(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.
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(3)
(4)
(5)
(6)
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737-300/400/500MAINTENANCE MANUAL
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.
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:
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.
(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.
Speed and Altitude Restrictions
(a)
The FMC contains provisions for all standard speed and altitude restrictions associated with airport departure and arrival sequences as well as the 250 knot below 10,000 feet restriction. Additionally the FMC is capable of implimenting entered speed and altitude restrictions associated with waypoints in the route.
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737-300/400/500MAINTENANCE MANUAL
(7)
Vertical Path Deviation
(a) For the descent phase modes, the FMC vertical steering process supplies a vertical path deviation to the EHSIs. The deviation normally equals the difference between the current reference altitude on the vertical reference path and current airplane altitude.
(8)
Path Descent Speed Targets
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