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C.  When ice clogs the engine main fuel filter, pressure differential across the filter builds up, the fuel filter differential pressure switch closes, and the fuel filter icing light and master caution light illuminate. Placing the fuel heat switch in the ON position, starts a timing device and opens the fuel heat valve, which permits high pressure compressor air to pass through the heater air tubes and heat the fuel. The fuel heat valve, after remaining open for one minute, will automatically close and the fuel heat switch will return to OFF. (See figure 2.) Should a fuel heat switch malfunction occur, the switch may be overridden to the OFF position. The warm fuel passes through the fuel filter and melts the ice. When the ice is melted, the fuel filter differential pressure switch opens and de-energizes the fuel filter icing light. The fuel heating system is designed to be used intermittently. One minute of operation every 30 minutes is recommended, when No. 1 tank fuel temperature is at 0 C or below, to restore the filter to its maximum filtration capacity.
2.  Fuel Heater
A.  The fuel heater (P&W fuel deicing heater) is an air-fuel heat exchanger. (See figure 1.) The heater consists of a housing containing a core composed of air tubes, a series of baffles, and a fuel bypass valve. The heater is mounted on the fuel control unit between the boost and main stages of the engine-driven fuel pump.
B.  The fuel flowing to the main engine fuel filter passes through the heater at all times. It is heated only when the fuel heat valve is open and permits high pressure compressor (13th stage) bleed air to pass through the hearer air tubes. To obtain uniform heating of the fuel, it is baffled around the air tubes. In the event the heater becomes clogged, a bypass valve permits the fuel to bypass the heater and flow directly through the fuel filter to the engine.
502 
Feb 15/67  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  73-14-0 Page 1 

H29210 H26633
Engine Fuel Deicing System Component Location  502 
73-14-0  Figure 1  May 15/68 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

563  Engine Fuel Deicing System Schematic 
May 15/77  Figure 2  73-14-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

P&WA 73-14-0, Page 1, Dec 15/62; Page 2, Sep 15/63
3.  Fuel Heat Valve
A.  The fuel heat valve (P&W fuel deicing air shutoff valve) is used to control the high pressure compressor (13th-stage) bleed airflow through the fuel heater (Fig. 1). It consists of a butterfly valve and an electrical actuator (motor). The valve is operated by a fuel heat switch, located on the forward overhead panel. There is one valve for each engine located at the 2 o'clock position slightly forward of the engine mount ring.
4.  Fuel Filter Differential Pressure Switch
A.  The fuel filter differential pressure switch (P&W differential fluid pressure switch) senses the pressure differential across the engine main fuel filter. It is mounted directly on the filter on the engine fuel control unit (Fig. 1). If the engine fuel filter becomes clogged, pressure differential across the filter builds up. When the pressure differential reaches 4.4 to 5.8 psi, the switch closes and illuminates the fuel filter icing light on the forward overhead panel. When the pressure differential again decreases, the pressure switch opens and the fuel filter icing light goes off.
5.  Operation
A.  When ice is present in the fuel it will eventually clog the engine main fuel filter and cause the pressure differential across the filter to increase (Fig. 2). When the pressure differential reaches
4.4 to 5.8 psi, the fuel filter differential pressure switch will close and illuminate the fuel filter icing light on the forward overhead panel and the master caution light. Placing the fuel heat switch to ON position opens the fuel heat valve and starts a timer. High pressure compressor bleed air (13th-stage) passes through the valve into the heater air tubes and heats the fuel. To obtain a uniform heating of the fuel, baffles in the heater core direct the fuel around the air tubes in a controlled flow pattern. The heated fuel flows to the fuel filter and gradually melts the ice clogging the filter. When all the ice is melted, the switch opens, de-energizing the filter icing light. One minute after being turned on, the hot airflow through the heater is stopped by the automatic return of the fuel heat switch to the OFF position. Whenever the fuel heat valve is open, the valve open light on the forward overhead panel will illuminate dimly. When the valve is in transit, the light illuminates brightly, and when the valve closes, the light extinguishes.
　
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