Icing Conditions in Flight
Icing Conditions in Flight
Refer to FCOM PRO-SUP-30 Ice and Rain Protection.
Icing conditions occur when low temperatures are accompanied by precipitation. Icing of the aircraft is one of the most dangerous flight hazards. Ice formation on the ground and its effects are explained in OM Part A; Chapter 8.2.4. Pilots must be aware that flight in icing conditions involves additional hazards. In particular, they must understand the peculiarities of in-flight icing conditions and its effect on aeroplane performance and handling as well as the use and limitations of aeroplane de-ice and anti-ice equipment.
Types of Ice
The usual cloud sized small droplets tend to follow the aerodynamic flow around the wing. They are swept away, but larger, heavier droplets tend to impinge aft of the leading edge. The larger (and hence heavier) the droplet (drizzle or rain-sized), the further aft it will impinge. Furthermore, large droplets are relatively warm and as a result tend to run back and spread out in liquid state before freezing. This effect is known as flowback, and its effect depends directly upon size, temperature and velocity of water droplets and wing surface temperature. Impingement and flowback result in ice collecting not only on the leading edge, but also on the upper and lower surfaces of a wing, where there may be no ice protection.
Rime ice is associated with stratiform clouds, and forms when droplets are small and the temperature relatively cold (-15°C or colder). Not all of the droplets follow the airflow and some impact upon the wing and tailplane leading edges with little or no flowback, freezing quickly into rime ice. Therefore, leading edge de/anti-icing equipment is relatively effective against this type of ice.
Clear Ice is associated with cumuliform clouds and forms when the temperature is relatively warm (-10°C or warmer). Droplet sizes are relatively large (drizzle or rain sized), and they tend to impinge further aft and flow back before freezing to form sheet ice on the upper and lower surfaces of the wing. These areas may not be protected by de/anti-icing equipment. Any deposit greater than light can significantly degrade performance, causing problems in roll and pitch control with, in the extreme case, uncontrollable pitch up and stall. Clear ice can form on aerofoil surfaces with few visual cues to the pilot. The absence of the usual indications, such as ice formation on the windshield posts and/or wipers, does not mean that clear ice is not forming.
The terms freezing drizzle or freezing rain in a terminal forecast/report indicate a strong probability of conditions for clear ice.
Mixed Ice is a combination of clear and rime ice with all the hazards of both. It forms when droplets vary in size (e.g. drizzle in stratiform cloud) and the temperature is in the range -10°C to -15°C. The proportional make-up varies with the weather system.
Ice Accumulation Rates
Traces of Ice: Ice becomes perceptible, but is of no consequence and does not affect the performance of the aeroplane. It should be reported by pilots for meteorological purposes.
Light Icing: The rate of accumulation may create a problem if extended flight in this condition occurs. It can be safely handled by the aeroplanes de/anti-icing equipment. No restriction to operations provided the systems are used.
Moderate Icing: The rate of accumulation is such that even short encounters become potentially hazardous. The aeroplane's de/anti-icing equipment is designed to safely handle it. However, it should be a signal to the pilot to alter his flight path so as to avoid further exposure.
Severe Icing: Icing condition in which the rate of accumulation is such that the de/antiicing equipment fails to reduce or control the ice accumulation. Flight path shall be immediately changed to establish more favourable conditions or land as soon as possible.
Exposure to Severe Icing Severe icing is often associated with super-cooled large droplets (i.e. freezing drizzle or rain). Flight in these conditions is not covered by icing certification rules. Droplets covered by icing certification envelopes are so small that they are usually below the threshold of detectability.
Important: Refer to the OM Part B for specific information regarding handling techniques if inadvertently encountering severe icing conditions.
The most effective means of identifying severe icing conditions are cues that can be seen, felt or heard. This includes visual inspection of aeroplane surfaces, e.g. wings or windscreens.
At temperatures near freezing it may be possible to detect large droplets splashing or splattering upon impact with the windscreen. When exposed to severe icing in the form of supercooled large droplets, perform the following actions:
The formation of ice on the aeroplane structure could create a situation from which the pilot might have difficulty recovering and, in some instances, may not be able to recover at all. For this reason, the following paragraphs describe two particular types of control upsets likely to be encountered during flight in severe icing conditions.
Roll upset is an uncommanded and uncontrolled roll phenomenon which may occur as a consequence of, or prior to, a wing stall due to anomalous forces that cause the ailerons to deflect or because the ailerons have lost effectiveness. Deflection of ailerons or loss of aileron effectiveness may be caused by ice accumulation in a sensitive area of the wing aft of the de-icing boots. It occurs under unusual circumstances associated with super-cooled large droplets and, rarely, normal cloud droplets in a very narrow temperature range near freezing. Roll upset can result from severe icing conditions even without the usual symptoms of ice accumulation or a perceived aerodynamic stall. Pilots can minimize the chance of a roll upset by being sensitive to cues that identify severe icing conditions and by promptly exiting the severe icing conditions before control or handling characteristics of the aeroplane are degraded to a hazardous level.
Tailplane stall is another significant hazard of airframe icing. Sharp edged surfaces are more susceptible to collecting ice than large blunt ones. For this reason, the tailplane may begin accumulating ice before the wings and can accumulate faster. There have been reports of ice on the tailplane without any visible ice on the wing. This can occur if the tailplane has not been or cannot be de-iced. A tailplane stall occurs when, as with the wing, the critical angle of attack is exceeded. Since the horizontal stabilizer counters the natural nose down tendency caused by the centre of lift of the main wing, the aeroplane will react by pitching nose down, sometimes uncontrollably, when the tailplane is stalled.
Application of flaps can aggravate or initiate the stall. Use caution when applying flaps during approach if there is the possibility of icing on the tailplane. Perhaps the most important characteristic of a tailplane stall is the relatively high airspeed at the onset and, if it occurs, the suddenness and magnitude of the nose down pitch. A stall is more likely to occur when the flaps are approaching the fully extended position, after nose down pitch and airspeed changes following flap extension, or during flight through wind gusts. 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. Airspeed, at any flap setting, in excess of the aeroplane manufacturer's recommendations for the flight and environmental conditions, accompanied by uncleared ice contamination on the tailplane, may result in a tailplane stall and uncommanded pitch down from which a recovery may not be possible. A tailplane stall may occur at speeds less than the flaps extension speed.
Refer to FCOM PRO-SUP-30 Ice and Rain Protection.
Icing conditions occur when low temperatures are accompanied by precipitation. Icing of the aircraft is one of the most dangerous flight hazards. Ice formation on the ground and its effects are explained in OM Part A; Chapter 8.2.4. Pilots must be aware that flight in icing conditions involves additional hazards. In particular, they must understand the peculiarities of in-flight icing conditions and its effect on aeroplane performance and handling as well as the use and limitations of aeroplane de-ice and anti-ice equipment.
Types of Ice
The usual cloud sized small droplets tend to follow the aerodynamic flow around the wing. They are swept away, but larger, heavier droplets tend to impinge aft of the leading edge. The larger (and hence heavier) the droplet (drizzle or rain-sized), the further aft it will impinge. Furthermore, large droplets are relatively warm and as a result tend to run back and spread out in liquid state before freezing. This effect is known as flowback, and its effect depends directly upon size, temperature and velocity of water droplets and wing surface temperature. Impingement and flowback result in ice collecting not only on the leading edge, but also on the upper and lower surfaces of a wing, where there may be no ice protection.
Rime ice is associated with stratiform clouds, and forms when droplets are small and the temperature relatively cold (-15°C or colder). Not all of the droplets follow the airflow and some impact upon the wing and tailplane leading edges with little or no flowback, freezing quickly into rime ice. Therefore, leading edge de/anti-icing equipment is relatively effective against this type of ice.
Clear Ice is associated with cumuliform clouds and forms when the temperature is relatively warm (-10°C or warmer). Droplet sizes are relatively large (drizzle or rain sized), and they tend to impinge further aft and flow back before freezing to form sheet ice on the upper and lower surfaces of the wing. These areas may not be protected by de/anti-icing equipment. Any deposit greater than light can significantly degrade performance, causing problems in roll and pitch control with, in the extreme case, uncontrollable pitch up and stall. Clear ice can form on aerofoil surfaces with few visual cues to the pilot. The absence of the usual indications, such as ice formation on the windshield posts and/or wipers, does not mean that clear ice is not forming.
The terms freezing drizzle or freezing rain in a terminal forecast/report indicate a strong probability of conditions for clear ice.
Mixed Ice is a combination of clear and rime ice with all the hazards of both. It forms when droplets vary in size (e.g. drizzle in stratiform cloud) and the temperature is in the range -10°C to -15°C. The proportional make-up varies with the weather system.
Ice Accumulation Rates
Traces of Ice: Ice becomes perceptible, but is of no consequence and does not affect the performance of the aeroplane. It should be reported by pilots for meteorological purposes.
Light Icing: The rate of accumulation may create a problem if extended flight in this condition occurs. It can be safely handled by the aeroplanes de/anti-icing equipment. No restriction to operations provided the systems are used.
Moderate Icing: The rate of accumulation is such that even short encounters become potentially hazardous. The aeroplane's de/anti-icing equipment is designed to safely handle it. However, it should be a signal to the pilot to alter his flight path so as to avoid further exposure.
Severe Icing: Icing condition in which the rate of accumulation is such that the de/antiicing equipment fails to reduce or control the ice accumulation. Flight path shall be immediately changed to establish more favourable conditions or land as soon as possible.
Exposure to Severe Icing Severe icing is often associated with super-cooled large droplets (i.e. freezing drizzle or rain). Flight in these conditions is not covered by icing certification rules. Droplets covered by icing certification envelopes are so small that they are usually below the threshold of detectability.
Important: Refer to the OM Part B for specific information regarding handling techniques if inadvertently encountering severe icing conditions.
The most effective means of identifying severe icing conditions are cues that can be seen, felt or heard. This includes visual inspection of aeroplane surfaces, e.g. wings or windscreens.
At temperatures near freezing it may be possible to detect large droplets splashing or splattering upon impact with the windscreen. When exposed to severe icing in the form of supercooled large droplets, perform the following actions:
- Disengage the autopilot and hand-fly the aeroplane. The autopilot may mask important cues or may self-disconnect and present unusual attitudes or control conditions;
- Advise ATC and promptly exit the condition, using control inputs that are as smooth and small as possible;
- Change heading, altitude or both to find an area that is warmer than freezing, substantially colder than the current ambient temperature, or free of clouds;
- When severe icing conditions exist, reporting may assist other crews in maintaining vigilance. Submit a PIREP of the observed icing conditions. It is important not to understate the conditions or effects of the icing observed.
The formation of ice on the aeroplane structure could create a situation from which the pilot might have difficulty recovering and, in some instances, may not be able to recover at all. For this reason, the following paragraphs describe two particular types of control upsets likely to be encountered during flight in severe icing conditions.
Roll upset is an uncommanded and uncontrolled roll phenomenon which may occur as a consequence of, or prior to, a wing stall due to anomalous forces that cause the ailerons to deflect or because the ailerons have lost effectiveness. Deflection of ailerons or loss of aileron effectiveness may be caused by ice accumulation in a sensitive area of the wing aft of the de-icing boots. It occurs under unusual circumstances associated with super-cooled large droplets and, rarely, normal cloud droplets in a very narrow temperature range near freezing. Roll upset can result from severe icing conditions even without the usual symptoms of ice accumulation or a perceived aerodynamic stall. Pilots can minimize the chance of a roll upset by being sensitive to cues that identify severe icing conditions and by promptly exiting the severe icing conditions before control or handling characteristics of the aeroplane are degraded to a hazardous level.
Tailplane stall is another significant hazard of airframe icing. Sharp edged surfaces are more susceptible to collecting ice than large blunt ones. For this reason, the tailplane may begin accumulating ice before the wings and can accumulate faster. There have been reports of ice on the tailplane without any visible ice on the wing. This can occur if the tailplane has not been or cannot be de-iced. A tailplane stall occurs when, as with the wing, the critical angle of attack is exceeded. Since the horizontal stabilizer counters the natural nose down tendency caused by the centre of lift of the main wing, the aeroplane will react by pitching nose down, sometimes uncontrollably, when the tailplane is stalled.
Application of flaps can aggravate or initiate the stall. Use caution when applying flaps during approach if there is the possibility of icing on the tailplane. Perhaps the most important characteristic of a tailplane stall is the relatively high airspeed at the onset and, if it occurs, the suddenness and magnitude of the nose down pitch. A stall is more likely to occur when the flaps are approaching the fully extended position, after nose down pitch and airspeed changes following flap extension, or during flight through wind gusts. 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. Airspeed, at any flap setting, in excess of the aeroplane manufacturer's recommendations for the flight and environmental conditions, accompanied by uncleared ice contamination on the tailplane, may result in a tailplane stall and uncommanded pitch down from which a recovery may not be possible. A tailplane stall may occur at speeds less than the flaps extension speed.
Icing Conditions in Flight
Reviewed by Aviation Lesson
on
11:30 AM
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Reviewed by Aviation Lesson
on
11:30 AM
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