Aircraft Ice Control Systems
- 1 Aircraft Ice Control Systems
- 1.1 Types of Ice Encountered During Flight
- 1.2 Ice or Frost Forming on Aircraft Creates Two Basic Hazards:
- 1.3 Ice Prevention; Several means to prevent or control ice formation are used in aircraft today:
- 1.4 Wing and Horizontal and Vertical Stabilizer Anti-Icing Systems
- 1.5 Icing Effects on Aircraft
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Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft surfaces (particularly leading edges), such as wings, propellers, rotor blades, engine intakes, and environmental control intakes.
If ice is allowed to build up to a significant thickness it can change the shape of airfoils and flight control surfaces, degrading the performance, control or handling characteristics of the aircraft.
An ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it can grow to a dangerous thickness.
In evaluating Aircraft Ice Control Systems, its very obvious that Rain, snow, and ice are transportation’s long-time enemies. Flying has added a new dimension, particularly with respect to ice. Under certain atmospheric conditions, ice can build rapidly on airfoils and air inlets.
On days when there is visible moisture in the air, ice can form on aircraft leading edge surfaces at altitudes where freezing temperatures start.
Water droplets in the air can be supercooled to below freezing without actually turning into ice unless they are disturbed in some manner. This unusual occurrence is partly due to the surface tension of the water droplet not allowing the droplet to expand and freeze.
However, when aircraft surfaces disturb these droplets, they immediately turn to ice on the aircraft surfaces.
Types of Ice Encountered During Flight
The two types of ice encountered during flight are clear and rime. Clear ice forms when the remaining liquid portion of the water drop flows out over the aircraft surface, gradually freezing as a smooth sheet of solid ice.
Formation occurs when droplets are large, such as in rain or in cumuliform clouds. Clear ice is hard, heavy, and tenacious. Its removal by de-icing equipment is especially difficult.
Rime ice forms when water drops are small, such as those in stratified clouds or light drizzle. The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread over the aircraft surface.
The small frozen droplets trap air giving the ice a white appearance. Rime ice is lighter in weight than clear ice and its weight is of little significance. However, its irregular shape and rough surface decrease the effectiveness of the aerodynamic efficiency of airfoils, reducing lift and increasing drag. Rime ice is brittle and more easily removed than clear ice.
Mixed clear and rime icing can form rapidly when water drops vary in size or when liquid drops intermingle with snow or ice particles.
Ice particles become embedded in clear ice, building a very rough accumulation sometimes in a mushroom shape on leading edges.
Ice may be expected to form whenever there is visible moisture in the air and temperature is near or below freezing. An exception is carburetor icing, which can occur during warm weather with no visible moisture present.
Ice or Frost Forming on Aircraft Creates Two Basic Hazards:
- The resulting malformation of the airfoil that could decrease the amount of lift.
- The additional weight and unequal formation of the ice that could cause unbalancing of the aircraft, making it hard to control. Enough ice to cause an unsafe flight condition can form in a very short period of time, thus some method of ice prevention or removal is necessary. Figure 15-1 shows the effects of ice on a leading edge.
Ice Prevention; Several means to prevent or control ice formation are used in aircraft today:
- Heating surfaces with hot air
- Heating by electrical elements
- Breaking up ice formations, usually by inflatable boots
- Chemical application
Equipment is designed for anti-icing or for de-icing. Anti-icing equipment is turned on before entering icing conditions and is designed to prevent ice from forming.
A surface may be anti-iced by keeping it dry, by heating to a temperature that evaporates water upon impingement, or by heating the surface just enough to prevent freezing, maintaining it running wet.
De-icing equipment is designed to remove ice after it begins to accumulate typically on the wings and stabilizer leading edges.
Wing and Horizontal and Vertical Stabilizer Anti-Icing Systems
The wing leading edges, or leading edge slats, and horizontal and vertical stabilizer leading edges of many aircraft make and models have anti-icing systems installed to prevent the formation of ice on these components.
The most common anti-icing systems used are thermal pneumatic, thermal electric, and chemical. Most general aviation (GA) aircraft equipped to fly in icing conditions use pneumatic deicing boots, a chemical anti-ice system.
High-performance aircraft may have “weeping wings.” Large transport-category aircraft are equipped with advanced thermal pneumatic or thermal electric anti-icing systems that are controlled automatically to prevent the formation of ice.
Icing Effects on Aircraft
Ice build-up increases drag and reduce lift. It causes destructive vibration and hampers true instrument readings. Control surfaces become unbalanced or frozen. Fixed slots are filled and movable slots jammed.
Radio reception is hampered and engine performance is affected. Ice, snow, and slush have a direct impact on the safety of flight. Not only because of degraded lift, reduced takeoff performance, and/ or maneuverability of the aircraft, but when chunks break off, they can also cause engine failures and structural damage.
Fuselage aft-mounted engines are particularly susceptible to this foreign object damage (FOD) phenomenon. Wing mounted engines are not excluded, however. Ice can be present on any part of the aircraft and when it breaks off.
There is some probability that it could go into an engine. The worst case is that ice on the wing breaks off during takeoff due to the flexing of the wing and goes directly into the engine, leading to surge, vibration, and complete thrust loss.
The light snow that is loose on the wing surfaces and the fuselage can also cause engine damage leading to surge, vibration, and thrust loss.
Whenever icing conditions are encountered, the performance characteristics of the airplane deteriorate. Decreased rate of climb must be anticipated.
Not only because of the decrease in the wing and empennage efficiency but also because of the possible reduced efficiency of the propellers and increase in gross weight.
Abrupt maneuvering and steep turns at low speeds must be avoided because the airplane stalls at higher-than-published speeds with ice accumulation.
On final approach for landing, increased airspeed must be maintained to compensate for this increased stall speed.
After touchdown with heavy ice accumulation, landing distances may be as much as twice the normal distance due to the increased landing speeds.
In this chapter, ice prevention and ice elimination using pneumatic pressure, application of heat, and the application of fluid is discussed.
The ice and rain protection systems used on aircraft keep ice from forming on the following airplane components:
- Wing leading edges
- Horizontal and vertical stabilizer leading edges
- Engine cowl leading edges
- Propeller Spinner
- Air data probes
- Flight deck windows
- Water and waste system lines and drains
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