C. Lateral Guidance (LNAV)
(1)
Lateral guidance shall be computed with respect to paths between waypoint and curved transitions between each path segment. The guidance function will provide smooth maneuvering while in segment transitions, path captures, holding patterns, offset path, path recaptures and direct-to operations. The LNAV software shall provide lateral guidance roll angle commands to the Autopilot Flight Director System (AFDS) for lateral airplane control.
(2)
Lateral guidance commands must be transmitted on a general purpose bus while LNAV is engaged, otherwise, the output will be zero with a SSM that shows the no computed data (NCD) state.
(3)
LNAV Engagement
(a)
Engagement of LNAV indicates LNAV has been selected on the AFDS MCP and the FMC controls lateral movement of the airplane.
(b)
Engagement shall be possible when: 1) A flight plan has been entered, activated and executed through the CDU. 2) The LNAV arm/operational digital discrete from the MCP is
set to true. 3) The airplane satisfies the LNAV capture criteria. 4) The current calculated airplane altitude is greater than 50
feet AGL. 5) Data required to compute the lateral guidance command is valid. 6) No more than one engine has failed.
(c)
LNAV capture shall be possible if: 1) The crosstrack distance 2.5 nm or less. 2) The crosstrack distance from which the FMC lateral control
laws can initiate path capture when on an intercept heading.
(d)
When the FMC receives the LNAV engagement signal, it first checks to see weather engagement conditions exist and does it,s initializaton. When ready to engage, the FMC sets its SSM status from NCD to normal operation and synchronizes to the current bank angle. 1) When the LNAV mode is deselected on the MCP the FMC will
set its steering command to zero and set the SSM to NCD. If the steering commands become invalid while LNAV is engaged, the airplane will continue flying at the FMCs derived steering command at the time of failure until the FMC becomes valid again.
D. Vertical Guidance
(1) The FMC will control vertical movement (VNAV) of the airplane through the flight profile. The VNAV function will provide pitch commands to the AFDS and thrust and speed commands to the autothrottle function. These commands shall provide smooth maneuvering through all mode transitions, flight phase transitions, path tracking maneuvers and unexpected vertical maneuvers.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
(2)
The FMC will fly to the MCP SELECTED ALTITUDE. This selected altitude will override the profile target altitude. VNAV control will ensure that the airplane stays well in a tolerable offset of the MCP selected altitude.
(3)
VNAV Engagement and Disengagement
(a)
Engagement of VNAV will be possible if: 1) Gross weight, cost index and cruise altitude are entered on the CDU's PERF INT page. 2) Required data from the ADC, IRS and internal records are valid. 3) The VNAV arm/operational digital discrete from the mode
control panel (MCP) is set true. 4) No more than one engine failed.
(b)
When the pilot pushes the VNAV button on the MCP the FCC determines if engagement is possible. If so, the VNAV button illuminates and the VNAV arm/operational signal is set true and received by the FMC. The FMC checks its thrust management takeoff status for engagement conditions.
(c)
Engagement of autothrottle is similar to that of VNAV arm/operational. The A/T ARM on the MCP must be set to ARM position and VNAV arm/operational button must be illuminated and conditions within the thrust management function must be met in order for A/T VNAV mode to engage.
(d)
Disengagement of VNAV can be accomplished through pilot action on the MCP, by selecting another mode or pressing the lighted VNAV button. Disengagement is also accomplished through FMC No Computed Data situations. Re-engagement will not be possible until the VNAV arm/operational discrete goes low then high again.
(e)
If disengagement occurs within the FMC the thrust management, airspeed, mach, and pitch commands will remain the same value at the time of disengagement.
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