EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

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 ALL  ú ú 26 Page 32 ú Jan 20/98
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
D00427
A
757
MAINTENANCE MANUAL

 2)  If the SEL switch is pressed first, the AFDS commands generated are always in the direction the HDG SEL knob is turned.
 (d)  Figure 12 shows an airplane at heading 000 (A) turning to a heading of 080 (D). After the SEL switch has been pressed, the airplane (B) rolls to a B/A proportional to the heading error but is limited by the B/A limit switch on the MCP. The F/D command bars on the EADI are centered indicating that correct roll command is being applied. At C, the heading error has reduced to 10 degrees right requiring a reduction in B/A to permit smooth rollout at 80 degrees. The EADI shows reduced roll attitude and roll left command. The heading of 080 at D is equal to the selected heading so the AFDS automatically engages into heading hold and maintains the selected heading.
 (12)
 Heading Hold/Heading Select Mode Logic

 (a)  
The heading hold mode is initiated by pressing the HDG HOLD switch/light. If adjusted heading is valid, the heading hold mode armed logic is generated (Fig. 10). When the heading hold capture criteria logic is zero, indicating a B/A greater than the threshold, the roll attitude washout logic is generated. When the heading capture criteria logic goes high, a heading hold request is generated. One second later, heading hold engage is generated and the washout logic goes to zero. The heading hold mode logic is generated and this initiates the command heading/track control and the heading proportional command logic signals.

 (b)  
Heading select mode enable logic is generated by pressing the HDG SEL switch. The heading select engage valid logic is generated when a selected heading is entered through the HDG SEL knob. These two logic signals produce the heading select mode engaged logic which then produces the heading/track and heading proportional logic.

 (13)
 Heading Hold/Heading Select Signal Processing

 (a)  
Heading/Track command processing computes the heading proportional command, the heading integral command, and the flight director heading command. It permits synchronization of heading computation software when the A/P is not engaged in a heading control mode. It selects the correct heading reference for the heading control mode in use and ensures that turn commands produce the correct turn direction for the mode in use. Gain programming is applied as required (Fig. 11).

 (b)  
Initial Conditions - Heading Hold Mode

 1)  When the airplane is on a given heading, magnetic heading is applied as roll attitude reference to both the autopilot and flight director turn direction controls. The heading command feedback from the heading command processor (the heading proportional command) is also magnetic heading. The output of both turn direction controls is zero and the heading integral command and the flight director command are both zero (i.e., no aileron command and F/D roll bar is centered).
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 22-13-00
 ALL  ú ú 16 Page 33 ú Jan 20/98
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL

 (c)  
Roll F/D Command - Heading Select Mode

 1)  When the HDG SEL button is pressed, the heading select mode is engaged and the selected heading from the MCP HDG SEL knob is compared with the magnetic heading. The difference is applied to both turn direction controls. Output from the F/D turn direction control is scaled to produce the F/D heading command. This is then modified by other inputs: roll attitude; roll attitude rate; and, if LNAV is engaged, horizontal steering commands. This produces the Roll F/D command which drives the roll command bar on the EADI.

 (d)  
Aileron Servo Command

 1)  Selected heading is integrated and limited by the B/A switch setting and the heading rate command filter. This output is summed with heading angle command and becomes the heading proportional command. The integrator output is also summed with magnetic heading and developed into the heading integral command. Localizer and runway alignment commands are added, when appropriate, and the roll rate integral command and delta roll attitude command are produced. The latter is differentiated and added to the former to produce the roll attitude command.
　
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