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2.28.2 Engine Fuel System Selector Control. Each fuel system has a selector valve which is manually operated through the ENG FUEL SYS selector lever on the over-head engine control quadrant (Figure 2-10). There is an ENG EMER OFF T-handle on each side of the quadrant which is arranged so that pulling the handle engages the ENG FUEL SYS selector lever, bringing it to OFF. The ENG FUEL SYS selectors are connected to the fuel selec-tor valves with low-friction .exible push-pull cables. Each lever can be actuated to three positions: OFF, DIR, and XFD. With the selectors at OFF, the control valves are closed, allowing no fuel .ow to the engines. When the selectors are moved forward to DIR, the selector valves are opened, providing fuel .ow for each engine from its indi-vidual fuel tank. If a tank is empty, or you wish to equalize fuel in the tanks, the ENG FUEL SYS selector of the engine that normally feeds from the empty or low-level tank is moved to XFD. This connects that engine to the other tank through the crossfeed system. A check valve in each crossfeed line prevents air from an inoperative engine’s fuel line crossing to the operating one.
2.28.3 Fuel Filter. The engine fuel .lter has a bypass valve and bypass warning device. The .lter is mounted on the forward left side of the engine accessory gear box. An impending bypass warning is incorporated on the .lter housing in the form of a popout button. The bypass valve opens to assure continuous fuel .ow with a blocked .lter. At the same time the valve opens, an electrical switch closes to light the #1 or #2 FUEL FLTR BYPAS caution legend. Power to operate the bypass warning system is from the No. 1 and No. 2 dc primary buses through circuit break-ers marked NO. 1 and NO. 2 ENG WARN LTS, respec-tively.
2.29 ENGINE FUEL PRIME SYSTEM.
NOTE
Priming engines using sump mounted fuel boost pumps is described in paragraph
8.41.3.

A toggle switch on the upper console, marked FUEL PUMP, FUEL PRIME, OFF and APU BOOST (Figure 2-6), when moved to FUEL PRIME, energizes the prime/ boost pump and solenoid valves to each main engine fuel supply line and to the solenoid valve for the APU fuel feed system. An advisory indication is displayed during this mode by a legend marked PRIME BOOST PUMP ON. Prime pump capacity is not enough to prime an engine when the opposite engine is running. Engines should there-fore be primed individually with both engines off. The prime/boost pump is actuated and the engine prime valve is opened whenever the engine starter is operating. This pro-vides fuel pressure to aid in a successful engine start. When the engine speed reaches starter dropout speed, engine fuel prime valve will close and the prime/boost pump will also stop operating if the FUEL PUMP switch is OFF. Power to operate the prime boost system is from the battery bus through a circuit breaker marked FUEL PRIME BOOST.
2.30 FUEL QUANTITY INDICATING SYSTEM.
All internal fuel is continuously gaged with the FUEL QTY gage system (Figure 2-8). The system consists of two tank unit sensors (probes), one in each tank, a dual channel fuel quantity gage conditioner, and a dual channel low-level warning system. The tank units are connected to the fuel quantity gages marked FUEL QTY 1-2 on the central display panel. A separate total fuel quantity readout nu-merically displays the total quantity of fuel on board. The system may be checked out by pressing the FUEL IND TEST pushbutton on the miscellaneous switch panel. The vertical scales of the FUEL QTY indicator and the digital readout should show a change, and the #1 and #2 FUEL LOW caution legends should .ash. When the button is released, the scales and digital readout will return to the original readings. The fuel quantity indicating system is powered by the No. 1 ac primary bus through a circuit breaker, marked NO. 1 AC INST.
2.30.1 Fuel Low Caution Legend. Two low-level sen-sors, one on each probe, provide signals which activate two low-level caution legends indicating #1 FUEL or #2 FUEL. Those legends .ash when the fuel level decreases to approximately 172 pounds in each tank. The illumination of these legends does not mean a .xed time period remains before fuel exhaustion, but is an indication that a low fuel condition exists The fuel-low caution legends are powered by the No. 1 dc primary bus through a circuit breaker marked FUEL LOW WARN.
　
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