TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 9 - Ice eliminating Equipment

SECTION IX: ICE ELIMINATING EQUIPMENT

51. General.-De-icer systems are used to prevent the continued accumulation of ice on airplane wings, tail surfaces, radio loop, and masts. This is accomplished by the periodic inflation of rubber shoes attached to the leading edges of these units.

52. Description.-a. De-icer system's can be classified as those served by one vacuum pump (single-engine airplane) and those served by two vacuum pumps (multiple-engine airplanes). In general, the de-icer system for the single-engine airplane employs the following units: a vacuum pump, an oil separator, an air-control valve, an air-filter unit with relief valve, an electrically driven distributor valve, and the inflatable rubber shoes and connecting tubing. A schematic drawing of this system is shown in figure 53.

b. The discharge side of an engine-driven vacuum pump is used to provide the operating pressure of the de-icer system. These pumps are usually of the rotary, four-vane positive-displacement type and are lubricated from the engine oiling system.

c. The purpose of the oil separator is to remove the oil from the pump discharge and return it to the crankcase. It is usually made of brass and has tube connections for inlet, air discharge, and oil outlet. It contains no moving parts and in most types incorporates a strainer, also a restriction in the oil outlet.

d. The control valve is essentially a selector valve having two settings: the on position which directs the discharge of the pump to the system, and the off position which directs the pump output to the atmosphere or engine exhaust stack.

e. The air filter is an auxiliary separator used to purge the air of the excess oil not removed by the main oil separator, and the condensed oil vapors accumulated in this part of the system before it is directed by the distributing valve to the rubber shoes. This is accomplished by passing the air from the inlet at the bottom through a chamber of copper wool and a perforated strainer to the outlet at the top of the filter. A system relief valve is incorporated in the oil outlet at the base of the filter. This valve controls the operating pressure of the de-icer system and is generally set to open at a pressure of 7 pounds per square inch.

FIGURE 53.-De-icing system for single-engine airplane.

The purpose of the distributing valve is to furnish air periodically under pressure to each of the cells of the rubber shoes. The rotor of the valve is driven by a fractional horsepower motor through a set of reduction gears, the cycle of operation being approximately 40 seconds. The outlets on the distributor are connected to the various cells so that the order of their inflation and deflation presents a minimum disturbance to the flying characteristics of the airplane.

g. A de-icer shoe is made of soft rubber and consists essentially of an inflatable tube area through its central portion, bordered by elastic zones, the outer margins of which are attached to the airplane. On the wings the inflatable area consists of parallel tubes, each from 1 3/4 to 3 inches wide in the flat or deflated condition. These tubes may be straight or tapered, depending on the contour of the wing. For the wings, three or more such tubes and their respective stretch areas are required, sometimes separated upon sections. The tail group de-icer on small airplanes makes use of a sine-curve tube (two separate tubes manifolded together and separated by a scalloped or sinecurve seam). The empennage surfaces of the larger airplanes are fitted with de-icers similar to those used on the wings. Figure 54 shows a section of these shoes in the inflated and deflated condition.

FIGURE .-54.-De-Icer shoe.

53. Operation.-a. A de-icing system employing a three-way control valve is placed into operation by starting the distributor valve motor and setting the three-way valve so that the air from the exhaust side of the vacuum pump is directed through the air filter and the distributor valve. In stopping the operation of the de-icing system the three-way control valve is set to bypass the air from the vacuum pump overboard instead of through the system. The distributor valve motor is stopped by opening the distributor valve motor switch only, after the valve has been allowed to operate an additional 2 or 3 minutes and the air is completely expelled from the cell.

b. The de-icer system for multiple-engine airplanes (fig. 55) has two vacuum pumps and oil separators. Check valves are placed in the lines between, the separators and the manifold to connect the output of these pumps to the system. These assure system pressure f rom either pump in case of failure of the other. In some cases the simple de-icer system is further modified as shown in figure 56. In this case a four-way control valve is employed and the vacuum line of one of the pumps is selected through a selector valve to give a more positive deflation of the boots. In the on position the system control

FIGURE 55.-De-icing system for multiple-engine airplane.

valve connects the pressure side of the vacuum pumps to the pressure inlet of the distributor valve and closes the electric switch to operate the distributor valve motor. In this position the vacuum line is connected to the discharge side of the distributor valve, giving a more positive deflation of the shoes. In the off position the control valve connects the pressure side of the pumps to an overboard vent, the electric switch is open, and the distributor valve motor is stopped. The vacuum line in this case is connected to both sides of the distributor valve to complete the application of vacuum to all the boots and insure their lying flat when not operating.

54. Maintenance.-De-icer shoes form a part of the contour of the airfoils on which they are installed, and the aerodynamic characteristics of the airfoils may be seriously affected and a distinct flying hazard produced, should any rupture of the rubber take place in flight. Careful and frequent visual inspection must be relied upon to determine the extent of continued serviceability. Repairs of punctures are made by patches and cement supplied for this purpose. Gasoline may

FIGURE 56.-De-icing system for multiple-engine airplane employing vacuum deflation.

be used to remove engine oil from the de-icer boots, but it should be wiped dry at once with a clean rag and not allowed to evaporate. The de-icer shoes may be washed with soap and water as a part of the regular cleansing of the airplane. In no case is the polished dark brown or black surface on the de-icer shoes to be removed except for the purpose of applying a patch. Since de-icer shoes are made of soft flexible rubber that is easily punctured, care must be taken not to drag heavy gasoline hose over the shoes or to lean maintenance ladders or platforms against them unless the ladders are fitted with sponge rubber pads at the points of contact with the shoe. After flights during which the de-icer boots have been operated and previous to servicing the airplane, metal chains reaching to the ground are placed in contact with each rubber boot to draw off any possible static charge.