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Guidance Material

S742.76(21), S743.98(268), S744.115(31) & S745.124(40)

Airborne Icing Ground/Flight Training Programmes

Note 1: Air operators may use the material below as the basis for the ground training programs for airborne icing required by CASS 722.76(21), 723.98(268), 724.115(31), and 725.124(40). Seasonal operators, or operators that employ pilots on short contracts, may apply to their POI for a reduced training requirement as an acceptable means of compliance with the applicable CASS. The reduced requirement may range from no training to an abbreviated version of the guidance material. Before approving a reduced training program POIs must be satisfied that the operator's activity presents minimal risk of encounter with icing conditions.

Note 2: The information provided below is not as technically complete as it would be if it was intended for comprehensive teaching of the subject matter. It is provided to raise pilots' awareness of the real dangers associated with flight into icing conditions, along with some tips to recognize and avoid situations which might end tragically.

Introduction

Flight in icing conditions is an inescapable fact of life for Canadian operators conducting all-weather operations. In fact, given Canada's tremendous geographic extent, it is probably true that there is no time during the year that icing is not forecast somewhere in the country. Our record for safe operations in icing conditions is excellent, and our environment ensures that every IFR-rated pilot will receive initial exposure to airborne icing sometime during the first year of all-weather operations, and annual recurrent exposure every year thereafter. However, a result of several high profile accidents in the US over the past few years, we have a greater knowledge about the risks inherent in airborne icing than we have ever had before. This guidance material is intended to ensure that this information is available to all Canadian commercial pilots, to enable air operators and pilots to make better informed decisions concerning operations in airborne icing conditions.

As is discussed below, there are many factors involved in determining an aeroplane's capability to operate in icing conditions, and not all aeroplanes are equal in this regard. Nevertheless, it is probably a truth that there is never any benefit to be gained from continued operation in icing conditions, regardless of aeroplane de-/anti-icing capability. Pilot workload is increased, performance is degraded, and fuel consumption will increase through operation of engine anti-ice equipment and/or evaporative airframe de-/anti-icing systems dependent on bleed air. If the option of changing altitude and/or route to exit the conditions is available, it should almost always be used.

1. Certification Basis

All transport category aeroplanes in Canadian commercial service certificated for flight into known icing conditions have been certificated to the standard contained in Appendix C to Chapter 525 of the Airworthiness Manual (AWM). This is identical to Appendix C to Federal Aviation Regulation (FAR) 25, the universally accepted standard for icing certification, and is also applied to aeroplanes certificated to standards other than transport category-for example, to AWM 523/FAR 23 Normal or Commuter category aeroplanes.

Airborne icing is a very complex issue. There are environmental, aircraft design features and flight phase factors that determine the type and severity of the accumulation. The aircraft design features include the extent and type of ice and angle of attack. The environmental component consists of liquid water content, temperature and median volume diameter (or droplet size). These parameters all contribute to the potential icing severity. Therefore, the environmental component can only be described as icing potential which, when a particular aircraft is flown through it, determines the intensity or severity of the icing encounter. The Appendix C envelopes are the design and certification bases for icing conditions inside clouds; however, even conditions even in clouds sometimes exceed these envelopes. In addition, icing conditions potential outside of cloud, such as freezing rain/drizzle are not covered by the Appendix C envelope. Finally, design and certification of anti-icing and de-icing equipment is conducted only with respect to the requirements of Appendix C. Therefore, considerable judgement byFlight crews therefore require aircrew is required when icing potentialconsiderable judgment when there is the potential to encounter conditions that may be outside the design and certification limits and flight regime and configuration of a particular aircraft.

To fully understand what certification for flight into known icing means, it is necessary to know what in-flight icing conditions are covered by the Appendix C envelope and, even more important, what conditions are not covered. Appendix C covers maximum continuous icing conditions containing water droplets up to 40 microns in median volume diameter, and maximum intermittent icing conditions containing droplets up to 50 microns. It is not possible to relate these droplet sizes directly to the meteorological terms for freezing precipitation with which pilots are most familiar, such as ZR, ZL-, ZR+, etc. (TAF/METAR terminology FZRA, -FZDZ, +FZRA). Freezing precipitation may contain spectra of droplet sizes that can have a droplet median volume diameter as high as 1000 microns. What this means in practical terms is that the ice protection equipment on aeroplanes certificated to Appendix C may not be adequate to cope with all icing conditions encountered. That is not to say that some aeroplanes cannot successfully fly through these conditions, at least for periods of limited duration. There is considerable operational experience suggesting that some can do so successfully.

2. Airborne Icing Definitions and Terminology - Icing Environment

Ground Icing - Icing accumulated while an aircraft is on the ground up to the point of rotation, or after touchdown.

Airborne Icing - Icing accumulated while the aircraft is in flight; that is, between rotation, when all protection from ground-applied anti-icing fluids ceases, and touchdown.

3. Airborne Icing Definitions and Terminology - Icing Intensity

The following are general, universally accepted definitions. They cannot, however, be applied without reservation to all aircraft types. For example, a trace of ice may present no problem to most aircraft, but may require action by the crew of an aircraft with wings sensitive to contamination of any sort as a result of their advanced design. In all cases, the AFM/AOM must be consulted as the final authority.

Trace - Ice becomes perceptible. Rate of accumulation is slightly greater than the rate of sublimation. AIP Canada MET 2.4. It may not be hazardous for most aircraft, even though de-icing and anti-icing equipment is used, unless encountered for an extended period of time (over 1 hour). It may, however, be hazardous for aircraft with wings that suffer significant performance degradation with relatively minor amounts of roughness on the leading edge. AIP Canada MET 2.4.

Light - The rate of accumulation may create a problem if flight is prolonged in the environment (over 1 hour). AIP Canada MET 2.4.

Moderate - The rate of accumulation is such that even short encounters become potentially hazardous, and use of de-icing or anti-icing equipment or diversion is necessary. AIP Canada MET 2.4.

Severe - Rate of accumulation is such that use of de-icing or anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary. AIP Canada MET 2.4.

4. Airborne Icing Definitions and Terminology - Types of Icing

Rime Ice - Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets. AIP Canada MET 2.4. Commonly found in stratiform clouds with a low concentration of supercooled water droplets in low temperatures. It adheres to leading edges of airfoils, antennae and windshields, and shows little tendency to spread. Easily removed by aircraft de-icing systems. AIP Canada AIR 2.12.3.

Clear Ice - Glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets. AIP Canada MET 2.4. It is commonly found in cumuliform clouds with a high concentration of supercooled water droplets at temperatures at or just below 0°C. Tends to spread back from the area of impingement, due to the longer freezing time. Adheres firmly and is not easily removed. AIP Canada AIR 2.12.3. Research aircraft have found that icing associated with freezing precipitation conditions will normally appear as clear or mixed ice.

Mixed Icing - A mixture of clear and rime icing, as the term implies. It has the characteristics of both, can form rapidly and, since rime particles are embedded in clear ice, can build a very rough accumulation. FAA AC91-51A. Research aircraft have found that icing associated with freezing precipitation conditions will normally appear as clear or mixed ice.

5. Aerodynamic Effects of Airborne Icing

Commercial pilots are familiar with the classic aerodynamic effects of ice accumulation on an aeroplane in flight. These can include:

• Reduced lift accompanied by significant increases in drag and increases in weight. Studies have found that for some wing designs, dramatic increases in drag can occur with apparently apparently insignificant amounts of rough ice.
• Increases in stall speed and reduced stall angle of attack as ice alters the shape of an airfoil and disrupts airflow. A contaminated wing may stall abruptly for example, during the flare without any of the aerodynamic warnings that aircrew might expect to precede a stall, or before stall warning systems such as stick shakers activate.
• Reduced thrust due to ice disrupting the airflow to the engine and/or degrading propeller efficiency.
• Control restrictions due to water flowing back into control surfaces and freezing.

Recently, however, attention has been drawn to two additional aerodynamic effects of airborne icing, about which little information has previously been available. These are Roll Upset and Tail Plane Stall, and will be discussed in some detail below.

6. Roll Upset

Roll upset describes an uncommanded and possibly uncontrollable rolling moment caused by airflow separation in front of the ailerons, resulting in self-deflection of unpowered control surfaces. It is associated with flight in icing conditions in which water droplets flow back behind the protected surfaces before freezing and form ridges that cannot be removed by de-icing equipment. Roll Upset has recently been associated with icing conditions involving Supercooled Large Droplets (SLD); however, it theoretically can also occur in conventional icing conditions when temperatures are just slightly below 0°C. The roll upset can occur well before the normal symptoms of ice accretion are evident to the pilot, and control forces may be physically beyond the pilot's ability to overcome. Pilots may receive a warning of incipient roll upset if abnormal or sloppy aileron control forces are experienced after the auto-pilot is disconnected when operating in icing conditions.

Corrective Actions

If the severe icing conditions are inadvertently encountered, pilots should consider the following actions to avoid Roll Upset:

• Disengage the auto-pilot and hand fly the aeroplane. The auto-pilot may mask important clues or may self disconnect when control forces exceed limits, presenting the pilot with abrupt unusual attitudes and control forces.
• Reduce the angle of attack (AOA) by increasing speed. If turning, roll wings level.
• If flaps are extended, do not retract them unless it can be determined that the upper surface of the wing is clear of ice. Retracting the flaps will increase the AOA at any given airspeed, possibly leading to the onset of roll upset.
• Set appropriate power and monitor airspeed/AOA. A controlled descent is better than an uncontrolled descent.
• Verify that wing ice protection is functioning symmetrically by visual observation if possible. If not, follow the procedures in the AFM/AOM.

7. Tail Plane Stall

Since the rate at which ice accumulates on an airfoil is related to the shape of the airfoil, and thinner airfoils have a higher collection efficiency than thicker ones, ice may accumulate on the horizontal stabilizer at a higher rate than on the wings. Ice has in fact been reported on the tailplane with none at all visible on the wings. Tailplane stall occurs when its critical AOA is exceeded. Because the horizontal stabilizer produces a downward force to counter the nose down tendency caused by the centre of lift on the wing, stall of the tailplane will lead to a rapid pitch down. Application of flaps, which may reduce or increase downwash on the tailplane depending on the configuration of the empennage (i.e., low set horizontal stabilizer, mid-set, or T-tail), can aggravate or initiate the stall. Pilots should therefore be very cautious in lowering flaps if tailplane icing is suspected. Abrupt nose-down pitching movements should also be avoided, since these increase the tailplane AOA and may cause a contaminated tailplane to stall. In all cases, the AFM/AOM for each aircraft must be consulted to determine the proper action to be taken.

Tailplane stall can occur at relatively high speeds, well above the normal 1G stall speed. The pitch down may occur without warning and be uncontrollable. It is more likely to occur when the flaps are selected to the landing position, after a nose down pitching manoeuvre, during airspeed changes following flap extension, or during flight through wind gusts.

Symptoms of incipient tailplane stall may include:

• Abnormal elevator control forces, pulsing, oscillation or vibration.
• An abnormal nose down trim change (NOTE: May not be detected if the auto-pilot is engaged).
• Any other abnormal or unusual pitch anomalies (possibly leading to pilot induced oscillations).
• Reduction or loss of elevator effectiveness (NOTE: May not be detected if the auto-pilot is engaged).
• Sudden change in elevator force (control would move nose down if not restrained).
• A sudden, uncommanded nose down pitch.

Corrective Actions

If any of the above symptoms occur, the pilot should:

• Plan approaches in icing conditions with minimum flap settings for the conditions. Fly the approach "on speed" for the configuration.
• If symptoms occurred shortly after flap extension, immediately retract the flaps to the previous setting.
• Increase airspeed as appropriate to the reduced flap setting.
• Apply sufficient power for the configuration and conditions. Observe the manufacturer's recommendations concerning power settings. High power settings may aggravate tailplane stall in some designs.
• Make any nose down pitch changes slowly, even in gusting conditions, if circumstances allow.
• If equipped with a pneumatic deicing system, operate several times to attempt to clear ice from the tailplane.

Warning: Once a tailplane stall is encountered, the stall condition tends to worsen with increased airspeed and possibly may worsen with increased power settings at the same flap setting. At any flap setting, airspeed in excess of the manufacturer's recommendations for the configuration and environmental conditions, accompanied by uncleared ice on the tailplane, may result in tailplane stall and an uncontrollable nose down pitch. Tailplane stall may occur at speeds below Maximum Flap Extended Speed (V_FE)..

Warning: The procedures for recovery from roll upset and tailplane stall are almost exactly opposite. Improper identity of the event and application of the wrong recovery procedure will make an already critical situation even worse.

Warning: The information above concerning roll upset and tailplane stall is necessarily general in nature, and may not be applicable to all aircraft configurations. Pilots must consult the AFM and AOM for their aircraft type to determine type specific procedures for these phenomena, and training programmes should be revised where necessary to reflect type-specific procedures where they differ from the above.

8. Ice Bridging versus Residual Ice

Several generations of pilots operating aeroplanes with pneumatic de-icing boots have been cautioned against the dangers of ice bridging. Pilots were, and are advised against activation of the de-icing boots before sufficient ice has built up on the leading edge generally between ¼ and 1 inch out of concern that the ice would form around the shape of the inflated boot, resulting in the boot inflating and deflating under a shell of ice, making de-icing impossible. Despite the widespread belief in this phenomenon within the pilot community and its coverage in numerous technical publications, its existence cannot be substantiated, either technically or anecdotally. At a recent conference held in Cleveland to investigate ice bridging, the major manufacturers of de-icing boots reported that they had been unable to reproduce ice bridging under any laboratory/wind tunnel conditions, and that any operational report of ice bridging investigated by them had been determined to be a report of residual ice.

Residual ice is the ice remaining on a boot surface after an inflation cycle. Wind tunnel tests have shown that a higher percentage of the ice on a boot breaks away if the ice is allowed to build up to ¼ to 1 inch prior to boot activation. Even in this case, some ice may adhere to the boot after inflation, and be removed after a subsequent boot cycle. If, however, the boots are inflated with a thin layer of ice on the boot surface, as little as 40% of the ice may be removed during the inflation cycle. This is not ice bridging, but residual ice. When pneumatic boots with an automatic cycle are selected "Oon" with a thin layer of ice on the boots, typically some residual ice will remain on the boots after the first and second inflation/deflation cycles, but be totally cleared following the third or fourth cycle. If the boots are left on automatic, the clearing pattern will repeat every third or fourth cycle. To repeat, the ice remaining on the boots under such circumstances is not evidence of ice bridging; it is evidence of residual ice.

9. Effects of Residual Ice

Any contamination on a wing leading edge will degrade performance. The degree of degradation depends on many factors, and can be quite dramatic on modern high performance airfoils at the low end of the speed range. While the effect on performance of what might appear to be insignificant amounts of residual ice may not be noticeable to the flight crew between boot cycles at cruising speed, it could seriously degrade performance as speed decreases, for example, while slowing the aircraft to configure for landing or in the landing flare, resulting in stalls at low altitude or unexpectedly hard landings. For this reason, pilots should respect guidance in the AFM concerning the minimum airspeeds to be maintained in icing conditions, and ensure that there is no residual ice on the boots prior to landing by cycling the boots passing the outer marker if IFR, or at some convenient time on final if operating VFR.

10. Operational Use of Pneumatic De-Icing Boots

Pilots of aeroplanes fitted with pneumatic de-icing boots will find direction on operational use of the boots in the AFM. In most cases the AFM will direct pilots to delay operation of the boots, either in the manual mode or automatic mode (if fitted), until ¼ to 1 inch of ice has built up on the leading edge. As mentioned above, this guidance is almost universally included to prevent the occurrence of ice bridging. In its report on the fatal accident of a Comair EMB120 in January 1997, the National Transportation Safety Board (NTSB) of the United States has concluded that a small amount of rough ice had built up on the wing as the aircraft slowed to configure for an approach, but Tthis small amount was, however, sufficient to cause the aircraft to stall without warning as speed decreased. As a result, the NTSB recommends that, for modern turboprop aeroplanes:

".leading edge deicing boots should be activated as soon as the aeroplane enters icing conditions because ice bridging is not a concern in such aeroplanes and thin amounts of rough ice can be extremely hazardous."

Unless specifically prohibited by the AFM, it is recommended that pilots of turbine-powered aeroplanes equipped with pneumatic de-icing boots with an automatic cycle, select the boots on automatic as soon as the aeroplane enters icing conditions. The boots should be left on until the aeroplane has departed the icing conditions. If the automatic boots have a FAST/SLOW option, the FAST option should be selected for moderate and severe icing conditions.

11. Monitoring the Auto-pilot in Icing Conditions

When the autopilot is utilized in icing conditions, it can mask changes in performance due to the aerodynamic effects of icing that would otherwise be detected by the pilot if the aeroplane were being hand flown. It is highly recommended that pilots disengage the autopilot and hand fly the aircraft when operating in icing conditions. If this is not desirable for safety reasons, such as cockpit workload or single-pilot operations, pilots should monitor the autopilot closely. This can be accomplished by frequently disengaging the autopilot while holding the control wheel firmly. The pilot should then be able to feel any trim changes and be better able to assess the effect of any ice accumulation on the performance of the aeroplane. It is highly recommended that pilots disengage the autopilot and hand fly the aircraft when operating in icing conditions. If this is not desirable for safety reasons, such as cockpit workload or single-pilot operations, pilots should monitor the autopilot closely. This can be accomplished by frequently disengaging the autopilot while holding the control wheel firmly. The pilot should then be able to feel any trim changes and be better able to assess the effect of any ice accumulation on the performance of the aeroplane.

12. The Freezing Rain/Freezing Drizzle/SLD Environment

The classical mechanism producing freezing rain and/or freezing drizzle aloft involves a layer of warm air overlaying a layer of cold air. Snow falling through the warm layer melts, falls into the cold air, becomes supercooled, and freezes on contact with an aircraft flying through the cold air. Freezing rain and freezing drizzle are therefore typically found near warm fronts and trowals, both of which cause warm air to overlay cold air. Freezing rain or freezing drizzle may also occur at cold fronts, but are less common and would have a lesser horizontal extent due to the steeper slope of the frontal surface. The presence of warm air above has always provided a possible escape route to pilots who have encounteredencounter classical freezing precipitation aloft through a climb into the warm air. Furthermore, most pilots consider the classical freezing precipitation aloft condition to be a phenomenon restricted to low altitudes.

Recent research has revealed that there are other non-classical mechanisms that produce freezing precipitation aloft. Flights conducted by National Research Council (NRC) aircraft on Canada's East Coast have encountered freezing drizzle at altitudes up to 11,500 ft ASL and at temperatures down to -11^°C. American research aeroplanes have encountered freezing drizzle at temperatures of -15^°C and an altitude of 4500 metres (approximately 15,000 ft) ASL. There was no temperature inversion-that is, no warm air aloft-present in either case. The fatal accident involving the American Eagle ATR72 at Roselawn IN is believed to have occurred in a non-classical icing environment, with the fatal ice accumulating while holding at an altitude of 10,000 ft ASL in total air temperatures around 2^°C (ambient air temperature of -2^°C).

The non-classical mechanism is complex, involving many factors, and meteorologists in Canada and the US are expending considerable effort in improving their ability to accurately forecast non-classical conditions. Meanwhile, pilots must be aware that severe icing may be encountered in conditions unrelated to warm air aloft. They must also understand that, if non-classical freezing drizzle is encountered in flight, the escape route of a climb into warmer air may not be immediately available. Notwithstanding, climbing remains, however, the preferred escape route. It should allow the aircraft to reach an altitude above the formation region, while a descent may keep the aircraft in freezing precipitation. It should be noted that, while ascending, the aircraft might get closer to the source region with smaller droplets, higher liquid water contents, and conventional icing.

13. Detecting SLD Conditions in Flight

Visible clues to flight crew that the aircraft is operating in SLD conditions will vary from type to type. Manufacturers should be consulted to assist operators in identifying the visible clues particular to the type operated. There are, however, some general clues of which pilots should be aware:

• Ice visible on the upper or lower surface of the wing aft of the area protected by de-icing equipment. Irregular or jagged lines of ice or pieces that are self-shedding.
• Ice adhering to non-heated propeller spinners farther aft than normal.
• Granular dispersed ice crystals or total translucent or opaque coverage of the unheated portions of front or side windows. This may be accompanied by other ice patterns on the windows such as ridges. Such patterns may occur within a few seconds to one half minute after exposure to SLD.
• Unusually extensive coverage of ice, visible ice fingers or ice feathers on parts of the airframe on which ice does not normally appear.
• Significant differences between airspeed expected and airspeed attained at a given power setting.

Additional clues significant at temperatures near freezing:

• Visible rain consisting of very large droplets. In reduced visibility selection of landing/taxi lights "On" occasionally will aid detection. Rain may also be detected by the audible impact of droplets on the fuselage.
• Droplets splashing or splattering on the windscreen. The 40-50 micron droplets covered by Appendix C are so small that they cannot usually be detected; however freezing drizzle droplets can reach sizes of 0.2 to 0.5 mm and can be seen when they hit the windscreen.
• Water droplets or rivulets streaming on windows, either heated or unheated. Streaming droplets or rivulets are indicators of high liquid water content (LWC) in any sized droplet.
• Weather radar returns showing precipitation. Whenever the radar indicates precipitation in temperatures near freezing, pilots should be extra alert for other clues of SLD.

14. Flight Planning/Reporting

Pilots must take advantage of all information available to them to plan flight through known icing conditions, especially when some of the classical conditions for freezing precipitation, such as cold fronts and trowals, are forecast or known to be present. As well as FAs, TAFs, and METARs, pilots should ask for pertinent SIGMETs and any PIREPs received along the planned route of flight. Significant Weather Prognostic charts should be studied, if available. Weather information should be analyzed to predict where icing is likely to be found, and to determine possible safe exit procedures should severe icing be encountered. Air operators should also encourage pilots to routinely pass detailed PIREPs whenever icing conditions are encountered, but especially whenever severe icing is involved.

Date modified:

2010-05-03

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