B. Flight Plan Management
(1) The primary structure of a flight planned route consists of a sequence of waypoints or legs. A waypoint is completely defined in terms of its latitude, longitude, and altitude, but in some cases these parameters may be affected by other operational variables. The FMCS implementation recognizes two distinct types of waypoints called fixed and floating. Fixed waypoints, which are the most common of the two waypoints, are defined by a fixed geographic position. Floating waypoints whose positions are dependent upon airplane operating state and external variables such as wind. The lateral flight plan is determined by fixed and floating waypoints and is computed by lateral guidance processes. The active TO waypoint may be a fixed or floating waypoint. Floating waypoints are displayed on the CDU with the identifier enclosed in parentheses.
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A 737-300/400/500MAINTENANCE MANUAL
DME 2
DME Slant Range Crossing Angle
Figure 14
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(2)
In addition to fixed and floating waypoints, the system implementation takes into account in its profile predictions the existance of points of importance to the vertical performance calculations. These points are like the floating waypoints in that they occupy positions dependent upon airplane operating state, but these points do not appear on any of the RTE or RTE LEGS displays and their existence is transparent to the crew using the system, except for Top of Climb (T/C), Top of Decent (T/D), and End of Decent (E/D) advisories. They influence the system only by their presence being accounted for in time and distance predictions.
(3)
The desired lateral path is defined by the sequence of fixed and floating waypoints. The waypoints may have been extracted from the navigation data base or manually inserted. For each of these waypoints, the profile prediction processing computes and assembles the following: predicted altitude, time-of-arrival, speed, and fuel remaining at the waypoint, distance from the preceding waypoints, and course of the flight path between the waypoints.
(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.
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