output of the roll resolve (RS) at summing point 7 and applied to summing point 8 through bank rate limiter. The bank rate limiter limits the roll rate command to  4 degrees per second in this mode.

(c)  
The heading error signal at summing point 8 is summed with tachometer feedback signals and applied to the roll computer through the motor amplifier and the power amplifier. The amplified signal drives the roll computer in a direction to allow the roll resolver RS to cancel the heading error at summing point 7. When the roll computer is driven by the heading error signal, the roll CT develops an error signal. The roll CT error signal results in aileron displacement as described in roll attitude hold mode. The aileron displacement produces a banked turn in the direction required to reduce the heading error and roll CT signals to a null.

(5)  
Heading Hold and Heading Off Modes

(a)  
The heading hold mode is selected on the control panel whenever the heading switch is in the center position and mode selector is in MAN position. Heading hold mode may be used during the intercept phase of lateral beam operation (VOR/LOC) but will be automatically disengaged at lateral radio beam capture. Heading hold will be in effect after engaging the roll system except during CWS out of detent, after lateral beam engagement or whenever HDG SEL or HDG OFF are selected. An additional requirement for heading hold mode engagement is that the bank angle sensed by the roll sensor must be less than 5 degrees. However, once the mode is engaged, CWS bank angles greater than 5 degrees will not cause mode disengagement. The heading switch will be in the center position (heading hold) whenever the heading logic circuit (Fig. 8) removes voltage from the heading switch holding coil thus allowing the spring-loaded switch to return to center from the HDG SEL position.

(b)  
During the heading hold mode, the autopilot will maintain the airplane compass heading that exists one second after the mode is engaged. Any heading error output of the heading CT (which is clamped during heading hold) is applied past de-energized switch RS-6 to summing point 1 of Fig. 6 and amplified. The heading error signal is applied to an ADC Q-potentiometer (heading Q-potentiometer) which schedules signal gain as a direct function of airspeed to provide optimum control at all airspeeds. The signal is then applied to summing point 2. The heading error signal is applied to summing point 7 through the bank angle limiter which provides bank angle commands limiting of the heading error signal. For the heading hold mode, the bank angle command limit established by the bank angle limiter is 25 degrees. The mode of operation determines whether a bank angle command limit of 4, 8, or 25 degrees is

570 
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Feb 01/76  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-0 Page 33 

HDG SEL and NAV Logic Circuit  5A5 
22-11-0  Figure 8  May 20/82 
Page 34 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

utilized, and the corresponding bank angle command limit is applied by the bank angle limiter. The heading error signal, which is now a bank angle command, is summed with the output of the roll computer resolver at summing point 7. The result of the summation is applied to the bank rate limiter. The bank rate limiter limits the roll rate command through the roll computer. For heading hold mode, the rate command limit is 7 degrees per second. The mode of operation determines whether a rate command limit of 1.3, 4, 7, or 13 degrees per second is utilized. The output signal from the bank rate limiter is summed with tachometer feedback signals at summing point 8. These tachometer feedback signals provide damping for the roll computer servo loop. The signal from summing point 8 is amplified by the motor amplifier. The amplified signal drives the roll computer in a direction which causes the resolver (RS) signal to cancel the heading error signal at summing point 7.
(c)  
When the roll computer is driven by the heading error signal, the computer CT develops an error signal. The CT error signal results in aileron displacement as described in roll attitude hold mode. The aileron displacement produces a banked turn in the direction required to reduce the heading error and roll CT signals to a null.

(d)  
When the autopilot enters the localizer approach on-course submode of operation, the heading computer is clamped (heading hold configuration). Error signals from the heading CT are fed to summing point 2 as previously described except that the signal gain is reduced by energized switch RS-7 and an integrated heading term is supplied to summing point 2 by the lagged roll-integrator. The summation of heading error signals with the inverted integral of heading error signals provides mode damping for the localizer on-course submode of lateral radio beam operation.
　
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