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not calculated, the FMCS sets the lateral guidance outputs to no
computed data (NCD). The DFCS disconnects the LNAV mode
and reverts to control wheel steering (CWS).
Vertical Guidance Commands
The vertical guidance function calculates and sends airspeed or
vertical speed targets to the DFCS. Airspeed, N1 limit and mode
commands are also calculated and sent to the A/T system.
These are the two basic modes for vertical guidance:
* Airspeed (VNAV SPD)
* Path (VNAV PATH).
In the airspeed mode, the DFCS controls the airplane to a target
FMCS airspeed while thrust is set to an N1 climb limit rating or
idle. Both the speed target and the thrust limit values are
provided by the FMCS. VNAV SPD will show on the flight mode
annunciators (FMA).
In the path sub-mode, the DFCS controls the airplane to a target
FMCS vertical speed. This path will be calculated for a specified
speed/thrust profile. In descents, the default will be idle thrust
and economy speeds. VNAV PATH will show on the FMAs.
FMCS - FMC FUNCTIONAL DESCRIPTION - GUIDANCE SUB-FUNCTION
EFFECTIVITY
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For descent, a vertical path deviation display is sent to the
common display system. The deviation shows the difference
between the reference altitude on the vertical reference path
and the current airplane altitude.
CDS Function
This function provides the common display system (CDS) with
display/background data to accurately show the airplane’s
position with respect to the flight plan path.
The following data is sent to the CDS:
* FMC present position (Lat & Long)
* Distance to go to a waypoint
* Groundspeed
* Estimated time of arrival
* Desired track
* Cross track distance
* Vertical deviation
* Range to altitude
* Navigation sensor position (IRS, GPS, radio)
* V1 speed
* VR speed
* Speed targets
* Waypoint bearing
* True track angle
* Magnetic track angle
* Wind speed and direction
* Drift angle
* Flight path angle
* Waypoint, navaid, airport symbology
* Discrete word.
The discrete word contains the following data:
* To/from
* IRS source
* IRS position difference
* N1 limit mode
* Reduced thrust derate
FMCS - FMC FUNCTIONAL DESCRIPTION - GUIDANCE SUB-FUNCTION
EFFECTIVITY
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FMCS - FMC FUNCTIONAL DESCRIPTION - GUIDANCE SUB-FUNCTION
FMCS - FMC FUNCTIONAL DESCRIPTION - GUIDANCE SUB-FUNCTION
EFFECTIVITY
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FMCS - FMC FUNCTIONAL DESCRIPTION - GUIDANCE SUB-FUNCTION
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General
LNAV and VNAV logic uses inputs from several sources to
engage the LNAV and VNAV modes.
The FMCS guidance sub-function needs these logic inputs to
complete its guidance command output path to the DFCS and
the A/T system.
LNAV/VNAV Mode Activation
To engage LNAV or VNAV, push the LNAV or VNAV switch on
the mode control panel (MCP). The MCP then sends the mode
requests by way of the flight control computers (FCCs) to the
guidance sub-function in the FMC.
FMC LNAV Logic
These are the inputs to the FMCS necessary to engage the
LNAV mode:
* ADIRU in the Nav mode and the data is valid
* LNAV capture criteria is satisfied (The present airplane track
will intercept the active flight leg at an angle of 90 degrees or
less and the crosstrack deviation is less than 3 nautical
miles)
* Active route (no discontinuity to the next waypoint)
* Localizer mode is not active
* Go-around mode is not active
* LNAV arm/operate (MCP).
FMC VNAV Logic
These inputs to the FMCS are necessary to engage the VNAV
mode:
* Airplane in the air
* ADIRU in the Nav mode and the data is valid
* Performance data (ZFW or GW, fuel reserves, cost index and
cruise altitude) is valid
* Glideslope mode is not active
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