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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 EHSIs during both automatic and manual steering of the airplane whenever the EFIS control panel mode selector is in the NAV or MAP mode.
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737-300/400/500MAINTENANCE MANUAL
 D. Vertical Guidance and Steering
 (1)  
The vertical guidance functions provide the vertical profile transition guidance required for the generation of airspeed or vertical speed targets to the DFCS. Coordinated speed, N1 limit/targets, and mode commands are also provided to the autothrottle system. When the VNAV mode is active during an airplane path descent, the FMCS steering functions continuously compute a rate and magnitude limited vertical speed command for the autopilot pitch channel. This vertical speed command is based upon a measured value of altitude error and a nominal altitude rate.

 (2)  
Performance Speed/Altitude Profile Guidance

 (a)  One mode of vertical guidance involves control of the airplane's airspeed by the autopilot to FMC computed target values, while thrust is set at the climb N1 limit rating, or at idle. Both speed target and thrust limit data are furnished by the FMC. Climbs and descents flown at the economy speed schedules are the most common application of this mode. Constant speed profiles based on manually entered speed schedules may also be used. These profiles are constrained by DFCS mode control panel altitude (MCP ALT) setting. When descents of this type are flown, the speed legend (SPD) appears in the title line of the descent displays to indicate a SPEED (airmass) descent is selected.

 (3)  
Desired Flight Path Guidance

 (a)  A second basic mode of vertical guidance involves control of its vertical path by a vertical speed command from the DFCS and is used only in descent. The vertical path is computed for a specified speed/thrust profile. This concept uses idle thrust and economy speeds as the most common mode of operation. When descents of this type are called for, the PATH legend appears in the descent display title line to indicated that a descent path referenced to specific geographic positions is selected.

 (4)  
VNAV Mode Transitions

 (a)  
VNAV mode transitions from on to off cause no new FMC predictions or flight path changes.

 (b)  
VNAV mode transitions can occur as a result of operator action or as a result of path computations. Two examples of the DFCS-FMCS interaction are:

 

 (5)  
Altitude Selection and Vertical Path Capture

 (a)  The vertical profile parameters which determine the system's vertical guidance requirements are the predicted or desired altitudes at each of the waypoints in the active flight plan. The selected altitude set on the MCP acts as a limiter. The MCP ALT always determines the airplane's current altitude limits, in both the climb and descent.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH EFIS ú

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (b)  The maximum descent Flight Path Angle (FPA) for predicting interception of the vertical reference path is 6 degrees for airplane operation above 10,000 feet and 4 degrees below 10,000 feet. If the predicted late descent path does not intercept the vertical reference path prior to the last waypoint, the system resets to a SPEED descent mode and displays a DES PATH UNACHIEVABLE message on the CDU. Longitudinal steering also is constrained by these limits when generating the vertical speed command to the DFCS.
　
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