.
Understanding the need for a clean aircraft requires knowledge of:
* Adverse effects of ice, snow, or frost on aircraft performance and flight characteristics, including:
decreased thrust, decreased lift, increased stall speed, trim changes, and altered stall characteristics and handling qualities;
* Various procedures available for aircraft ground deicing and anti-icing, including the use and effectiveness of freezing point depressant (FPD) fluids;
* Capabilities and limitations of these procedures in various weather conditions;
* Critical areas of aircraft such as the wings, propellers, control surfaces, airspeed, altimeter, rate of climb, and flight attitude instrument systems; and To achieve compliance with the clean aircraft concept, it is imperative that takeoff not be attempted in any aircraft unless the pilot-in-command (PIC) is certain that critical components of the aircraft are free of frozen contaminants. The revised rules in Parts 121, 125, and 135 of the FAR are intended to achieve implementation of the clean aircraft concept. The new regulations require that the operator develop specific procedures for the PIC. Those procedures may require having, in place, specific procedures, qualified personnel, and adequate equipment, and supplies.
FAA AC 20-117 provides general information for the basic understanding of aircraft ground deicing issues and philosophy, including the definition of frozen contaminants and how they can affect aircraft performance and flight characteristics. The information contained herein is intended for basic understanding purposes and as a quick-reference guide for pilots of small aircraft (commuter, air taxi, and general aviation). For aircraft type specific procedures, pilots should refer to the aircraft flight manuals or other manufacturer documents developed for that particular type aircraft.
PRACTICES FOR PILOTS TO ACHIEVE A
CLEAN AIRCRAFT * The ultimate responsibility for the safety of the flight rests with the pilot in command of the aircraft.
* For FAR Parts 135 and 125 operations a pretakeoff
contamination check must be completed within 5 minutes prior to beginning takeoff.
* The fact that FAR's require that pretakeoff contamination checks be completed at least 5 minutes prior to beginning takeoff does not mean that the aircraft will always be safe for takeoff for a 5 minute period, or any other specific period of time. Under some weather or operational conditions (as described later), the time of effectiveness of FPD fluids may be less than 1 minute. Under those conditions, it is recommended that takeoff be delayed until the weather conditions abate and then additional checks should be conducted just prior to initiating takeoff roll to achieve compliance with the clean aircraft concept.
* 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 activities 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, surfaces, and instrumentation sensors.
* Ensure that a thorough post-deicing/anti-icing check is performed as part of the deicing/anti-icing process.
* Be knowledgeable of the function, capabilities, limitations, and operations of the ice protection systems installed on your aircraft.
* Be aware that the time of effectiveness of FPD deicing or anti-icing treatments can only be estimated because of the many variables that influence this time (holdover time).
* The holdover times of deicing/anti-icing fluids should be used as guidelines and should not be relied upon as the sole basis for a decision to takeoff.
* Deicing and anti-icing should be performed at the latest possible time before taxi to the takeoff position.
* Accumulation of 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.
* Do not start engines until it has been ascertained that all
ice deposits have been removed. Ice particles shed from rotating components (such as propellers) may damage the aircraft 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.
* It is not advisable to take off if snow or slush is observed splashing onto critical areas of the aircraft, such as wing leading edges, or trailing edge flaps during taxi.
* FPD fluids used during ground deicing are not intended for, and do not provide, ice protection during flight.
FROZEN CONTAMINANTS AND THEIR CAUSES Frozen contaminants in the form of ice, snow, or frost can form and accumulate on exterior surfaces of an aircraft on the ground.
These contaminates may be caused by weather and or operational conditions conducive of icing, generally described as follows:
Aircraft on the ground or in flight are susceptible to accumulation of ice formations (FROZEN CONTAMINANTS) under various atmospheric and operational conditions. It is generally accepted that icing conditions (during flight or ground operations) can occur and ice protection systems or procedures should be activated when OAT is below 50 degrees F (10 degrees C) and visible moisture in any form is present or when there is standing water, ice, or snow on the runway and/or taxiways.
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:
* Supercooled Clouds. Clouds containing water droplets (at ambient temperatures below 32 degrees F) that have remained in the liquid state. Supercooled water droplets are very small (generally in the range of 5 to 100 micrometers) and will freeze upon impact with another object. Water droplets can remain in the liquid state at ambient temperatures as low as -40 degrees F. The rate of ice accretion and shape of ice formed on an aircraft component are dependent upon many factors such as cloud liquid water content, ambient temperature, droplet size, and component size, shape, and velocity.
NOTE: One micrometer (micron) is one millionth of one meter or 0.00003937 inches.
* Ice Crystal Clouds. Clouds existing usually at very cold temperatures where moisture has frozen to the solid or
crystal state.
* Mixed Conditions. Clouds at ambient temperatures below 32 degrees F containing a mixture of ice crystals and supercooled water droplets.
* Freezing Rain and Drizzle. Precipitation existing within clouds or below clouds at ambient temperatures below 32 degrees F where rain droplets remain in the supercooled liquid state. Freezing rain is generally differentiated from freezing drizzle as a function of droplet size where rain droplets range from 500 to 2000 microns and freezing drizzle droplets range less than 500 microns.
AIRCRAFT ON THE GROUND, when parked or during 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:
* Frozen precipitation such as snow, sleet, or hail.
* Residual ice from a previous flight. Such contaminants may exist on leading edges of wings, empennage, trailing edge flaps, and other surfaces.
* Operation on ramps, taxiways, and runways containing moisture, slush, or snow. Residual ice or slush accumulated on airframe components during landing and taxi operations on contaminated runways, taxiways and ramps, can remain in place if low temperatures and other weather conditions exist unless identified and removed. Contaminants of this type are commonly found in wheel wells, on landing gear components, trailing edge flaps, undersurfaces of wings and horizontal stabilizers, and other components.
* Supercooled ground fog and ice fog. Similar to supercooled clouds found at altitude but caused by advection or night time cooling and existing near ground level.
* Blowing snow. Snow blown by ambient winds, other aircraft or ground support equipment from snow drifts, other aircraft, buildings, or other ground structures.
* Recirculated snow. Snow made airborne by engine, propeller, or rotor wash. Operation of jet engines in reverse thrust, reverse pitch propellers, and helicopter rotor blades are common causes of snow recirculation.
* High relative humidity. Conditions that may produce frost formations on aircraft surfaces having a temperature at or below the dew or 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 and other ice formations can also occur on upper wing surfaces in contact with cold fuel. On some aircraft clear
ice formations can occur that are difficult to detect.
* Frost. Frost, including hoar frost, is crystallized deposit, formed from water vapor on surfaces which are at or below 0 degrees C (32 degrees F).
* Underwing Frost. Operational experience as well as research experiments with several aircraft have indicated that underwing frost formations do not generally influence aircraft performance and flight characteristics as severely as leading edge and upper wing frost; however, it must be understood that some aircraft designs may be more sensitive to underwing frost than others and particular aircraft could be unsafe with underwing frost. It is required that underwing frost be removed unless the FAA Aircraft Certification Office accepts the aircraft manufacturer's data for such operations.
* Polished Frost. FAR 135 and other rules for small aircraft allow takeoff with frost formations on the wing surfaces if the frost is polished smooth, thereby reducing the amount of surface roughness. It is recommended that all wing frost be removed by means of conventional deicing process, however, if polishing of frost is desired, the aircraft manufacturers' recommended procedures should be followed.
* Clear Ice Phenomena. Some aircraft have experienced formations of clear ice on the upper surfaces of wings in the vicinity of integral fuel tanks. Such ice is difficult to see and in many instances can not be detected other than by touch with the bare hand or by means of a special purpose ice detector. These phenomena typically occur on aircraft that have flown high-altitude missions for a sufficient time to cold-soak fuel in integral tanks, and the fuel remaining in these tanks, after landing, is sufficient to contact upper wing skins causing clear ice to form when rain, drizzle, wet snow, or high humidity is present (at, above, or below freezing ambient temperatures). Upperwing frost can also occur under conditions of high relative humidity.
Other potential locations of frozen contamination * Areas under leading edge slats and portions of trailing edge flaps (e.g.; leading edges and upper surfaces of multisegment fowler flaps) might not be exposed to anti-icing fluids during the deicing/anti-icing process. Such unprotected areas may be exposed and susceptible to icing during precipitation or high relative humidity conditions, in taxi, takeoff queue, or takeoff configurations.
* Leading edges of wings, empennage, slotted flaps, engine air inlets, etc.; of arriving aircraft may contain residual ice formations from previous flights. If ambient conditions are not such that these formations would be dissipated by natural means, or removed by means of a deicing process, they will remain and can have significant effect upon aircraft performance and flight characteristics during
subsequent operations.
* Wing flap tracks, landing gear wheel wells, control bays, control seals, engine cowl inlets, etc, * Ports, orifices, vents, air and fluid drains.
* Propellers, and other rotating components during ground operations are exposed to conditions similar to those of forward flight. Some aircraft require operation of inflight ice protection equipment while on the ground. Others may prohibit, or inhibit by design, operation of such equipment during ground operations.
THE EFFECTS OF CONTAMINATION Test data indicate that ice, snow, or frost formations having thickness and surface roughness similar to medium or course sandpaper on the leading edge and upper surfaces of a wing can reduce wing lift by as much as 30 percent and increase drag by 40 percent. See figure 1. As illustrated in Figure 2, greater surface roughness can increase these values. Some aircraft are more susceptible to the effects of surface roughness than others.
---------------------------------------------------------------- Figure 1. Typical Effect of Contamination on Lift and Drag ¢FIGURE NOT INCLUDED| ---------------------------------------------------------------- As illustrated in Figure 2, greater surface roughness can increase these values. Some aircraft are more susceptible to the effects of surface roughness than others.
---------------------------------------------------------------- Figure 2. Typical Effect of Wing Surface Contamination on Airplane Stall Speed ¢FIGURE NOT INCLUDED| ---------------------------------------------------------------- Changes in lift and drag significantly increase stall speed, reduce controllability, and alter aircraft flight characteristics. Thicker or rougher frozen contaminants can have increasing adverse effects on lift, drag, stall speed, stability and control, and aircraft performance 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 made certain that the critical surfaces and components 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 frost may accumulate and cause surface roughness on an aircraft and affect the anti-icing abilities of FPD fluids. These factors, among others, include: ambient
temperature; aircraft surface (skin) temperature; deicing fluid type, temperature, and concentration; relative humidity; and wind velocity and direction.
Snow, frost, slush, and other ice formations on other components of the aircraft, can cause undesirable local air flow disturbances, or restriction of air and fluid vents. They can cause mechanical interference and restricted movement of flight controls, flap, slat, speed brake, landing gear retraction, and other mechanisms which are necessary for safe flight.
Ice formations on turbine engine and carburetor air intakes can cause a power loss, and if dislodged and ingested into the engine, can cause engine damage and/or failure.
Ice formations on external instrumentation sensors, such as pitot/static ports, and angle of attack sensors can cause improper indications or improper operation of certain systems and components that may be critical to safe flight.
KEY POINTS The following list provides key points regarding operations in ground icing conditions and aircraft deicing and anti-icing procedures for small aircraft.
* Most ground deicing-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.
* 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 AEA, SAE and ISO Type II, are generally more effective in the anti-icing process because the final fluid film thickness is greater.
* Anti-icing should be performed as near to the takeoff time as possible to minimize the risk of exceeding the useful life or time of effectiveness of the anti-icing fluid.
* 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 _________________________________________________________________ AEA, SAE and ISO Type I All 18 degrees F AEA, SAE and ISO Type II above 19 degrees F 50 degrees F AEA, SAE and ISO Type II below 19 degrees F 13 degrees F ** OAT - Outside Air Temperature _________________________________________________________________ WARNING: SOME DEICING/ANTI-ICING FLUIDS MAY NOT BE APPROVED FOR USE ON CERTAIN AIRCRAFT. YOUR AIRCRAFT SHOULD NOT BE DEICED OR
ANTI-ICED WITH FLUIDS OR PROCEDURES NOT APPROVED FOR USE ON YOUR AIRCRAFT TYPE. AEA/SAE/ISO TYPE I FLUIDS SHOULD NOT BE USED IN THE CONCENTRATE FORM. THEY SHALL BE DILUTED WITH WATER BEFORE USE IN ACCORDANCE WITH MANUFACTURER'S INSTRUCTIONS.
* Undiluted SAE and ISO Type II fluids contain no less than 50 percent glycols and have a freeze point of -32 degrees C (-25.6 degrees F) minimum. Diluted solutions have higher (warmer temperature) freeze points.
* SAE and ISO Type II fluids have a longer time of effectiveness 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.
* Flight tests performed by manufacturers of large transport category aircraft have shown that most SAE and ISO Type II fluid flows off lifting surfaces by rotation speeds (V sub R) on the order of 85 knots or greater. Most fluid remaining dissipates during 2nd segment climb. Some large aircraft experience performance degradation due to fluid residue and may require weight or other takeoff compensation. Degradation of takeoff and climb performance, induced by Type II fluids, may be significant on smaller airplanes.
* Flight tests with some small and large airplanes have indicated that the test pilot does not usually notice FPD fluid induced changes in performance (lift-off speed, lift-off deck angle, best angle of climb, best climb rate) and flight characteristics (stall margin, control margins, and stability margins) even though these changes have occurred and could be dangerous.
* Propwash from operating propellers can cause rapid degradation (blowoff) of FPD; e.g., SAE Type II fluids on wing and other surfaces within the slipstream.
* Some fluid residue may remain throughout the flight in aerodynamically quiet areas. The aircraft manufacturer
should have determined that this residue (in aerodynamically quiet areas) will have no significant adverse effect on aircraft performance, handling qualities, or component operation. However, this residue should be cleared periodically.
* Windshield wipers may alone be a good indication that the aircraft is clean. However, if windshield wipers are iced, it might indicate that other critical aircraft components are no longer clean and are also contaminated.
* Deicing procedures and equipment developed for large transport airplanes may not be appropriate for some smaller aircraft.
* Conditions that are conducive to aircraft icing during ground operations include:
- Precipitation in the form of snow, freezing rain drizzle, sleet, and hail.
- High relative humidity and low aircraft skin temperature.
- Blowing or recirculated snow, other ice crystals or water droplets.
- Splashing of water or slush.
* Certain conditions can cause ice to remain on the aircraft even though ground conditions, other than ambient temperature, are not conducive to ground icing. All residual frozen contamination must be removed prior to subsequent takeoff.
- Residual ice formations may remain on leading edges of wings and other surfaces following flight operations in airborne icing conditions. The aircraft should always be inspected for residual ice formations and these ice formations must be removed (properly deiced) prior to departure.
- Pneumatic boots, commonly used on small airplanes of the type used in many FAR 135 operations, may retain some residual ice on leading edge boots and aft of the boots during and following flight in icing conditions.
COLD WEATHER PREFLIGHT INSPECTION PROCEDURES * Pilot preflight inspection/cold weather preflight inspection procedures. This is the normal walk-around preflight inspection conducted by a pilot. This inspection should be used to note any aircraft surface contamination and initiate any required deicing/anti-icing operations.
* A thorough preflight inspection is more important in temperature extremes because those temperature extremes may affect the aircraft or its performance. At extremely low temperatures, the urge to hurry the preflight of the aircraft is natural, particularly when the aircraft is
outside and adverse weather conditions exists, which make the preflight physically uncomfortable for the pilots. This is the very time to perform the most thorough preflight inspection.
* Aircraft areas that require special attention during a preflight during cold weather operations depend on the aircraft design and should be identified in the certificate holder's training program. The preflight should include, at a minimum, all items recommended by the aircraft manufacturer. A preflight should include items appropriate to the specific aircraft type. Generally, those items may include:
- Wing leading edges, upper and lower surfaces.
- Vertical and horizontal stabilizing devices, leading edges, upper surfaces, lower surfaces, and side panels.
- Lift/drag devices such as 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, and exhausts.
- Landing gear.
* Blowing Snow. If an aircraft is exposed to blowing snow, special attention should be given to openings in the aircraft where snow can enter, freeze, and obstruct normal operations. The following openings should be free of snow and ice before flight:
- Pitot tubes and static system sensing ports.
- Wheel wells.
- Heater intakes.
- Engine air intakes and carburetor intakes.
- Elevator and rudder controls.
- Fuel vents.
POST-DEICING/ANTI-ICING CHECKS Post-deicing/anti-icing checks should be performed as part of the deicing and anti-icing process. Generally, the following items should be checked, as applicable to the aircraft type and recommended by the manufacturer.
* Wing leading edges, upper surfaces, and lower surfaces;
* Vertical and horizontal stabilizing devices, leading edges, upper surfaces, lower surfaces, and side panels;