FUEL FEED LOW PRESSURE INDICATING SYSTEM - DESCRIPTION AND OPERATION
1. General
A. The fuel feed low pressure indicating system provides an indication when the output pressure of any fuel boost pump falls below a preset value. The system consists of six pressure actuated switches and six indicator lights, one for each fuel boost pump, together with connecting fuel lines. If the outlet fuel pressure of a fuel boost pump falls to the preset value its fuel low pressure switch will be actuated; this will illuminate the appropriate indicator light on the overhead panel.
2. Fuel Low Pressure Switches
A. On airplanes with low pressure switches mounted on wing leading edge beams, fuel feed low pressure switches control the fuel low pressure indicating lights on the overhead panel. Each switch assembly consists of a cylindrical housing containing a diaphragm and a switch. The housing has two ports, one for inlet pressure and one for venting. The pressure inlet port is connected to a sensing line coming from a fuel boost pump check valve. The vent port is connected to a line leading overboard. The fuel feed low pressure switches are located on wing leading edge beams (Fig. 1).
B. On airplanes with low pressure switches mounted on wing front spar, fuel feed low pressure switches control the fuel low pressure indicating lights on the overhead panel. Each switch has one inlet port for a pressure sensing line coming from a fuel boost pump. The fuel feed low pressure switches are located on wing front spar (Fig. 1).
C. Tank No. 1 and 2 Boost Pump Low Pressure Switches
(1)
When pump output pressure drops to 5.0 psig, the switch closes to complete the circuit to energize the fuel low pressure indicating light.
(2)
When pump output pressure is increased to 7.0 psig, the switch opens and the indicating light is extinguished.
D. Center Wing Tank Boost Pump Low Pressure Switches
(1)
When pump output pressure drops to 14 (+ 1) psig, the switch closes to complete the circuit to energize the fuel low pressure indicating light.
(2)
When pump output pressure is increased to 16 psig, the switch opens and the indicating light is extinguished.
3. Operation
A. The low pressure indicating light switches are actuated by direct pressure from the applicable boost pumps. (See Fig. 2.)
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Fuel Feed Low Pressure Indicating System Component Location 5C8
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5B8 Fuel Feed Low Pressure Indicating System Component Location
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Fuel Feed Low Pressure Indicating System Diagram 5C1
28-42-0 Figure 2 Jun 20/85
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B. Low pressure fuel from any boost pump in any of the tanks will turn on an applicable low pressure light (Fig. 3). Low pressure from both fuel boost pumps in the same tank will turn on the applicable low pressure lights and, in addition, the fuel annunciator light and the master caution light will illuminate. When the master caution light is pressed in, the circuit is rearmed and the master caution and fuel annunciator lights are extinguished. Pressing the fuel annunciator light will reilluminate the master caution and fuel annunciator lights provided the low pressure condition remains (Ref Chapter 33, Master Warning and Caution Lights).
C. When the fuel in the center tank is depleted, the center tank boost pump low pressure lights are illuminated. The low pressure lights for the center tank boost pumps are extinguished when the boost pumps are turned off. The low pressure lights for the No. 1 or 2 tank boost pumps are illuminated when low fuel pressure exists or when the boost pumps are turned off.
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Boost Pump Low Pressure Indicating Light Circuit 513
28-42-0 Figure 3 May 15/78
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