Aircraft ELECTRO THERMAL PROPELLER DEICING

 ELECTRO THERMAL Propeller Deicing

Many of the modern propellers installed on both reciprocating and turboprop engines are deiced with an electro thermal deicer system. Rubber boots with heater wires embedded in the rubber are bonded to the leading edges of the propeller blades, and electrical current is passed through these wires to heat the rubber and melt any ice that has formed, so centrifugal force and wind can carry the ice away.

The boots in some installations are made in two sections on each blade. Current flows for about a half minute through the outboard section of all blades and then for the same time through the heaters on the inboard section of all of the blades. The time the current flows have been proven by flight tests to be sufficient to allow ice to form over the inactive section and long enough to loosen the ice from the section that is receiving the current.

The complete propeller deicer system, shown in Figure 6.7, consists of the following components:

1.         Electrically heated deicers bonded to the propeller blades.

2.         Slip-ring and brush block assemblies that carry the current to the rotating propeller.

3.         Timer to control the heating time and sequence of the deicing cycle.

4.         An ammeter to indicate the operation of the system.

5.         All of the wiring, switches and circuit breakers necessary to conduct electrical power from the aircraft electrical system into the deicer sys­tem.

The slip-ring assembly is mounted on the propeller either through a specially adapted starter gear, or attached to the spinner bulkhead or the crankshaft flange. The brush block is mounted on the engine so the three brushes will ride squarely on the slip rings. The timer controls the sequence of current to each of the deicers. The sequence of heating is important, to provide the best loosening of the ice so it can be carried away by the centrifugal force. It is also important that the same portion of each blade be heated at the same time, to prevent an out-of-balance condition.

Inspection, Maintenance, And Troubleshooting Of Rubber Deicer Boot Systems

Inspection, Maintenance, And Troubleshooting Of Rubber Deicer Boot Systems

The most important part of deicer boot main­tenance is to keep the boots clean. Wash the boots with a mild soap and water solution, and if any cleaning compounds have been used with the aircraft, wash all traces of them off the boots with lots of clean water. Oil or grease may be removed by scrubbing the surface of the boot lightly with a rag damp with benzoil or lead-free gasoline, and then wiping it dry before the solvent has had a chance to soak into the rubber. During inspection, the surface deice system should always be check­ed for boot condition and security and condition of plumbing. The inspection should also contain a thorough operational check of the system.

Repairs that can be made to deicer boots include refurbishing the surface of the boot, repairing scuff damage to the surface of the boot, repairing damage to the tube area, and repairing tears in the fillet area. All of these repairs are detailed in the manufacturer's service manuals, and these in­structions must be followed in explicit detail. A repair on a scuff or surface damaged deicer boot will be discussed for familiarization purposes. This is the most common type of damage encountered and is usually caused by scuffing the outer surface of the deicer boot. The area can be repaired by selecting a patch of ample size to cover the entire damaged area. The area around the repair must be cleaned and buffed so the area is roughened. Assure the repair area is clean and apply one even coat of cement on the patch and on the damaged area. Allow to set until the cement becomes tacky, then apply the patch, being careful to avoid trap­ping air under the patch. Roll the patch down and allow to set, this can take as much as 4 hours, but the boot can be checked for inflation in about 20 minutes. There are other types of damage that cannot be covered here, so always use the manufacturers maintenance manual when repair­ing deicer boots or their systems.

Construction And Installation Of Deicer Boots

Construction And Installation Of Deicer Boots

There are several configurations of deicer boots, but all accomplish their work in the same way. They allow the ice to form and then break it off as the tubes inflate. Figure 6.6 shows some of the more commonly used configurations.

Some boots use span-wise tubes that inflate alternately, and some inflate simultaneously. Other configurations of boots have chord-wise tubes that may inflate either alternately or simultaneously. The config­uration of the tubes is determined by flight test and, naturally, only the specific boot that is ap­proved for the aircraft should be used.

When rubber deicer boots were first developed, adhesives had not been developed to the state they are today, and these boots were installed on the leading edge of the surfaces with machine screws driven into Rivnuts installed in the skin. This type of installation can be identified by a narrow metal fairing strip that covers the screw heads at the edges of the boots. Almost all of the newer boot installations fasten the boot to the surface with adhesives so that there is no need for Rivnuts and screws. When removing or installing a deicer boot, be sure that you follow the instructions in the aircraft service manual or the deicer boot

manufacturer's information in detail and do not make any substitutions in the method or material. Deicer boots may be removed by softening the adhesive with the recommended solvent and care­fully applying tension to peel the edges of the boot back from the surface. Keep the separation area wet with solvent and the boot may be carefully pulled away.

Installation begins by preparing the leading edge of the surface. Remove all of the paint and primer. Clean both the surface and the back side of the boot thoroughly. Apply the adhesive to the back side of the boot and to the leading edge. Secure the hoses to the boot and position the boot in place and press it tightly to the surface with a roller. The actual process is considerably more complicated than this, and since it must be done in strict accordance with the manufacturer's recommenda­tions, no attempt will be made here to elaborate on the details.

Deicing System Components (typical)

Deicing System Components (typical):

Some of the main components in a pneumatic deicer system are the air pump (vacuum pump), vacuum regulator, pressure control valve, timer module, and deicer boots. The vacuum pump is normally used to create a vacuum for operating the flight instruments. The output side of the pump provides air pressure which is used to inflate the deicer boots. The oil separator, previously de­scribed, is used only with the older style vacuum pumps and its purpose is to separate the oil from the air to prevent oil from being blown overboard and deterioration of the deicer boots. The amount of vacuum applied to the deice boots and the instruments is controlled by the vacuum regula­tor. Likewise, the amount of pressure allowed in the system is controlled by the pressure control valve. The timer module is normally activated by a switch in the cockpit which sequences the deicer boots through one complete deice cycle. On some systems the center deice boot cells or tubes are activated by the timer, then deflated, and the outer tubes are inflated. Normally, after one complete cycle, the system returns to off, which applies vacuum to the cells or tubes until another cycle is called for by the pilot. Other components include filters, valves and miscellaneous tubing and lines.

Aircraft Deicing System Components

Deicing System Components (typical): Some of the main components in a pneumatic deicer system are the air pump (vacuum pump), vacuum regulator, pressure control valve, timer module, and deicer boots. The vacuum pump is normally used to create a vacuum for operating the flight instruments. The output side of the pump provides air pressure which is used to inflate the deicer boots. The oil separator, previously de­scribed, is used only with the older style vacuum pumps and its purpose is to separate the oil from the air to prevent oil from being blown overboard and deterioration of the deicer boots. The amount of vacuum applied to the deice boots and the instruments is controlled by the vacuum regula­tor. Likewise, the amount of pressure allowed in the system is controlled by the pressure control valve. The timer module is normally activated by a switch in the cockpit which sequences the deicer boots through one complete deice cycle. On some systems the center deice boot cells or tubes are activated by the timer, then deflated, and the outer tubes are inflated. Normally, after one complete cycle, the system returns to off, which applies vacuum to the cells or tubes until another cycle is called for by the pilot. Other components include filters, valves and miscellaneous tubing and lines.

Source Of Operating Air

1.         On many smaller turbine aircraft the source of pneumatic air is from the turbine engine com­pressor bleed air. This air is under pressure and with the use of a regulator the pressure is made suitable for inflating the deicer boots. It can also be used to create a vacuum, by using a venturi. This vacuum, or negative pressure, is used to hold the boots down smoothly to the leading edge during the deflation cycle.

2.         The air for inflating the boots can also come from the exhaust of the engine-driven air pump (instrument system vacuum pump). Some of these pumps are of the "wet" type which uses engine oil taken into the pump through holes in the mounting flange to lubricate pump elements and also for sealing purpose. Some oil is accompanied with the output air and this oil is removed by an oil separator and sent back into the engine crankcase before the air can be used to inflate the deicer boots.

3.         Newer dry-type pumps are used for many in­stallations, and these pumps do not require an oil separator as they use carbon vanes which make the pump self-lubricating.

4.         Some deicing systems that are used only oc­casionally inflate the boots from a cylinder of compressed air that is carried just for this purpose.

Rubber Deicer Boot System

 Airline flying was hindered in the early days of aviation because of aircraft ice accumulation. Pilots did not dare fly into clouds where ice could exist. But with improved instruments and radio, and with the introduction of newer models of aircraft, flight into icing conditions did occur. And to remove the ice, the B.F. Goodrich Company developed a rubber deicer boot that was installed on the leading edges of the wings and the empen­nage. An example of just such an aircraft..

Principle Of Operation: A rubber boot containing several longitudinal tubes is fastened to the leading edge of the surface, and air from the discharge of the engine driven vacuum pump is passed through an oil separator to remove the oil that has been used to lubricate and seal the pump. Newer types of vacuum pumps do not need to use oil separators. This air is now passed through a timer-operated distributor valve into the tubes in a sequential manner. As can be seen  , the boot is installed on the leading edge of a wing with all of the tubes deflated. When they are deflated, suction from the suction side of the pump or from an ejector around the pump discharge line holds the tubes evacuated, so air flowing over the boot will not cause the tube to distort the shape of the leading edge of the wing. , the center tube is inflated and any ice that has formed over it will crack.

The center tube now deflates, , and the outer tubes inflate and push up the crack­ed ice so air flowing over the wing will get under it and blow it off of the surface. All of the tubes now deflate and are held tight against the boot by suction until the ice reforms, and then the cycle repeats itself. 

The cycle of operation causes the tubes to inflate in a symmetrical manner so the disruption of lift during the inflation will be uniform and will not

cause any flight control problems. The manufac­turer of the aircraft has determined by flight tests the proper cycle time for the operation.

The larger aircraft that use this type of deicing system have an electric motor-driven timer to operate solenoid valves that, when the system is turned on, will continually cycle the system through all of the tubes, and then provide the proper duration of rest time to allow the ice to form over the boots; then the cycle is repeated. Any time the tubes are not inflated, suction is applied to them as mentioned earlier.

A smaller aircraft deicing system , does not use the elaborate timer, but is turned on by the pilot when they detect an accu­mulation of ice on the leading edges that should be removed. When the deicing switch is turned on, the boots will cycle through one, two, or three operating cycles, depending upon the design of the system, and then the tubes will be connected to the vacuum side of the air pump to hold them tight against the leading edge.

DEICING SYSTEMS: GENERAL

Deicing Systems: GENERAL

As said in week 1, deicing systems remove the ice after it has formed, by the use of pneumatic deicer boots on the leading edges of wings and tails. Propeller deicing uses heating elements that are cycled to melt the accumulated ice and allow it to be removed by centrifugal force.

We have just noticed that an anti-icing system prevents the formation of ice on the protected component, but it has been found that for keeping the wings and tail surfaces of some of the slower airplanes free of ice, it is more effective to allow the ice to form on the surface and then crack it so the airflow over the surface will carry the ice away. This is more effective than melting the ice on the leading edge, because when this is done, the water flows back to an unheated portion of the surface and re-freezes, forming a ridge that becomes an effective aerodynamic spoiler.

AIRCRAFT ICE AND RAIN PROTECTION (CONTD)

The previous week illustrated some measures against ice accumulation, using the methodology of ant-icing.

This week discusses measures against ice using the methodology called de-icing.

AIRCRAFT CHEMICAL ANTI-ICING

Chemical Anti-icing

Certain surfaces and components of an aircraft may be coated with either isopropyl alcohol, or a mixture of ethylene glycol and alcohol. Either of these chemicals lowers the freezing point of the water at the surface of the aircraft, and at the same time makes the surface slick to prevent ice from getting a good grip on the surface.

Chemical anti-icing is normally done to the carburettors, the propellers, and to the windshield from a tank of anti-icing fluid carried in the aircraft. Ground chemical anti-icing is done by spraying all of the surfaces with ethylene glycol before the aircraft takes off, and will be discussed later in this chapter. Rubber de-icer boots are often sprayed with a silicon spray that gives the rubber an extremely smooth surface so the ice cannot adhere to it.

Propeller anti-icing, shown in Figure 5.7, uses isopropyl alcohol which is sprayed onto the leading edges of the propeller blades, preventing icing. The alcohol is stored in a tank from which it is pumped to the propeller when needed. The pump is driven by an electric motor which is controlled by a rheostat. By controlling the pumps speed through the rheostat, the pilot can control the amount of alcohol flowing to the propeller. Each propeller has a slinger ring that uses centrifugal force to dis­tribute the alcohol to the blade nozzles. The length of time this system can be used is limited by the amount of alcohol the tank can carry.