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737-300/400/500MAINTENANCE MANUAL
(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.
(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.
(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.
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH DUAL FMC ú
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737-300/400/500MAINTENANCE MANUAL
(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.
(7)
Lateral Offset Function
(a) The FMC has the capability of selecting parallel lateral path to either the left or right of the original path. A lateral offset can be activated for the active leg, or can be pre-planned. Offsets are specified in increments from 0.1 to 99.9nm. The flight crew can initiate, change, or cancel a lateral offset at any time. The offset path may be modified while flying the offset path. When an offset is active, the OFST light on the CDU will be on.
D. Vertical Guidance and Steering
(1)
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