22-31-00
 ALL  ú ú 07 Page 39 ú Nov 12/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 (4)  
For normal operation, with the PMC ON, the forward limit is 65 degrees, and with PMC OFF the forward limit is 55 degrees. During the reversion mode or when the related N1 sensor is invalid, the forward limit is dependent on altitude, temperature, and PMC status, ranging from 35 degrees to 57 degrees.

 (5)  
At altitudes above 400 feet and when fast engine response is not required, the aft limit for the integrators is set to -5 degrees, insuring that the throttles will be driven into the aft stop. Below 400 feet radio altitude or when the throttles should be advanced to improve response time, the integrator aft limit is moved forward to 10 degrees.

 (6)  
The difference between throttle position commands is limited. With PMC ON, or during a dual channel approach, the command split is limited to 8 degrees. This prevents disengagement due to differing commands, as would occur during an engine failure. With PMC OFF and not dual channel Autopilot operation, the split limit is increased to 15 degrees.

 (7)  
The servo loop responds to the output rates and drives the throttle levers.

 (8)  
The servo loop is inhibited when the A/T is in arm, disengaged, during validity inhibit (see engage/disengage), throttle hold, if N1 becomes invalid during track operation, or if the thrust reversers are deployed during retard. Also during throttle hold, power is removed from the servo control and motor.

 (9)  
The throttle position command is compared to thrust lever angle feedback to generate a rate error signal. This signal is limited, then used as the throttle servo rate command. Normally set to 6 degrees/second, the limiter is opened for 12 seconds after a TO/GA switch is pressed, allowing throttle advancement rate up to 15 degrees/second.

 (10)
 If N1 limit is exceeded, a large drive aft command is provided, driving the thrust lever back. If this occurs during a dual channel approach, the autothrottle will disengage as soon as the throttle lever split reaches 10 degrees if flare arm or flare engage are annunciated.

 (11)
 A throttle rate command, damped by throttle rate feedback, drives the servo actuator motor through the servo control. The servo control is an ac powered triac bridge circuit that serves as a variable duty cycle switching amplifier to control the motor.

 (12)
 During altitude hold, when flight conditions are relatively stable, it is desirable to minimize throttle activity. This is partially accomlished by operating the speed command function at low gain. Also, small short-term errors are crossfed to the Autopilot as an altitude bias; thus, providing limited speed control through the elevators. The bias is limited to 50 feet.

 (13)
 For example:  if a minor disturbance causes airspeed to decrease, the crossfeed provides the autopilot with a ramping altitude bias, rate limited to 5 feet/second. This will result in a descent rate of up to 300 feet/second restoring the loss airspeed. Such a command as washed out within 100 seconds and is limited to a total altitude change of 50 feet.

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 22-31-00
 ALL  ú ú 09 Page 40 ú Nov 12/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (14) Altitude bias crossfeed is inhibited if the bank angle exceeds 11.5 degrees.
 B. Electromagnetic Clutch Control (Fig. 14)
 (1)  
General

 (a)  The A/T servo actuator is connected to the throttle cables through an electromagnetic clutch. When the A/T is engaged, the clutch is energized unless the thrust levers are restrained. Restraint may result from the thrust levers being driven into the aft stop or by action of the pilot.

 (2)  
Clutch Control

 (a)  The clutch receives 28-volt dc excitation through S2. A dc return for the clutch is provided through S3, when the A/T is engaged. With both S2 and S3 closed, the clutch is energized, connecting the servo actuator motor to the throttle cables. This allows the A/T to position the engine power lever and to back-drive the throttle levers.

 (3)  
Torque Switch

 (a)  
If the A/T drives the throttle lever into the aft mechanical stop, or the pilot restrains A/T movement of the thrust levers, or the pilot attempts to manually position the thrust lever, the resultant torque is sensed through the torque switch mechanism and the torque switch opens. When the torque switch opens, the clutch is de-energized. A torque switch logic is provided to the A/T computer, arming the torque switch bypass function.
　
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