In-flight Icing

Information gathered from Skybrary

Definition

In-flight airframe icing occurs when supercooled water freezes on impact with any part of the external structure of an aircraft during flight.

Description

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 to -40°C (except in cumulonimbus [Cb] clouds, where supercooled large droplets [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 Effects

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 stabilizer. 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 recognized 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.

The Airframe Ice Accretion Process

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

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 stabilizers, as well as restrict 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 ice

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 are defined as those with a diameter greater than 50 microns”—The World Meteorological Organisation”

“Supercooled Large Droplet....[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

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

Intercycle ice is that which forms between cyclic activation of a mechanical or thermal de-icing 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 Adverse Aerodynamic Effects of Accreted Ice

The aerodynamic effects of accreted ice on the continued safe flight of an aircraft is 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.

Icing in Cloud and Precipitation

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.

Types of In-flight Airframe Icing Accidents

There are two main origins of accidents and serious incidents involving airframe icing:

1. General aviation aircraft that are not equipped with ice protection systems but are flown in icing conditions may encounter enough icing at cruise altitudes to overwhelm the aircraft power reserve, leading to an inability to maintain altitude and/or airspeed. In mountainous terrain, this very often leads to a stall followed by a loss of control when the pilot attempts to maintain altitude over the high terrain. Alternatively, a collision with terrain may result when altitude cannot be maintained. Regardless of the type of terrain, any aircraft without airframe ice protection systems which is flown in icing conditions can quickly encounter a stall and loss of control due to the excessive drag and loss of lift which ice accretion can bring.
2. Aircraft, predominantly propeller-driven, which rely on wing and tail ice protection by de-icing, principally by pneumatic deicing boots and are operated in icing conditions which exceed the capability of the protection. In these cases, if the angle of attack increases in the presence of an abnormal ice loading either as a result of attempting to maintain a climb with limited power and a relatively high load or, more suddenly, when configuration is changed during the approach to land, a stall and loss of control can result from which recovery may not be possible at low level.

Solutions

• flight planning. For aircraft without airframe ice protection systems, operation in icing conditions should be avoided. This can only be assured if operating in visual meteorological conditions (VMC) and flight in freezing precipitation will not occur, or in instrument meteorological conditions (IMC), when temperatures will be above freezing and flight in freezing precipitation will not occur. It is particularly important that the cruise portion be planned so as to avoid icing at high altitudes above mountainous terrain.

• operation of ice protection systems. Care should be taken to operate the wing and tailplane ice protection systems in accordance with the manufacturer's specification. In recent years, there have been significant changes in procedures for effective operation of pneumatic ice protection systems, and these instructions should not be ignored in favour of popular notions such as ice bridging.

• approach and landing. Pilots operating ice-protected aircraft should consider the effects of any residual ice which may be present during approach and landing, since it may degrade performance substantially and lead to abnormal responses to configuration changes.