Icing

What is a deicing system?

A deicing system is one in which ice is allowed to build up on a surface and is then removed, e.g., with pneumatic leading-edge boots.

What is an anti-icing system?

Ail anti-icing system is one in which ice is prevented from building up on a surface, e.g., thermal or electrical anti-icing systems on engine cowls.

Icing

What is icing?

The formation of ice (icing) is the change of the state of water to a solid form when the temperature is less than the freezing point of water, i.e., 0°C.

Note: There are three states of water: liquid (water drops), gas (water vapor), and solid (ice). Ice can form from either of the other two states of water:

1. Water vapor, by sublimation, causing hoar frost.

2. Water droplets, by freezing rain or by freezing supercooled water drops (SWDs).

Note: Airframe icing occurs in freezing clouds when SWDs are present, causing rime and/or clear ice.

What is sublimation?

Sublimation is the process of turning water vapor immediately into ice when the dewpoint/actual temperature is less than 0°C. The usual result of sublimation is hoar frost, which is in fact regarded as a type of icing.

Note: The dew point is now called the frost point.

What are supercooled water droplets (SWDs)?

Supercooled water droplets (SWDs) are liquid water drops sized between 40 and 2000 |xm that exist in a nonfrozen liquid form in the atmosphere at temperatures down to as low as -45°C, well below the normal freezing point of water (0°C). This is known as being supercooled. Such supercooled water drops will freeze only partially on contact with a colder, subzero surface, i.e., the skin of an aircraft or a propeller blade.

Usually, a SWD will turn progressively into ice as it is washed back along the colder aircraft surface. This is so because the SWDs release latent heat (into the remaining water) as it starts to freeze , which reduces the rate of freezing of the remaining water/liquid, allowing it to stay in a liquid state for longer, which in turn gets spread backwards by the airflow along the aircraft’s surface. As the remaining liquid/water comes into direct contact with another part of the subzero surface, it in turn freezes progressively. This can cause long trails of ice to build up on and from the leading edges, which the deicing systems of the aircraft must be capable of removing. For every one degree of super cooling, Vm of the water drop will change to ice on impact.

Obviously, the larger the supercooled water drops, the more severe is the icing.

Note: SWDs do not exist below approximately -45°C, where any moisture is already an ice crystal in the atmosphere, and do not stick to the aircraft’s airframe. Therefore, airframe icing is only possible between 0 and -45°C.

What conditions present an icing risk?

Icing from water drops that are present in clouds and as rain pose a risk to aircraft when the total air temperature (in flight) or outside air temperature (on the ground) is between +10°C and -40/45°C.

What hazards to aviation does icing cause?

Ice buildup on an aircraft or within its engine induction system can be a serious hazard to flight safety, particularly at slow flight phases of flight, e.g., takeoffs and landings, because of the following effects:

1. Adverse aerodynamic performance effects. Ice buildup on an aircraft’s wings seriously disrupts the airflow pattern, causing it to separate at a significantly lower angle of attack than it would from a clean wing. This results in

a. Reduced stalling angle

b. Reduced maximum lift capability

c. Increased stall speed

d. Increased drag

e. Reduced lift (loss of lift) at a given angle attack, especially at higher angles of attack

2. Control surface effects. Ice formation may freeze on control surfaces, leading to restricted movement and even loss of control in extreme circumstances.

3. Increase in aircraft weight effects. The buildup of ice on an aircraft will increase the overall weight of the aircraft, with all the associated effects of higher weights, i.e., higher stall speed, extra lift required, etc. In addition, this uncontrolled increase in weight may change

a. The position of the aircraft’s center of gravity

b. The balance of the various control surfaces and propellers, causing severe vibration and/or control difficulties

4. Reduced engine power. Ice buildup in the jet engine intake or piston engine carburetor can restrict the airflow into the engine, causing a power loss and even engine failure in extreme circumstances.

5. Vent blockage effects. Ice buildup on pitot and static probes will produce errors in the aircraft’s pressure-driven flight instruments, airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI).

6. Degraded navigation and radio communication effects. If ice builds up on navigation and radio aerials, then these systems will be degraded and unreliable. An ice-laden aircraft may even be incapable of flight. If the ice or frost buildup on the leading edges and upper surfaces of the wings is too great, then the lift produced is reduced and insufficient to balance the increased aircraft weight. In addition, if the ice builds up on the engine intake, the power output may be degraded to the extent that it is insufficient to balance the increased drag from the ice on the airframe.

What is carburetor icing?

Ice formation can occur in the engine induction system and in the carburetor of piston engines, particularly in the venturi and around the throttle valve, where acceleration of the air can produce a temperature fall by as much as 25°C. This, combined with the heat absorbed as the fuel evaporates, can cause serious icing, even when there is no visible moisture present. Such a buildup of ice in a carburetor can disturb or even prevent the flow of air and fuel into the engine manifold, causing it to lose power, run roughly, and even to stop the engine in extreme circumstances. This effect is called carburetor icing. Throttle icing (i.e., around the throttle valve) is more likely to occur at low power settings (e.g., descents), when the partially closed butterfly creates its own venturi cooling effect.

What considerations should you take for taking off in icing conditions?

Your considerations for taking off from a contaminated runway with snow or slush in icing conditions starts from your preflight preparations through the startup, taxi, actual takeoff, and the after-takeoff phases of flight. Operations from a snow- or slush-covered runway involve a significant element of risk. Therefore, operations from contaminated runways should be avoided wherever possible. However, if you are committed to a departure in these conditions, then consider the following:

1. Deice the aircraft either on standby prior to engine start or by taxiing through a deicing station.

2. Ensure that engine anti-ice is switched on during ground operations in icing conditions. Use carburetor heat and propeller deicing if appropriate.

Note: In the absence of guidance from your aircraft’s flight manual, engine icing is considered possible if the outside air temperature (OAT) is +10°C or less with visible moisture.

3. During taxiing in icing conditions, the use of reverse thrust on podded engines should be avoided because this can result in ice contamination of the wing leading edges and slats. For the same reason, keep a good distance behind the aircraft ahead. In no circumstances should an attempt be made to deice an aircraft by positioning it in the wake of the engine exhaust of another aircraft.

4. Before takeoff, ensure that the wings are not contaminated by ice or snow. Operate all the appropriate anti- and deicing systems, e.g., engine, wing, propeller, carburetor, probes, etc., before and after takeoff in accordance with the aircraft’s flight manual. Takeoff power should be monitored on more than one instrument, e.g., EPR and Nt gauges. This is in case of icing on any engine probes that causes an overreading on the main engine power instrument. Monitoring a second instrument can cross-check the reliability of the main instrument.

What are the two main types of anti-icing fluids used for deicing on the ground?

1. Type 1 fluids (unthickened). These fluids have a high glycol content and a low viscosity. The deicing performance is good, but they provide only limited protection against refreezing.

2. Type 2 fluids (thickened). These fluids have a minimum glycol content of approximately 50 percent and, due to the thickening agent, have special properties that enable the fluid to remain on the aircraft surfaces, known as holdover time. The deicing performance is good, and in addition, protection is provided against refreezing and/or buildup of further accretions when exposed to freezing conditions. Therefore, they are also anti-icing fluids. Their anti-ice holdover time depends on the following factors:

a. Type of snow

b. Wet or dry snow (Note that wet snow will result in a shorter holdover time)

c. Airframe temperature

d. Outside air temperature (OAT)

e. Amount of precipitation The deicing fluid applied would be described as follows: Type of fluid, i.e., I or II percent of fluid to water mixture, holdover time expected.

In-flight precautions for icing conditions

Icing during flight can occur at any time of the year. Therefore, a pilot should take the following precautions when icing conditions are possible:

1. Pilots should avoid icing conditions for which the aircraft is not approved.

2. Keep probe heating on when airborne.

3. Crews should visually check for the buildup of ice on the airframe at regular intervals and select wing anti-icing when required.

4. Crews should monitor the air temperature and signs of visible moisture regularly and select engine anti-icing when required.

Note: Switch on and off the engine anti-icing systems separately to guard against simultaneous engine flameouts from ice ingestion.

5. Be aware of the influence of ice buildup on the performance and controllability.

a. The performance effect of wing contamination includes

(1) Reduced stalling angle of attack

(2) Increased stalling speed

(3) Reduced maximum lift capability

(4) Reduced amount of lift at a given angle of attack, especially at higher angles of attack

b. Controllability difficulties of the aircraft due to ice buildup can be manifested in many different situations. For example, the deployment of flaps may cause a reduction in longitudinal controllability when ice is present on the tailplane. When this happens, the tailplane can stall with subsequent loss of control, from which recovery may be impossible in the time/height available. Therefore, if tailplane icing is suspected, then it would be wise not to deploy flaps. Also be aware of a lateral imbalance if you were to only deice one wing; on some aircraft types, this can occur with an engine failure and the cross-bleed closed.

What conditions would you expect icing, and when should you turn on the engine anti-icing?

You should expect icing in the following conditions and therefore turn on the engine anti-icing systems.

1. On the ground. When the outside air temperature (OAT) is +10°C or lower with visible moisture present.

Note: Static air temperature (SAT) measurements usually are considered to represent ground OAT.

2. In flight:

a. During the climb and cruise. When the total air temperature (TAT), which is SAT plus the kinetic effect of the aircraft, is colder than +10°C but warmer than -40°C, with visible moisture present.

Note: You should not turn on the engine anti-icing systems in the climb or cruise when the SAT is colder than -40°C. This is so because only ice crystals are present in the atmosphere at these extreme temperatures. These crystals do not pose an icing threat because they have no liquid state that can stick to the aircraft.

b. Descent. When the TAT is colder than +10°C with visible moisture present.

Note: Even with a SAT colder than -40°C. This is so because in a descent the temperature will increase, and the SAT will become warmer than -40°C, creating a real risk of supercooled water droplet icing. Therefore, the engine anti-icing should be switched on at the top of descent whenever the TAT is colder than +10°C and visible moisture is present.

When would you expect carburetor icing in a piston engine?

Carburetor icing should be expected in a piston engine when the outside air temperature (OAT) is between — 10°C to +30°C with a high humidity and/or visible moisture present in the air. However, carburetor/throttle icing is most likely to occur between +10 and +15°C with a relative humidity greater than 40 percent.

Note: Carburetor icing can be found on a warm day in moist air, especially with descent power settings.

What actions should you take to prevent or remove carburetor/throttle icing?

You should use the carburetor heat system (hot air) at regular intervals to treat icing in the carburetor when in carburetor icing conditions. The carb heat system delivers hot air from the engine compartment into the carburetor that melts the buildup of ice. Note: It is also advisable to apply carb heat at the start of a descent to protect against throttle icing occurring and thereby ensuring that full power is available in the event of a go-around engine application. However, carb heat always should be switched off before the throttle is advance to ensure maximum thrust delivery, and therefore, it should be off in the final landing approach in anticipation of an emergency goaround near to the ground. However, the use of carburetor heat should be avoided when the OAT is colder than — 10°C because by applying carburetor heat you raise the air temperature in the carburetor into the icing temperature range, i.e., — 10°C to +30°C.

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