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Ice accreted on an aircraft structure and/or its engines and their air inlets as a result of flight in atmospheric icing conditions.
The freezing point of water is 0°C273.15 K
491.67 °R. If the local temperature near an airframe or powerplant external surface falls to zero or below, then ice may form from water droplets freezing on or immediately after contact with the aircraft surface.
Considerable quantities of atmospheric water continue to exist in liquid form well below 0°C. The quantity of "supercooled" water steadily decreases until, by about -40°C233.15 K
419.67 °R, most has frozen. The size of supercooled water droplets and the nature of the airflow around the aircraft surface, dictate how many of the droplets will impact the surface. The size of a droplet also determines what happens after impact, for example larger droplets tend to splash and break up into smaller ones. Finally, the size is related to the mass of water in the droplet, which determines the time required for the physical state change. Larger droplets which do not break up into smaller ones will take longer to freeze and can form a surface layer of liquid water before freezing occurs.
Adverse Aerodynamic Effects
Ice accretion on the airframe is likely to 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 center 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 i.e. manually operated, this change in pressure distribution can ultimately cause uncommanded control deflections that may not be able to be overpowered by the pilot.
Blockage of pitot tubes and static vents
Blockage of the air inlet to any part of a pitot static system can produce errors in pressure instruments such as Altimeters, Airspeed indicators, and Vertical Speed Indicators. The most likely origin of such occurrences to otherwise serviceable systems has always been the non-activation of the built-in electrical heating which these tubes and plates are provided with.
Ice forming on unheated aerials can degrade the performance of radios.
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. There are many documented cases of hazardous-sized pieces of so-called "blue ice" falling from aircraft and damaging property and occasional reports of such falls nearly injuring people. Most of these events are recorded where there is a high density of long haul commercial air traffic in the vicinity of a large airport which is located near a densely populated residential area. Many have been attributed to drain masts from aircraft galleys or toilet compartments which should have been heated to prevent ice formation but the heating systems have been found to have been faulty.
Types of Ice Accretion
Two types of ice accretion are generally recognised:
Rime ice is formed when small super cooled water droplets freeze on contact with a sub-zero surface. Because the droplets are small, they freeze almost instantly creating a mixture of tiny ice particles and trapped air. The ice deposit formed is rough and crystalline and opaque. Because of its crystalline structure, rime ice is brittle and appears white in colour from a distance.
Rime ice forms on the leading edges and can affect the aerodynamic characteristics of wings as well as forming an obstruction at engine air intakes. Rime may start with no particular shape, instead just coating the leading edge area roughly. As it develops, it may protrude forward into the airstream, although it is limited structurally in how much of a “horn” it can develop.
Clear, or Glaze ice is formed by larger supercooled droplets which take longer to freeze. This results in some degree of runback, fewer air bubbles, and leads to the ice accretion being 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.
In extreme cases and dependant on the temperature and droplet size, the ice accretion on a leading edge may take the shape of a “double ram’s horn” with protrusions on the upper and lower leading edges. These horns can occur at a variety of angles and across a wide range of locations around the leading edge. They may become somewhat large as glaze ice tends to have more structure than rime.
Cloudy or Mixed Ice is the description given to ice accretion which has features of both Rime and Clear/Glaze Icing. Because it forms in the wide range of conditions between those which lead to mostly Rime or mostly Clear/Glaze Ice, it is the most common and may tend to have a relatively more rime-like or glaze-like character and appearance.
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 Organization.
“Supercooled Large Drop (SLD). A supercooled droplet with 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 an SLD is large enough, its mass will prevent the pressure wave traveling 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 occur 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 Adverse Aerodynamic Effects of In flight Icing
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 the separate article: Aerodynamic Effects of In-Flight Icing.
Icing in Cloud and Precipitation
Any cloud containing liquid water can present a significant icing environment if the temperature is 0 °C273.15 K
491.67 °R 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 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.
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.
Types of In-flight Airframe Icing Accidents
There are two main origins of accidents and serious incidents caused by airframe icing:
- 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.
- Aircraft, predominantly propeller-driven, which rely on wing and tail ice protection by de-icing, principally 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.
- Flight Planning. For aircraft without airframe ice protection systems, operation in icing conditions should be avoided. This can only be assured if operating in VMC and flight in freezing precipitation will not occur, or in 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. There have been significant changes in recent years 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.
- Aircraft Handling. When operating with the possibility of significant ice accretion, careful management of angle of attack and drag are required. When configuration changes are made, the pilot should anticipate the possibility of rapid degradation in performance and/or handling characteristics. If this occurs, the angle of attack must be reduced immediately and aggressively. It is important to remember that there is no reason to expect any kind of warning, either from a stall warning system or through feel, buffet, or power requirements prior to a critical degradation.
- Approach and Landing. Pilots operating ice-protected aircraft should consider the effects of all forms of residual ice which may be present or acquired during approach and landing. Ice accreted during an approach can degrade performance substantially.
- Supercooled Water Droplets
- Piston Engine Induction Icing
- Freezing Rain
- Aircraft Ground De/Anti Icing
- Ice Protection Systems
- Ice Formation on Aircraft
- Aerodynamic Effects of In-Flight Icing
- Ice Contaminated Tailplane Stall
Accidents & Incidents
The following events held on the SKYbrary A&I database include reference to In-Flight Airframe Icing:
- B732, vicinity Washington National DC USA, 1982 (LOC HF WX GND) (On 13 January 1982, an Air Florida Boeing 737-200 took off in daylight from runway 36 at Washington National in moderate snow but then stalled before hitting a bridge and vehicles and continuing into the river below after just one minute of flight killing most of the occupants and some people on the ground. The accident was attributed entirely to a combination of the actions and inactions of the crew in relation to the prevailing adverse weather conditions and, crucially, to the failure to select engine anti ice on which led to over reading of actual engine thrust.)
- B712, en-route, Union Start MO USA, 2005 (HF LOC WX) (On 12 May 2005, a Boeing 717-200 being operated by Midwest AL on a scheduled passenger flight from Kansas City to Washington National and climbing in night IMC experienced a sudden loss of control from which recovery was only achieved after a prolonged period of pitch oscillation involving considerable height variation. An en-route diversion to Kirksville MO was then made without further event. None of the 80 occupants were injured and the aircraft was not damaged.)
- C208, vicinity Pelee Island Canada, 2004 (WX HF GND LOC) (On 17 January, 2004 a Cessna 208 Caravan operated by Georgian Express, took off from Pellee Island, Ontario, Canada, at a weight significantly greater than maximum permitted and with ice visible on the airframe. Shortly after take off, the pilot lost control of the aircraft and it crashed into a frozen lake.)
- AT72, en-route, Roselawn IL USA, 1994 (WX LOC) (On 31 October 1994, an ATR 72 operated by Simmons Airlines, crashed near Roselawn, Indiana, USA, following loss of control due to airframe icing.)
- AT43, en-route, Folgefonna Norway, 2005 (WX LOC) (On 14 September 2005, an ATR 42-320 operated by Coast Air AS experienced a continuous build up of ice in the climb, despite the activation of de-icing systems aircraft entered an uncontrolled roll and lost 1500ft in altitude. The crew initiated recovery actions, the aircraft was stabilised, and the flight continued without further event.)
- … further results
- Extract from Transport Canada Aviation Safety Letter 1/2007: The Adverse Aerodynamic Effects of Inflight Icing on Airplane Operation
- Flight Safety Foundation - Flight Safety Digest, January 1996: "Pilots Can Minimize the Likelihood of Aircraft Roll Upset in Severe Icing".
- Flight Safety Foundation - Flight Safety Digest, April 2005: "Understanding the Stall Recovery Procedure for Turboprop Airplanes in Icing Conditions"
- American Institute of Aeronautics and Astronautics:"Inflight Icing Educational Objectives for Air Carrier Pilots"
- American Institute of Aeronautics and Astronautics:"A Study of U.S. Inflight Icing Accidents and Incidents, 1978 to 2002"
- Aircraft Icing Handbook, Version 1 by Civil Aviation Authority of New Zealand
- AOPA Safety Advisor Leaflet: Icing (SA11-04/08)
- see also FAA "Lessons Learned from Transport Airplane Accidents": Inclement Weather / Icing