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 ú ú 03 Page 50 ú Nov 12/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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(3)

(4)

A
737-300/400/500MAINTENANCE MANUAL
If at any time LNAV capability is invalidated and lateral guidance outputs are lost, the FMC provides an alerting message, and sets the roll steering command to no computed data, and the LNAV to invalid. The autopilot disconnects the LNAV mode and reverts to Control Wheel Steering (CWS), until an alternative guidance mode is selected.Guidance Path Capture
 (a)  
An initial path capture maneuver may be required when the FMCS becomes coupled to the DFCS (MCP LNAV is pressed and turns on) or when the FMCS is already coupled and the FMCS executed flight plan is changed.

 (b)  
When the FMCS is not coupled to the DFCS (LNAV is off), and a valid flight plan exists, path capture criteria is tested every 2 seconds. When the FMCS is already coupled to the DFCS (LNAV is on), and the FMCS flight plan is changed, path capture criteria are tested with respect to the new flight plan. If LNAV is out of limits, the FMCS will disengage itself from the DFCS (by invalidating its roll command). The guidance path may be captured by the lateral steering function or by normal steering by the flight crew.

Waypoint Transitions, Leg Switching, and Path Capture Criteria

 (a)  
In defining the guidance maneuver requirements for leg-to-leg transitions at waypoints, the lateral guidance subroutine recognizes fly-over and non-fly-over transitions. Fly-Over (FO) transitions require the airplane to pass directly over the waypoint's geographic position. Non-Fly-Over (NFO) transitions, do not require the airplane to pass over the waypoint's geographic position.

 (b)  
Maneuvering requirements are also dictated by the magnitude of the course change involved in the transition. The turning maneuver required for a transition involving a course change of 135 degrees or less differs considerably from the required for a course change greater than 135 degrees. Transition switching point criteria are determined by the FMCS for NFO transitions to provide a roll lead-in point on the inbound leg to the waypoint. This maneuver produces a smooth, coordinated turn that intercepts the outbound leg without overshoot. During large course changes, the roll lead-in point is advanced to provide a turn that is tangent to a perpendicular defined by the inbound leg, which serves to limit the necessary overshoot. The subsequent path capture maneuver required to complete this path or any of the FO transition paths is subject to a 45 degree path intercept limitation.

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 ú ú 03 Page 51 ú Feb 25/87

BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
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Steering, Turns, and Bank Angle Limits

 (a)  
FMCS computed roll commands are designed to result in smooth coordinated turns at all leg-to-leg transitions. The radii of the automatically controlled transition turns are dependent upon the airplane's maximum estimated ground speed during the turn and upon speed dependent roll angle limiting functions.

 (b)  
When the LNAV mode is valid, the FMC steering process continuously computes an acceleration limited, rate limited, and magnitude limited roll angle command. This command is based upon measured values of track angle error, cross track deviation and a nominal roll angle. The nominal roll angle is determined by the airplane's ground speed and the required turn radius. The primary mode of guidance provides lateral path steering to a track defined as a segment of a great circle path. If steering along a constant heading path is required, the roll command is generated as a function of the measured heading error alone.

 (6)  
Lateral Deviation

 (a)  The lateral deviation of the airplane from the desired track is continuously computed by the FMCS whenever a flight has been entered. This deviation is displayed on the HSIs during both automatic and manual steering of the airplane whenever the transfer is in the NAV position. The HSI cross-track deviation has a normal sensitivity of 2 nautical miles per dot of deflection.
　
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