(b)
If heading hold capture criteria are satisfied when the CWS switch/light is pressed (and wheel force is zero), the CWS request pulse enables CWS heading request logic. If CWS heading enable is logic one (F/Ds off or LNAV engaged), the CWS heading request disables roll attitude sync and roll attitude capture and initiates heading hold logic.
(c)
After CWS engage, whenever wheel force is greater than |2 lbs, both CWS heading request and roll attitude capture are disabled. The airplane is in the CWS maneuver submode. When the wheel force is reduced to below |2 lbs, with roll attitude rate greater than |0.75 deg/sec, roll attitude sync and roll attitude capture become logic one and the airplane holds at that attitude.
(d)
The roll synchronization rate, roll command limit and roll mode change logic signals are all generated whenever roll attitude synchronization is logic one (after a 0.2 sec delay).
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MAINTENANCE MANUAL
(10) AIRPLANES WITH CONTROL WHEEL STEERING; Roll CWS Engage Signal Processing
(a)
Synchronization (A/P not engaged)
1) Prior to engagement of autopilot, the aileron servo command is synchronized to wheel position and roll attitude command is synchronized to roll attitude (Fig. 11).
(b)
Roll Attitude Hold Submode.
1) Assume bank angle is a constant 7 degrees and no wheel force is applied when the CWS Engage pushbutton is pressed. Roll attitude synchronization and roll attitude capture are both logic one and heading capture criteria is logic zero.
2) When local servos are engaged, A/P engage becomes logic one and roll attitude command has synchronized to 7 degrees. At A/P engage, the aileron trim integrator is switched to receive wheel rate but roll command limit has opened the path. The aileron servo command then remains unchanged from the synchronized value and the airplane holds the 7 degree bank angle.
(11) Heading Hold and Heading Select Modes (Fig. 12)
(a)
The MCP controls and displays associated with the heading modes are identified on figure 12.
(b)
The heading hold mode can be engaged by pressing the HDG HOLD switch/light. If the bank angle (B/A) is greater than the heading capture criteria threshold, the AFDS commands a roll to wings-level and then commands to heading hold. If the B/A is within the heading capture criteria threshold, the AFDS commands to heading hold when the HDG switch/light is pressed.
(c)
Heading select mode is used to change to a new heading. The HDG SEL knob and switch on the MCP control the selected heading and the heading select mode. The AFDS commands generated depend on whether the SEL switch is pressed before or after the SEL knob is turned to provide the selected heading. 1) If the HDG SEL knob is rotated to the desired heading
before the SEL switch is pressed, the AFDS commands a turn towards that heading in the direction of the smallest turn angle.
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757
MAINTENANCE MANUAL
KNOB HDG HOLD MODE REQUEST)
080 MODE CONTROL PANEL (EXAMPLE)
000
HDG HOLD WITH AIRPLANE IN A BANK BELOW HDG CAPTURE CRITERIA THRESHOLD
RESPONSE
CONTROL FOR HEADING HOLD DISPLAY UNTIL WINGS LEVEL AFDS STATUS FMA SHOW HDG HOLD
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