APU Airframe Controls Component Location
Figure 3 (Sheet 2)
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747-400 MAINTENANCE MANUAL
B. The APU controller contains both software and hardware Built-In-Test features. The software features include random access memory test, programmable read only memory checksum, input data range checks, interface flag check, and instruction check. Hardware check features include load compressor overspeed detector, load compressor inlet-overtemperature detector and high compressor overspeed detector, all independent of the processor, a loss of clock detector and watchdog timer.
C. The APU controller executes a program defined by the Programmable Read Only Memory. This program contains the commands necessary to acquire and convert the input data to digital words and evaluates the validity of the data. Discrete, analog, frequency and pneumatic inputs are all required, converted and utilized by the processor. The following is a list of the inputs to the APU controller.
~....................................................................... | Inputs to APU controller | ......................................................................... | Descrete | Analog | Frequency | ......................................................................... |íFire Shutdown |íECS Air Demand |íN1 Power Turbine Speed| |íStart |íCurrent Xfmr #1 |íN2 Gas Generator Speed| |íFrequency Raise |íCurrent Xfmr #2 | | |íFrequency Lower |íT1 Ambient Temp. | | |íAir Inlet-Door Open |íT6-T1 EGT | | |íMain Engine Start |íP1 Pressure | | |íAir Driven Hyd. Pump |íT13 Load Compressor | | |íECS Demand | Discharge-Air Temp. | | |íAPU Bleed Air |íT12 Load Compressor | | | Shut-off Valve | Inlet-Air Temp. | | |íAir-Ground |íDiverter Valve Control| | |íGround Self Test | LVDT | | |íHigh Fuel Filter |íOil Level | | | Delta-P |íOil Temperature | | |íLow Oil Pressure |íStepper Motor Position| | |íCommunication Mode | | | 2.......................................................................1
D. After acquiring the input data, the processor performs the required computations to develop control signals for the output interfacing circuitry.
E. APU controller outputs:
(1)
Pneumatic Loading Available
(2)
Generator Loading Available
(3)
APU Shutdown
(4)
APU Fault
(5)
Starter Contactor
(6)
Primary Shutoff
(7)
Secondary Shutoff
(8)
Stepper Motor Power
(9)
Fuel Flow Stepper Motor Signals
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(10)
Diverter Valve Control Unit Torque Motor Output
(11)
IGV Torque Motor Output
(12)
Diverter Valve Control LVDT Actuator
(13)
IGV Actuator LVDT Excitation
(14)
Stepper Motor Resolver Excitation
(15)
P1 Excitation
(16)
Oil Level Excitation (via ARINC 429 bus)
(17)
Ignition Power
12. Operation (Fig. 4)_________
A. When the APU master control switch is positioned to ON, the APU relay controls actuate supplying power to: start the APU DC fuel boost pump, open the APU fuel shutoff valve, open the air inlet door. At this point the APU controller is also supplied power. After the air inlet door is open, positioning the master control switch to START initiates engine rotation. The engine rpm is sensed by the N1 and N2 speed sensors and transmitted to the APU controller.
B. At 8% N2 speed, the APU controller supplies power to the ignition exciter and signals the fuel metering unit (FMU) to allow fuel flow into the combustion chamber. After ignition, combustion occurs and the engine accelerates with the assistance of the starter motor. At 40% N2 the APU controller turns off power to the starter motor and ignition exciter.
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