(b)  
N1 equalization is enabled with both engines above 50 percent

 N1 and the equalization rate limit is 1.0 degree/second.N1 Limit Select

 (a)  
When operating through the speed command function, the speed error rate command is limited to prevent the speed error rate command from causing the engine to exceed N1 limit.

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A
737-300/400/500MAINTENANCE MANUAL
 B. Speed Mode Logic
 (1)  
During speed mode operation, the Autothrottle controls the throttles to maintain the selected target airspeed, or minimum speed.

 (2)  
The speed mode is enabled by a speed request from the Autopilot, minimum speed logic, or a request for the throttles to be moved forward of the aft limit (throttle-up).

 (3)  
The speed mode is inhibited when N1 mode is enabled, or during landing retard, or if CAS is less than 60 knots (80 knots for airplanes with A/T computers P/N 10-62017-25 and subsequent), or throttle hold, or squat for two seconds and not in go-around.

 C. Retard 1 Mode Logic
 (1)  
During retard mode, the throttles are driven aft at a predetermined rate. Retard occurs during landing flare (Retard 1), or when autopilot descent is engaged with speed on the elevators (Retard 2).

 (2)  
Landing retard is set when radio altitude is less than 27 feet, or

 1.5 seconds after autopilot flare.

 (3)  
The mode set is inhibited if the flaps are not in the landing configuration, or if the Autothrottle is not in the speed mode.

 (4)  
Landing retard is reset when the Autothrottle is disengaged, the LRRA becomes invalid and DFCS not in flare mode, or the N1 mode is enabled.

 D. Retard 2 Mode Logic
 (1)  
Cruise descent retard is set after the receipt of a drive aft request from the Autopilot.

 (2)  
The throttles are driven to the aft stops, unless overriden by the pilot, or unless the speed mode or the N1 mode is selected. The retard is also stopped if the Autopilot transmits a large pitch down rate signal.

 21. A/T Servo Drive and Clutch Control__________________________________
 A. Servo Drive (Fig. 13)
 (1)  
The servo drive function generates the throttle commands, then through the throttle servo loop drives the throttles to the command position. Rate command inputs to the servo control are selected from either the speed/Mach command function or the N1 command function.

 (2)  
Either the N1 control error or the speed control error is selected, damped by throttle rate feedback, and after limiting provides the rate input to the throttle servo control. The rate is limited to 15 degrees/second during takeoff, reduced go-around, maximum go-around. Otherwise, rate is limited to 6 degrees/second.

 (3)  
The command integrators integrates the rate command inputs and compares this position with the actual position to generate a throttle rate demand to the servo control. When the Autothrottle is in an inactive mode, not responding to the integrator commands, the integrators are continuously synchronized to their respective throttle position. Variable forward and aft integrators are continuously synchronized to their respective throttle position. Variable forward and aft integrator output limits establish the command authority of the Autothrottle.

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A 737-300/400/500MAINTENANCE MANUAL
(FIG 14)D1667(FIG 5)
M927 A/T SERVOACTUATOR ASSY 1(STA 370, WL 200,
LBL 10)
D1669APC
SAME AS
B
M927
M928 A/T SERVOACTUATOR ASSY 2M917 AUTOTHROTTLE COMPUTER (E1-1)

(STA 370, WL 200,RBL 10) NOTES:

1
THE INTEGRATOR HAS VARIABLE UPPER AND LOWER LIMITS WHICH
CORRESPOND TO THROTTLE POSITION FORWARD AND AFT LIMITS

Autothrottle System - Servo Drive
Figure 13

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