[2] The inertial reference system provides position, rate and attitude inputs for the flight management computer system. The FMC calculates navigation and performance targets and other information. This information can be displayed on the EFIS for use by the pilot. In addition , the navigation and performance functions can be coupled to the digital flight control system and the autothrottle to provide automatic control of the airplane. This is the guidance function of the flight management computer.

[3] The ANCDU provides a limited navigation display on EFIS and can be coupled to the digital flight control system for lateral guidance. The ANCDU has no vertical guidance capability.

[4] The digital flight control system performs flight director and autopilot functions. Navigation is coupled from the flight management computer to the flight director and /or autopilot in the LNAV ( Lateral navigation ) mode. Performance is coupled to the digital flight control system and the autothrottle in the VNAV (Vertical navigation ) mode. Thus with both 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.
　
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