ICE DETECTION SYSTEMSIce protection systems should be operated only when necessary so that operational expenses and unnecessary wear can be minimized. By having some form of ice detection, the prevention or removal system operates only when needed. There are visual detection methods as well as several opti-cal and electronic methods of ice detection.

VISUAL DETECTIONWhen conditions are favorable for ice formation, flight crews routinely observe the aircraft structure during flight for ice buildup. For example, it is easy to look at the wing leading edge or check the windshield for early signs of ice accumulation. When flying at night, lights are necessary to illuminate aircraft structures.

ELECTRONIC DETECTIONIce detectors consist of a microprocessor circuit with an aerodynamic strut and probe extending into the slipstream. The probe vibrates at a pre-deter-mined frequency and when ice attaches to the probe, its frequency decreases. A microprocessor turns on an annunciator light when the probe reaches a preset minimum frequency value.After detecting ice, a heating element within the probe melts it away and continues to recheck for icing conditions. As long as the probe continues to sense icing at each check, the ICE annunciator remains on. The light will go out when ice is no longer detected.

ANTI-ICING SYSTEMSOnboard anti-icing systems are designed to prevent ice formation on certain portions of the aircraft. These are areas most affected by ice build-up, or where damage would occur if accumulated ice were to break free. Typically, anti-ice systems serve the components are around the engine intakes, on locations upstream from the engines, or on components that would not operate properly if allowed to ice over.

Propellers are frequently protected by anti-ice systems because ice can cause an unbalanced condition that makes continued flight difficult or impossible. Anti-icing systems use several different methods to remove the ice, typically by heating the surface or component with hot air, engine oil, or electric heating elements. Another type of system uses chemicals with low freezing points injected at the root of the blade, which flow outward by centrifugal force. This type of system often protects windscreens also.

THERMAL ANTI-ICINGHeated air can be directed through specially designed heater ducts in the leading edges of the wings and tail surfaces to prevent ice formation. Combustion heaters or heater shrouds around the engine exhaust system heat the air in reciprocating engine aircraft. Hot air is also routed to the carburetor when conditions are conducive to carburetor icing. Combustion heaters are controlled by thermo-cycling switches that turn off the flow of fuel when a certain temperature is reached and turn it back on when the heater has cooled.

Thermal de-icing is accomplished by directing hot air through a duct in the leading edge of the wing.

Turbine engines are susceptible to damage from chunks of ice breaking off from the engine inlet and drawn into the engine's compressor. Compressor bleed air heats most turbine engine intakes by circulating around the intake leading edge to prevent ice. Boeing 727 engines also have hot compressor bleed air directed through the inlet guide vanes, the engine bullet nose, the oil cooler scoop for the constant speed drive, and the inlet duct for the center engine.

Bleed air heats the structures associated with the engine intakes.

ELECTRIC ANTI-ICINGAircraft that may possibly encounter icing usually have electrically heated pitot tubes. Pitot heaters are so powerful that they should not be operated on the ground because they may burn out without an adequate flow of air over them. Their in-flight operation is monitored by indicator lights or through the ammeter. Current flow is high enough that the ammeter will deflect noticeably when the heater is on. A heated pitot tube prevents ice from plugging the entry portal.

CHEMICAL ANTI-ICINGSome aircraft surfaces and components may be coated with either isopropyl alcohol, methyl alcohol, or a mixture of ethylene glycol and alcohol. Chemicals lower the freezing point of water on the surface of the aircraft, and at the same time make the surface too slick for ice to collect upon it. Chemical anti-icing is normally used on carburetors, propellers, and windshields and is stored in a tank on the aircraft.

Propeller anti-icing uses isopropyl alcohol sprayed onto the leading edges of its blades. Alcohol is stored in a tank and pumped to the propeller when needed. A rheostat-controlled electric motor drives the pump. A pilot can control the amount of alcohol flowing to the propeller by controlling pump speed through the rheostat. Each propeller has a slinger ring that uses centrifugal force to distribute alcohol to the blade nozzles. The amount of alcohol the tank can carry limits the system operation.

RUBBER DE-ICER BOOT SYSTEM (DE-ICING SYSTEM)Airline flying was hindered in the early days of aviation because of aircraft ice accumulation. With improved instruments and radios, and the introduction of higher performance aircraft, flight into icing conditions could no longer be avoided. To remove the ice, B.F. Goodrich developed a rubber de-icer boot that was installed on the leading edges of the wings and empennage. This allowed aircraft to fully utilize their improved capabilities. De-icer boots are used on many aircraft to this day.

PRINCIPLE OF OPERATIONA rubber boot contains several tubes fastened to the leading edge of the surface to be protected. Low-pressure air passes through a timer-operated distributor valve into the tubes. On a sequentially operated system, the center tube inflates first and any ice formed over it will crack. The center tube deflates and the outer tubes inflate, pushing up the cracked ice. Air flowing over the wing gets under it and blows it off the surface. All tubes then deflate and suction holds them tight against the boot until the ice reforms, and the cycle is repeated. Alternating inflation of the boots keeps the overall disturbance of the airflow to a minimum.

Sequential inflation of the boot's tubes breaks up the ice so the airflow can sweep it away

GROUND DE-ICING OF AIRCRAFTAircraft ground de-icing/anti-icing plays a vital role in cold weather procedures to insure that an aircraft is free of ice, frost, and snow contamination before takeoff. Procedures range from a low-tech broom to sophisticated chemicals. Airlines most commonly use a truck-mounted mobile de-icer/anti-icer. These units generally consist of one or more fluid tanks, a heater to bring the fluid to the desired application temperature, an aerial device (boom and basket) to reach remote areas, and a fluid-dispensing system (including pumps, piping, and a spray nozzle). The dispensing system is capable of supplying fluid at various pressures and flow rates with an adjustable spray pattern at the nozzle.

Spraying an airplane with a mixture of iso-propyl alcohol and ethylene glycol removes frost and ice from the surface and prevents its refreezing for a period of time (holdover time).

FROST REMOVALWhen possible, remove frost from the wing and tail surfaces by brushing it off with a long handled T-broom. Better yet, prevent ice from forming on the surfaces by covering them with nylon or canvas when the airplane is secured for the night. Spray the surfaces with a de-icing solution of ethylene glycol and isopropyl alcohol just before flight to effectively remove all traces of frost.

SNOW REMOVALRemove wet snow with a brush or squeegee. Be careful not to push the snow into the gaps around control surfaces and doors. If the snow is sticking to the aircraft, removal by chemical means is necessary.

RAIN REPELLANT SYSTEMWINDSHIELD WIPER SYSTEMSWindshield wipers for aircraft are similar to those on automobiles except they must be able to withstand the air loads that are caused by high speeds of operation. Common problems encountered with the use of windshield wipers include insufficient wiper pressure caused by aerodynamic forces and wiper systems that fail to oscillate fast enough to keep the windshield free of moisture.

The rain control panel of a large transport aircraft controls the wiper speed as well as the rain repellant system.

CHEMICAL RAIN REPELLANTMany jet transport aircraft utilize a liquid chemical rain repellent that is sprayed on the windshield to prevent water from reaching the surface of the glass. The water forms into beads and the wind carries it away, leaving the glass free of water.

Rain repellent is a syrupy liquid contained in pressurized cans connected to the rain repellent system. If an aircraft is flying in rain too heavy for the windshield wipers, the pilot can depress the rain repellent button. A single timed application of the liquid will spray out onto the windshield, and the wipers will spread it out evenly over the surface.

Repellant liquid should never be sprayed onto the windshield unless the rain is sufficiently heavy. Too much repellent can smear the windshield and be difficult to see through. It is difficult to remove if sprayed onto a dry windshield.

The operating system consists of two pressurized containers of repellent and two DC solenoid valves that, once actuated, are held open by a time-delay relay. When the rain repellent push-button switch is depressed, fluid flows for the required period of time and then the valve closes until the push button is pressed again. Rain intensity determines the number of times to depress the button.

Chemical rain repellant systems are used primarily on large transport aircraft.

PNEUMATIC RAIN REMOVAL SYSTEMSIn pneumatic rain removal systems, high-pressure compressed air is ducted from the engine bleed air system into a plenum chamber. It is then directed up against the outside of the windshield as a high velocity sheet of air. This air blast effectively prevents the rain from reaching the windshield surface.