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B. Separate 115v ac and 28v dc inputs are provided to each of the two FCC's for the mach trim system. The 115v ac input provides outputs from a power supply of 115v ac for the fixed phase of the motor and 26v ac for the excitation input for the feedback synchro. The 28v dc input is used to release the MTA brake when in the mach region. When on the ground, or if the local FCC is not in control, the dc input energizes S1 and removes the output from this FCC to the mach trim actuator.
C. The command signal for the mach trim is the mach input from the DADC. The trim schedule detector in the FCC determines when the mach number has reached 0.615. A nose up command output is provided to the elevator from this point until the max input of 0.83 mach. The signal is summed at SP1 with the feedback signal from the mach trim actuator. Switch S1 will relax and the output will be available to the amplifier whenever airplane is in mach tuck region.
D. The output passes through the mach trim relay K6 in the DFCS accessory unit to the mach trim actuator. Power is also applied to the MTA brake which releases the brake and allows the control input to drive the MTA screw assembly. This also moves the motor of the feedback synchro and provides the feedback required to cancel the mach trim input. If the FCC B computers operates the system, then a ground output will be provided on the MTS SEL line which will energize relay K11 in the accessory unit and transfer control over to FCC B.
E. The P5-3 Flight Control Module contains the light for indicating a mach trim failure. The ground for the mach trim fail light passes through the two relaxed contacts of relays K1 and K2. If FCC B is controlling the mach trim function, the output of FCC A not selected from the logical inverter will provide a DC input to both sides of relay K1 which will cause it to be relaxed. As long as FCC B is operating properly, a ground is provided to the flight control module which will maintain K2 energized and there will be no warning. If a failure occurs, K2 will relax which will provide a ground for the mach trim fail light. This failure indication will also cause the master fail lights to illuminate. These lights can be reset. The recall button can be pressed which will cause a ground to be provided which, if a mach trim failure has occured, will cause the mach trim light to again illuminate.
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A 737-300/400/500MAINTENANCE MANUAL
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A
737-300/400/500MAINTENANCE MANUAL
19. Speed Trim System (Fig. 8)_________________
A. The Speed Trim system provides automatic stabilizer trim for positive speed stability during low speed high thrust conditions. The speed trim system is only operational while the autopilot is off and it is automatically terminated when the flaps are up. Either FCC can provide the speed trim commands and only one channel will be engaged at any one time. The system which will be in command is alternately chosen as a function of the squat switch input, i.e., if FCC A was in command on the last flight, the FCC B will be in command this flight.
B. The engine N-1 inputs are used to control the gain of the trim command signal. The two-engine inputs are summed, divided by 2 to get the average and then summed with stabilizer position gain control. The gain will go from zero to 30% N-1 to a gain of 1 at 70% N-1 and above.
C. The stabilizer position input is used as the reference at the time that the speed trim system was engaged and also to provide position inputs as it changes as a result of the computed airspeed input. The speed trim reference hold logic is produced when the speed trim system goes into operation. This logic causes switches S1, S2, and S3 to open, which provides reference stabilizer, CAS, and Altitude rate inputs. The stabilizer reference input also provides a gain control function which is combined with the N-1 input to provide overall circuit gain.
D. The Digital Air Data computer provides computed airspeed and altitude rate inputs. The computed airspeed is used to generate a stabilizer command input as a function of airspeed. This input can cause stabilizer movement of 8 degrees in the nose up direction as a result of the CAS changing from 90 knots to 250 knots. The altitude rate input provides an in phase signal referenced to the CAS command to increase the nose down command as the altitude rate increases.
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