H29211
Fuel Flow Indicating System Component Location 500
73-31-0 Figure 1 Nov 15/66
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
B. The transmitter is located in the fuel line downstream of the fuel control unit and does not appreciably effect the fuel pressure to the engines. The fuel pressure to the engines is the same whether the transmitter is functioning or is inoperative.
C. The transmitter consists of a housing containing a motor driven impeller and a fuel flow driven turbine. An impeller motor is located upstream of the impeller. A signal transmitting unit is located downstream of the turbine and is attached to the turbine with two restraining springs of different load characteristics. This permits the use of an indicator with fine index marks at the low side and coarser index marks at the high side resulting in greater sensitivity at low flow rates.
D. The impeller motor drives the impeller and produces a swirling motion of the fuel. The fuel then passes through the turbine and the fuel flow is straightened. The action of the fuel on the blades of the turbine produces a torque against the restraining springs which is directly proportional to the mass rate of fuel flow. Rotation of the restraining springs positions a permanent magnet rotor in the transmitter to a position corresponding to the fuel flow. An electrical signal is generated and transmitted to the indicator.
CAUTION: IT IS RECOMMENDED THAT THE "FUEL FLOW" CIRCUIT BREAKERS BE PULLED (OPEN) WHENEVER THE ENGINE FUEL FEED LINE IS DRAINED OR THE AIRPLANE IS OUT OF SERVICE FOR MAINTENANCE. THIS WILL PREVENT DAMAGE TO THE FUEL FLOW TRANSMITTER CAUSED FROM DRY OPERATION AND WILL PROLONG THE LIFE OF THE FUEL FLOW TRANSMITTER.
4. Fuel Flow Indicator
A. There are two fuel flow indicators in the fuel flow indicating system. One indicator is provided for each engine. Each indicator is hermetically sealed and is located on the center instrument panel.
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Nov 15/66 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 73-31-0 Page 3
B. The fuel flow indicator provides high accuracy fuel flow indications in the 500- to 6000-pph range. Less accurate fuel flow indications are shown in the 6000 - to 12,000-pph range. The dial face of the fuel flow indicator is graduated with 100-pph index marks up to 6000 pph. From 6000 - to 12,000 pph the graduations are 500 pph apart.
C. The fuel flow indicator consists essentially of a coil, a frame assembly, bearings, a dial face, a permanent magnet rotor, and a pointer contained in a can assembly.
D. The fuel flow indicator receives an electrical signal from the fuel flow transmitter. The fundamental frequency exciting current of the fuel flow indicator and the electrical signal received from the fuel flow transmitter creates flux across the core. The flux across the core aligns the permanent magnet rotor in the same angular position as the magnet in the fuel flow transmitter.
5. Operation
A. The main load control center furnishes 28-volt dc and 115-volt ac power for operation of the fuel flow indicating system. The fuel flow indicating system provides an electrical signal relative to fuel flow rate and transforms this signal into mechanical movement of the indicator pointer. The fuel flow indicating system does not contribute to radio noise.
B. Closing the FUEL FLOW circuit breakers on the FUEL SYSTEMS circuit breaker panel P6-3, energizes the fuel flow power supply. The solid state power supply changes the incoming 28 volt dc current into three-phase, four-cycle electrical output that drives the fuel flow transmitter impeller motor which drives the impeller through reduction gears and is at a constant speed regardless of fuel flow rate. Fuel passes into the fuel flow transmitter and is given a swirling motion by the rotating impeller (Fig. 2).
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73-31-0 Page 4 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Nov 15/78
H26690
520 Fuel Flow Indicating System Schematic
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