FMCS -FMCF FUNCTIONAL DESCRIPTION -INTRODUCTION
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FMCS -FMCF FUNCTIONAL DESCRIPTION -LNAV SUB-FUNCTION
General
The FMCF LNAV sub-function uses data from external systems and internal data from AIMS to calculate navigation data for the performance, guidance, and display control sub-function of the FMCF.
The LNAV sub-function supplies this data:
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Navigation update mode
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Position uncertainty
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Velocity vectors
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Wind data.
The active FMCF display control sub-function sends display data to the CDUs and the primary display function (PDF).
Navigation Update Mode
The FMCF has ten navigation update modes. The navigation update mode determines which sensors the FMCF uses to calculate the airplane position and velocity. These are the ten navigation update modes (not in order of priority):
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No nav -There are no valid FMCF sensors (GPS, radios, or ADIRU) necessary
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GPS only -The FMCF uses global positioning system (GPS) data
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ADIRU only -The FMCF uses ADIRU data
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VOR/DME/ADIRU -The FMCF filters VOR, DME, and ADIRU data
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DME/DME/ADIRU -The FMCF filters DME and ADIRU data
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GPS/ADIRU -The FMCF filters ADIRU data with GPS position data
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LOC/ADIRU -The FMCF filters localizer deviation with ADIRU data
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LOC/VOR/DME/ADIRU -The FMCF filters localizer deviation, collocated VOR/DME, and ADIRU data
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LOC/DME/DME/ADIRU -The FMCF filters localizer deviation, DME/DME, and ADIRU data
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LOC/GPS/ADIRU -The FMCF filters localizer deviation, GPS, and ADIRU data.
Position Uncertainty
The FMCF uses mathematical algorithms to calculate a position uncertainty value for each one of the navigation sensors (GPS, radios, ADIRU).
The FMCF selects only one navigation update mode at a time. The FMCF selects the navigation update mode with the least amount of position uncertainty.
Velocity Vector Calculation
The LNAV sub-function uses north, east, and vertical velocity components from the ADIRU and GPS position bias or radio position bias to calculate the velocity vectors. These are the outputs from the total velocity vector calculations:
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Ground speed
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Distance-to-go to next waypoint
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Cross-track error
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FMCS -FMCF FUNCTIONAL DESCRIPTION -LNAV SUB-FUNCTION
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True heading
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Magnetic track
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True track
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Flight path angle
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Magnetic heading
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Altitude.
Wind Data Calculation
The LNAV sub-function uses these inputs to calculate wind data:
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Flight path angle
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North and east velocity
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True heading
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ADIRU true airspeed (TAS).
The LNAV sub-function gives wind speed, wind direction, and wind angle. Wind speed is the difference between the track and airplane heading vector.
Wind direction and wind angle is the difference between ground speed and the true airspeed vector.
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FMCS -FMCF FUNCTIONAL DESCRIPTION -LNAV SUB-FUNCTION
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