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LNAV and VNAV engaged the guidance function of the flight
management computer controls the ailerons and elevators through the
autopilot and controls the throttles through the autothrottle.
Fig. 36-1 FMCS
[5] The basic function of FMCS is to compare a selected route (lateral
and vertical ) to airplane position and use this data to generate steering
and thrust requests to maintain the airplane on the requested route profile.
The FMC depends upon two separate data bases to perform its functions.
The performance data base is permanent and not accessible to the flight
crew. The navigation data base may be either permanent (the airline route
structure ) or temporary (entered by the flight crew ) or a combination of
permanent and temporary.
[6] The navigation data base contains all information required for
operation in a specified geographic area. This information includes data
for :
(1) VOR, DME, VORTAC and TACAN navigation aids.
(2) waypoints
(3) Airport and runways
(4) Terminal area procedures and approaches
(5) Enroute airways
(6) Holding patterns
(7) Company route structure.
Upon the expiration data of the navigation data base, a new navigation
data base is required for the FMC. The new navigation data base must be
valid for the next period of effectivity. Data base updates are to be
accomplished at intervals of 28 days by loading the new data through a
Data Base Loader (DBL), Which can be plugged into the airplane. The
loading operation can be performed in the airplane on the ground only
and in approximately 10 minutes.
The performance data base is a software model of the engine and the
airframe. It includes such information as engine thrust, fuel flow,
aerodynamic characteristics, and operating limits.
[7] The FMC receives digital and discrete inputs from several sensors
and other systems in the FMS. The FMC performs the computational,
logical, and input/output processing required by the airplane's flight and
performance systems. The outputs of the FMC are in ARINC 429 digital
format. Digital inputs are as follows:
 CDU provide the interface between the flight crew and the FMC.
 Each IRS provides present position, heading, attitude, velocity,
altitude, ground speed, and vertical speed to the FMC through an
ARINC 429 bus.
 Air data computer provides altitude, airspeed mach, and temperature
information to the FMC through an ARINC 429 bus.
 The pilot's clock provides Greenwich Mean Time ((GMT) to the
FMC.
 Data from the FCC is transmitted to the MCP (Mode Control Panel)
and then to the FMC.
 The digital flight control system (DFCS) mode control panel (MCD)
provides LNAV and VNAV engage mode logic, selected altitude data,
and flap position to the FMC.
 The fuel summation unit provides fuel quantity through the DAA.
 DME interrogators provide distance information to the FMC, through
the DAA.
 VHF navigation receivers provide deviation and bearing information
to the FMC, through the DAA.
In addition, the FMC monitors discrete input signals from the engine
bleed air , engine and wing anti-ice, engine fuel control, and air
conditioning systems. Turning on the bleed air, anti-ice, or air
conditioning may cause an engine thrust reduction. The FMC uses these
discrete signals to provide corresponding adjustments to calculated
performance values.
[8] The digital outputs of FMC are as follows:
 FMC outputs the N1 limit information to N1 indicators on the
primary engine display through the DAA.
 FMC tunes the VHF NAV systems. Tuning information is sent to
both VHF NAV systems through the DAA.
 FMC provides latitude and longitude of airplane present position to
the IRS. The present position is used to initialize the IRS. Latitude
and longitude coordinates are entered at the FMC ANCDU. The FMC
also provides set heading for the IRS attitude mode.
 FMC provides gross weight to the digital stall management computer
 FMC provides the active N1 limit to TMA on the primary engine
display.
 FMC sends data to the flight recorder system for recording
 Either the FMC or the ANCDU can be selected as the source for the
digital outputs to the DFCS flight control computer (FCC) and the EFIS
symbol generators. the FMC output provides the FCC with target
airspeed and mach, target altitude, target vertical speed, and roll
command. The ANCDU output provides roll command only. The FMC
sends navigation, guidance and navigation background data to the
symbol generators for display on the EHSI and EADI. The ANCDU
　
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