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 E.  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.
 F.  The output passes through the mach trim relay K6 in the AFC accessory unit (or IFSAU) 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.
 G.  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 either FCC mach tirm system fails, the respective relay (K1 or K2) relaxes. Both mach trim system failures are required to illuminate the mach trim light controlling the mach trim function. 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.
 19. Speed Trim System (Fig. 8)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

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 ú ú 05 Page 32 ú Nov 12/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
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.The engine No. 1 and 2 N1 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, as a function of the N1 signal, will go from zero at 60% N1 to a gain of 100% at 80% N1 and above.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 stabilizer reference input also provides a gain control function which is combined with the N1 input to provide overall circuit gain.The Digital Air Data computer provides computed airspeed. The computed airspeed is used to generate a stabilizer command input as a function of airspeed. Inertial vertical speed from the Inertial Reference Unit provides an in-phase signal referenced to the CAS command to increase the nose down command as the altitude rate increases.The flaps input provides a reference angle of airflow program for any particular flap position which is then compared to the actual alpha angle of the airplane. If the airplane approaches the stall condition, switch S1 will open, which will remove any more inputs from the speed trim system until the condition has been corrected.To enable speed trim system operation, the following conditions must all be met: Autopilot not engaged Flaps not up Flight Control computer, speed trim circuit valid More than five seconds elapsed after cessation of manual trim More than 10 seconds after unsquat.Prior to speed trim operation, STAB trim, CAS and inertial vertical speed are synchronized. The synchronized signal is used as a reference and any differance from that reference is the output signal. When the speed system is operational and the airspeed increases, there would be a command generated as a result of the difference between the airspeed reference and present airspeed. This is compared to the stabilizer position along with any assist from the altitude rate input. This signal passes through relaxed S1 to the gain amplifier. The amplifier has a variable gain which is dependant on stabilizer position at the time the speed trim system engaged (hold logic generated). The amplifier gain ranges from 100% at zero units of trim to zero gain at 5 units of stabilizer trim.
　
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