FMCS -FMCF FUNCTIONAL DESCRIPTION -GUIDANCE SUB-FUNCTION

General
The FMCF guidance sub-function calculates lateral and vertical guidance steering commands. This occurs when the guidance sub-function receives an active route (lateral flight plan) and active vertical flight plan.
There are two guidance sub-functions, one for AFDS and one for TMCF. They receive these inputs to calculate the guidance commands:
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LNAV sub-function (desired)

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Mode control panel (MCP)

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VNAV sub-function (desired)

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CDU

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Lateral position (actual)

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Vertical position (actual)

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Airspeed (actual)

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TMCF thrust (actual)

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LNAV Engage

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VNAV engage.

Guidance Sub-function -AFDS
The AFDS guidance sub-function uses desired lateral (route) and desired vertical flight plan data from the LNAV and VNAV sub-functions. It also uses actual airplane position data from the sensors. The guidance steering commands are the difference between the desired and actual.
The active FMCF sends the LNAV and VNAV steering commands to the AFDS for automatic roll and pitch control of the airplane during the climb, cruise, and the descent flight phases.
The active FMCF sends speed commands to the AFDS in the climb flight phase to control it to the VNAV sub-function target speed. It sends VNAV path commands to the AFDS in the cruise and the descent flight phases.
The CDU and mode control panel (MCP) is the main interface for preflight and enroute changes of the LNAV and VNAV flight plans. The LNAV and VNAV modes can arm on the ground and become active in the air.
Guidance Sub-function -TMCF (Thrust/Speed)
The TMCF guidance sub-function uses thrust or speed targets from the VNAV sub-function and compares them with the actual airplane thrust or speed. The difference between the target and the actual thrust or speed is the guidance command.
The active FMCF sends thrust steering commands to the TMCS to control the engine thrust in climb and descent. The FMCF sends speed commands to the TMCS to control the airplane speed in cruise.
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FMCS -FMCF FUNCTIONAL DESCRIPTION -GUIDANCE SUB-FUNCTION

VNAV Mode Summary
The AFDS and TMCS guidance sub-functions supply five basic VNAV modes to fly the airplane along the vertical flight profile. The AFDS controls the pitch axis and the TMCS controls the thrust. These are the five basic VNAV modes:
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VNAV speed/thrust

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VNAV path/speed

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VNAV path/idle

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VNAV altitude/speed

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VNAV speed/hold.

VNAV speed/thrust causes the AFDS to control to the FMCF speed. The TMCF controls the throttles to a thrust target.
VNAV path/speed causes the AFDS to control to a path (cruise altitude). The TMCF controls the throttles to a target airspeed in cruise and shallow path descents.
VNAV path/idle causes the AFDS to control to a set descent path from the top of descent to the end of descent. The TMCF moves the throttles to the idle position.
VNAV altitude/speed causes the AFDS to control to an altitude hold request from the MCP or an altitude override in climb or descent. The TMCF controls the throttles to a speed target.
VNAV speed/hold causes the AFDS to control to a speed target and the TMCF to remove servo power from the servomotors. This occurs in steep descents. Power loss to the servomotors allows manual throttle control with no resistance.
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FMCS -FMCF FUNCTIONAL DESCRIPTION -LNAV AND VNAV ENGAGE LOGIC

General
LNAV and VNAV engage logic uses inputs from several sources to engage the LNAV and VNAV modes.
The guidance sub-function needs the engage logic input to complete its steering command output path to the AFDS and the TMCS. The engage/operate logic also goes to the display control sub-function in the AIMS cabinets to cause the arm and engage modes to show on the primary display system.
LNAV/VNAV Engage Initiation
LNAV/VNAV engage starts with a push of the LNAV or VNAV switch on the mode control panel (MCP). The MCP sends these requests by way of the AFDCs to the guidance sub-function.
LNAV Engage Logic
These are the other inputs for LNAV engage logic:
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Software monitors valid