(b)
During the approach and capture phase of A/T speed operation, acceleration damping and wind shear compensation are provided.
(2)
Airspeed Error
(a)
Airspeed error is derived from selected airspeed, selected mach, or minimum speed error.
(b)
Selected airspeed, either FMC or MCP airspeed reference, is compared with CAS from the ADC. The difference is the airspeed error, used during low altitude operation. At higher altitude, when the Autopilot has selected mach for the speed reference, the difference between selected mach and mach from the ADC is used during speed mode operation.
(c)
During go-around operation, when the mode has transferred from reduced go-around to reduced GA (maintaining), the average N1 error is processed through the speed command channel. This allows acceleration damping, wind shear compensation, and minimum speed protection, while still controlling N1 to the reduced N1 limit.
(d)
The selected error is compared to the minimum speed error, and the error resulting in the highest airspeed is selected.
(e)
The airspeed error signal, after further processing, provides the throttle rate command and is also used to generate the fast/slow indicator command.
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A 737-300/400/500MAINTENANCE MANUAL
HARDWAREM917 AUTOTHROTTLE COMPUTER (E1-1)
Autothrottle System Speed/Mach CommandsFigure 12
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(3)
(4)
(5)
(6)
(7)
(8)
A
737-300/400/500MAINTENANCE MANUAL
Gain Adjustment
(a)
Airspeed gains are changed to suit the maneuvering requirements of the airplane. Gains from high to low are: approach, capture, and track.
(b)
The approach configuration uses a high gain configuration. Further, the gain is twice as high for forward throttle motion as for aft throttle motion. During flap motion, the gain is reduced to one quarter of the forward high gain to avoid unnecessary throttle motion. The Autothrottle uses a special low gain configuration in cruise with N1 feedback to track airspeed which keeps throttle and N1 activity to a minimum. Any condition which indicates maneuvering such as lowering the flaps, turning off the Autopilot altitude hold or altitude acquire mode, or minimum speed will cause the Autothrottle to drop out of the track gains and go to the higher capture gains.
(c)
In the absence of the approach or track gains, capture gains are used which result in moderate throttle activity for vertical speed, minimum speed or flap down operation.
Vertical Wind Compensation
(a)
During approach or reduced GA (maintaining) operation, a vertical wind component can cause throttle advancement up to the selected N1 limit. The vertical wind component is measured by comparing the air flight path angle (alpha minus pitch attitude) to the inertial flight path angle (filtered altitude rate compensated by normal acceleration).
N1 Damping
(a)
N1 damping introduces the average N1 error into the speed command function when in track configuration. Any standing N1 error is washed out so it does not introduce a static speed error, but provides dynamic damping. A function of angle of attack is also introduced, providing fast engine response to changes in drag due to changes in angle of attack.
Retard
(a)
During cruise descent retard or landing retard, the speed error is replaced by a retard rate command.
(b)
The retard rate during cruise descent is minus two degrees/second minus the flight path angle rate. During landing operation, the throttle position rate is adjusted so retard is complete in six seconds. After touchdown, the rate is increased to minus eight degrees/second.
Equalization
(a)
Equalization is used to keep the engine N1's balanced at high N1 and hold throttle stagger at low N1. This reduces rudder trim requirements caused by mismatched thrust.
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