Excerpt of 737-300/400/500 MAINTENANCE MANUAL STS logic.....
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 on 737-300 airplanes only, the speed trim system 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.
737-300 AIRPLANES;
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. 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.
ALL EXCEPT 737-300 AIRPLANES;
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 (For 737-300 airplanes only)
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 altitude rate (or inertial vertical speed on ALL EXCEPT 737-300 airplanes) 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.
The polarity detector determines whether the command is for nose up or nose down. The trim detector will not put out a command until the error signal is at least equal to 0.072 degrees of stabilizer command. This also provides the other required input to either gate 2 or 3 which
provides the nose up or down command. The output of the trim error detector remains until the error gets down to less than 0.070 degrees of
command error. The output then goes to zero and the stabilizer trim stops. Gates 2 and 3 also require that the speed trim system is in operation and that the local FCC has been selected for command of the system. The signal then passes to the nose up or nose down relay in the stabilizer trim servo.
Selection of FCC A or B is altered at each squat or selection of the operational system, if one system fails.
Last edited by T28B; 7th Nov 2018 at 18:29.
Reason: formatting clean up