(4)
The lateral path is first constructed as a series of legs, where each leg is the path between two waypoints. The profile prediction process uses the lateral guidance functions to compute the combination of great circle and turn segments that make up this path. The profile predictions utilize the climb wind profile estimated as well as any inserted wind for cruise altitude. Similarly for descent, wind is estimated and zero wind at the destination airport is assumed.
(5)
The performance plan that was defined along with the flight plan determines the performance advisory speed schedule and thrust that will be calculated by the FMC. The speed and altitude restrictions attached to the lateral waypoints in the climb and descent phases also constrain the airspeed and altitude profiles in the process of computing the vertical path. The profile predictions then use the performance calculations to compute altitude, time, airspeed and fuel for each leg. Thus, the vertical profile generated already accounts for speed and altitude restrictions.
C. Lateral Guidance and Steering
(1)
When a complete flight plan has been activated in the FMCS, the lateral guidance functions are responsible for its execution. Route segments may be flown as great circle tracks between flight planned waypoints, or as constant heading legs, depending upon flight plan criteria.
(2)
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.
(3)
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.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
(4)
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.
(5)
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.
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