(5)
When the GA mode is in affect, a different detector is used. The greater than 10% error is still required, however, the sampling time is now reduced. If the signal is present for 500 out of the 800 milliseconds in a sampling period, a command will be generated. The trim up or trim down command will continue until the less than 2% detector senses that the ratio of the command to the authority is down to this level. The output of the drop out detector sends a reset signal to the trim detectors which will stop the trim command and reset the detectors to zero. These detectors will be held in the reset mode if the A/P is in the GA mode. The input to the drop out detector is used to reset the GA trim detectors when in the GA mode.
B. Stabilizer Trim Interface (Fig. 11)
(1) The stabilizer trim system maintains proper airplane trim under varying flight conditions. These commands are provided by the FCC using the speed trim system with A/P disengaged at low speed or the A/P automatic trim.
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A 737-300/400/500MAINTENANCE MANUAL
TRIM ON DOWN LIMIT SW
R64 MAIN STABTRIM CONT RLY
Stabilizer Trim Commands Interface SchematicFigure 11
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A
737-300/400/500MAINTENANCE MANUAL
(2)
The stab trim clutch voltage first passes through the control column forward and aft switches. Pressure on the column in either direction will disengage the clutch. The signal then passes through the cruise trim cutoff switch on the control stand. This switch removes all stabilizer signals in the event of stabilizer runaway. The signal then goes to the stab trim servo where it provides the voltage for engaging the clutch.
(3)
The trim up/down commands also pass through the column cutoff and cruise trim cutoff switches. Prior to going to the trim servo, the signals pass through nose up or nose down limit switches which limit the amount of stabilizer travel which can be commanded by the FCC.
(4)
The power required by the trim servomotor is three phase. Switching of the power input provides the control for proper directional rotation. A trim down command energizes the trim down relay which applies power through relaxed contacts of the trim up and high speed relays. When the trim up command is received, both the trim down and trim up relays are energized. The trim up relay changes the power input to the motor to cause opposite rotation. The high speed motor is energized when the flaps are down to increase trim response.
C. Stabilizer Trim Warning Lights (Fig. 12)
(1)
The FCC's provide an output to warning light circuits based on operational status of stabilizer trim systems.
(2)
The speed trim warning annunciator only illuminates when both FCC's provide logic to speed trim light circuits. Warning light logic is based on input sensor being valid. This includes N1 inputs, stabilizer position, computed airspeed and altitude rate. There must not be a simultaneous trim up and tirm down command. Stab trim clutch wrap around must be present. Detector 2, which is a command response monitor which detects movement of the stabilizer after a stab trim command signal has been initiated.
(3)
The stabilizer Out of Trim Warning light is illuminated by three differenct detectors. A delay of 10 seconds prevents nuisance warnings. The Stab Out of Trim light is inhibited when GS engage and below a radio altitude of 100 feet. However, this inhibit logic does not extinguish the light if illuminated prior to passing through the 100 feet altitude. The stabilizer warning detector areas follows:
(a)
Detector 1. This unit detects a difference of 3° between the elevator position and elevator A/P actuator. This detector also activates a holding circuit which requires that the elevator A/P actuator commands be reduced to within 0.5° of the neutral shift position.
(b)
Detector 2. This unit detects movement of the stabilizer after the command is initiated. A stabilizer movement of greater 0.5° in 10 seconds is required to prevent a warning.
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