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In-Flight Airframe Icing occurs when supercooled water freezes on impact with any part of the external structure of an aircraft during flight.
Although the nominal freezing point of water is 0°C, water in the atmosphere does not always freeze at that temperature and often exists as a "supercooled" liquid. If the surface temperature of an aircraft structure is below zero, then moisture within the atmosphere may turn to ice as an immediate or secondary consequence of contact.
Considerable quantities of atmospheric water continue to exist in liquid form well below 0°C. The proportion of such supercooled water decreases as the static air temperature drops until by about -40°C (except in Cumulonimbus (Cb) Cloud where SLD may exist at even lower temperatures), almost all of it is in solid form. The size of supercooled water droplets and the nature of the airflow around the aircraft surface determine the extent to which these droplets will strike the surface. The size of a droplet will also affect what happens after such impact - for example larger droplets will often be broken up into smaller ones. Finally, since the size of a water droplet is broadly proportional to the mass of water it contains and this mass determines the time required for the physical change of state from liquid (water) to solid (ice) to occur, larger droplets which do not break up into smaller ones will take longer to freeze because of the greater release of latent heat and may form a surface layer of liquid water before this change of state occurs.
Airframe Icing can lead to reduced performance, loss of lift, altered controllability and ultimately stall and subsequent loss of control of the aircraft. Hazards arising from the presence of ice on an airframe include:
Adverse Aerodynamic Effects
Ice accretion on critical parts of an airframe unprotected by a normally functioning anti-icing or de-icing system can modify the airflow pattern around airfoil surfaces such as wings and propeller blades leading to loss of lift, increased drag and a shift in the airfoil centre of pressure. The latter effect may alter longitudinal stability and pitch trim requirements. Longitudinal stability may also be affected by a degradation of lift generated by the horizontal stabiliser. The modified airflow pattern may significantly alter the pressure distribution around flight control surfaces such as ailerons and elevators. If the control surface is unpowered, such changes in pressure distribution can eventually lead to uncommanded control deflections which the pilot may not be able to be overpower.
Blockage of pitot tubes and static vents
Partial or complete blockage of the air inlet to any part of a pitot static system can produce errors in the readings of pressure instruments such as Altimeters, Airspeed indicators, and Vertical Speed Indicators. The most likely origin of such occurrences to otherwise serviceable systems has been the non-activation of the built-in electrical heating which these tubes and plates are provided with, although in some cases, the detail design of pitot heads has made them relatively more vulnerable to ice accretion even when functioning as certificated. It is now also recognised that the effects of high level ice crystal icing can have what are usually transient effects on the effectiveness of normally functioning pitot probe heating.
Radio communication problems
Historically, ice forming on some types of unheated aerials has been the cause of degraded performance of radios but this has not been encountered in the case of modern radio equipment and aerials.
Surface Hazard from Ice Shedding
Ice shed during in-flight de icing is not of a size which could create a hazard should it survive in frozen form until reaching the ground below. However, there has been a long history of ice falls from aircraft waste drain masts, a few of which have caused minor property damage and occasionally come close to hitting and injuring people. The drain masts involved are those from aircraft galleys or toilet compartments which are normally heated to prevent ice formation but for some reason have not been operating as intended. Ice from toilet waste masts is often referred to as "blue ice". Most of these events have been recorded where there is a high density of long haul commercial air traffic inbound to a large airport which routinely overflies a densely populated residential area as it descends below the freezing level in the vicinity of the airport.
Ice accretion on an aircraft structure can be distinguished as Rime Icing, Clear/Glaze Icing or a blend of the two referred to as Cloudy or Mixed Icing:
Rime ice is formed when small supercooled water droplets freeze rapidly on contact with a sub-zero surface. The rapidity of the transition to a frozen state is because the droplets are small and the almost instant transition leads to the creation of a mixture of tiny ice particles and trapped air. The resultant ice deposit formed is rough and crystalline and opaque and because of its crystalline structure, it is brittle. It appears white in colour when viewed from a distance - for example from the flight deck when on a wing leading edge.
Since rime ice forms on leading edges, it can affect the aerodynamic characteristics of both wings and horizontal stabilisers as well as restricting engine air inlets. Rime may begin to form as a rough coating of a leading edge but if accretion continues, irregular protrusions may develop forward into the airstream, although there are structural limits to how much “horn” development can occur.
Clear or Glaze ice is formed by larger supercooled water droplets, of which only a small portion freezes immediately. This results in runback and progressive freezing of the remaining liquid and since the resultant frozen deposit contains relatively few air bubbles as a result, the accreted ice is transparent or translucent. If the freezing process is sufficiently slow to allow the water to spread more evenly before freezing, the resultant transparent sheet of ice may be difficult to detect. The larger the droplets and the slower the freezing process, the more transparent the ice.
Occasionally, certain temperature and droplet size combinations can lead to the formation of a “double ram’s horn” shape forward of the leading edge with protrusions from both the upper and lower leading edge surfaces. These horns have been observed to occur in a variety of forms in a wide range of locations along a leading edge and, because clear ice has a more robust structure than rime ice, they can reach larger sizes.
Cloudy or Mixed Ice
This blend of the two accreted ice forms in the wide range of conditions between those which lead to mostly Rime or mostly Clear/Glaze Ice and is the most commonly encountered. Its appearance will be determined by the extent to which it has been formed from supercooled water droplets of various sizes.
Some other terms which may be encountered in connection with airframe ice accretion include:
Supercooled Large Droplets (SLD)
"Supercooled large droplets (SLD) are defined as those with a diameter greater than 50 microns” - The World Meteorological Organisation.
“Supercooled Large Droplet (SLD)....[has] a diameter greater than 50 micrometers (0.05 mm). SLD conditions include freezing drizzle drops and freezing raindrops.2 - FAA AC 91-74A, Pilot’s Guide to Flight in Icing Conditions
If a SLD is large enough, its mass will prevent the pressure wave travelling ahead of an airfoil from deflecting it. When this occurs, the droplet will impinge further aft than a typical cloud-sized droplet, possibly beyond the protected area and form clear ice.
Droplets of this size are typically found in areas of freezing rain and freezing drizzle. Weather radar is designed to detect large droplets since they are not only an indication of potential in-flight icing but also updrafts and wind shear.
Runback ice forms when supercooled liquid water moves aft on the upper surface of the wing or tailplane beyond the protected area and then freezes as clear ice. Forms of ice accretion which are likely to be hazardous to continued safe flight can rapidly build up. Runback is usually attributable to the relatively large size of the SLD encountered but may also occur when a thermal ice protection system has insufficient heat to evaporate the quantity of supercooled water impinging on the surface.
Intercycle ice is that which forms between cyclic activation of a mechanical or thermal de-ice system. Accumulation of some ice when these systems are not 'on' is an essential part of their functional design. The time interval between 'on' periods is usually selectable between at least two settings. Any ice remaining after a de-icing system of this type has been selected off is sometimes referred to as residual ice.
The aerodynamic effects of accreted ice on the continued safe flight of an aircraft are a complex subject because of the many forms such ice accretion can take. In certain circumstances, very little surface roughness is required to generate significant aerodynamic effects and, as ice-load accumulates, there is often no aerodynamic warning of a departure from normal performance. Stall warning systems are designed to operate in relation to the angle of attack on a clean aeroplane and cannot be relied upon to activate usefully in the case of an ice-loaded airframe.
For further information, see this separate article: Aerodynamic Effects of In-Flight Icing.
Any cloud containing liquid water can present a significant icing environment if the temperature is 0 °C or less. Generally, cumuliform cloud structures will contain relatively large droplets which can lead to very rapid ice build up. Stratiform cloud structures usually contain much smaller droplets, although the horizontal extent of icing conditions within a stratiform cloud may be such that the accumulation in even a relatively short period of level flight can sometimes be considerable. The most significant ice accretion in any cloud can be expected to occur at temperatures below, but close to, 0˚C. In a stratiform cloud in temperate latitudes, the maximum ice accretion is often found near the top of the cloud and it may be unwise for some turboprop aircraft to remain at such an altitude for extended periods.
Any drizzle or rain which is encountered at temperatures of freezing or below is likely to generate significant ice accretion in a very short period of time, even if reasonable forward visibility prevails, and such conditions should be exited by any appropriate change of flight path.
Snow in itself does not present an icing threat, since the water is already frozen. However, snow can be mixed with liquid water, particularly cloud droplets, and, in some circumstances, can contribute to the accumulation of hazardous frozen deposits. This phenomenon may also occur in Cumulonimbus anvil clouds, where the ice crystals may be mixed with SLD to incur significant icing.
There are two main origins of accidents and serious incidents involving airframe icing:
The following events held on the SKYbrary A&I database include reference to In-Flight Airframe Icing:
On 14 November 2016, an ATR72-600 crew lost control at FL150 in severe icing conditions. Uncontrolled rolls and a 1,500 feet height loss followed during an apparent stall. After recovery, the Captain announced to the alarmed passengers that he had regained control and the flight was completed without further event. The Investigation found that the crew had been aware that they had encountered severe icing rather than the forecast moderate icing but had attempted to continue to climb which took the aircraft outside its performance limitations. The recovery from the stall was non-optimal and two key memory actions were overlooked.
In the early hours of 24 July 2014, a Boeing MD 83 being operated for Air Algérie by Spanish ACMI operator Swiftair crashed in northern Mali whilst en route from Ouagadougou, Burkina Faso to Algiers and in the vicinity of severe convective actvity associated with the ICTZ. Initial findings of the continuing Investigation include that after indications of brief but concurrent instability in the function of both engines, the thrust to both simultaneously reduced to near idle and control of the aircraft was lost. High speed terrain impact followed and the aircraft was destroyed and all 116 occupants killed.
On 9 September 2017, an ATR 72-500 crew temporarily lost control of their aircraft when it stalled whilst climbing in forecast moderate icing conditions after violation of applicable guidance. Recovery was then delayed because the correct stall recovery procedure was not followed. A MAYDAY declaration due to a perception of continuing control problems was followed by a comprehensively unstabilised ILS approach to Madrid. The Investigation concluded that the stall and its sequel were attributable to deficient flight management and inappropriate use of automation. The operator involved was recommended to implement corrective actions to improve the competence of its crews.
On 11 January 2017, control of a Cessna Citation 560 departing Oslo on a short positioning flight was lost control during flap retraction when a violent nose-down manoeuvre occurred. The First Officer took control when the Captain did not react and recovered with a 6 g pullout which left only 170 feet of ground clearance. A MAYDAY - subsequently cancelled when control was regained - was declared and the intended flight was then completed without further event. The Investigation concluded that tailplane stall after the aircraft was not de-iced prior to departure was the probable cause of the upset.
On 15 February 2013, an Embraer EMB-500 Phenom 100 crew lost control of their aircraft shortly before touchdown at Berlin Schönefeld when it stalled and crash-landed. The Investigation was not completed for almost six years but concluded that the stall was a result of ice accretion during an approach in icing conditions without activation of the airframe de icing system. It found poor crew awareness of both the ice and stall protection systems and, suspecting that this may be true of other type-rated pilots, accordingly made Safety Recommendations to key regulatory authorities concerning the type rating syllabus.
On 5 June 2017, a Saab 340B encountered an unexpected short period of severe in-cloud turbulence and icing soon after climbing through FL 100 on departure from Edinburgh and a temporary but constrained loss of pitch control occurred during which three successive Angle of Attack-triggered stick shaker activations occurred before the severity of the turbulence reduced and the intended climb could be resumed. The Investigation found that the crew had not responded to the problem in accordance with prescribed procedures and that at no time during the episode had they set Maximum Continuous Power to aid prompt and effective recovery.
On 13 December 2017, control of an ATR 42-300 was lost just after it became airborne at night from Fond-du-Lac and it was destroyed by the subsequent terrain impact. Ten occupants sustained serious injuries from which one later died and all others sustained minor injuries. The Investigation found that the accident was primarily attributable to pre-takeoff ice contamination of the airframe with an inappropriate pilot response then preventing an achievable recovery. It was found that significant airframe ice accretion had gone undetected during an inadequate pre-flight inspection and that there was a more widespread failure to recognise airframe icing risk.
On 21 December 2015, a Boeing 787-8 at FL400 in the vicinity of convective weather conducive to ice crystal icing penetrated an area which included maximum intensity weather radar returns. A very short period of erratic airspeed indications followed and the FCS reverted to Secondary Mode requiring manual flying. Since this Mode remained 'latched' and could therefore only be reset on the ground, it was decided that an en route diversion was appropriate and this was accomplished without further event. Boeing subsequently modified the FCS software to reduce the chances of reversion to Secondary Mode in short-duration unreliable airspeed events.
On 20 October 2013, a Boeing 757-200 Co-Pilot believed his aircraft was at risk of stalling when he saw a sudden low airspeed indication on his display during a night descent and reacted by increasing thrust and making abrupt pitch-down inputs. Other airspeed indications remained unaffected. The Captain took control and recovery to normal flight followed. The excursion involved a significant Vmo exceedance, damage to and consequent failure of one of the hydraulic systems and passengers and cabin crew injuries. The false airspeed reading was attributed by the Investigation to transient Ice Crystal Icing affecting one of the pitot probes.
On 3 October 2014, the crew of a Saab 340 in the cruise at FL150 in day IMC did not recognise that severe icing conditions had been encountered early enough to make a fully-controlled exit from them and although recovery from the subsequent stall was successful, it was achieved in a non-standard manner. The Investigation concluded that although both mountain wave effects and severe icing had contributed to the incident, the latter had been the main cause. Both crew understanding of airframe icing risk and supporting Operator and Manufacturer documentation on the subject were considered deficient.
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