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Thanks for the reply, but how are VNAV and RNAV actually used
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Modern aircraft with "glass" cockpits usually come equipped with one or more FMC's (Flight Management Computers). These computers have an internal database which consist of not only navigational charts but also an aircraft specific performance database.
Using inputs from GPS and/or IRS systems for positional updates, the FMC, using it's internal nav and performance databases, along with the aircraft's autopilot and autothrottle systems, can control all aspects of the flight. VNAV controls, or provides, vertical information in the climb, cruise and descent phases of flight along with any crossing and/or speed constraints within those 3 phases.
RNAV or LNAV (Lateral navigation), as we called it on the B757, uses the FMC's database and the autopilot/flt director to steer the aircraft on a prescribed course. All airways/navaids/fixes and approaches are within the FMC's database. Selecting "LNAV" tells the autopilot/flt director to follow the route selected in the database the pilot has chosen. This route is also depicted on the pretty little moving map in front of the pilot.
Selecting "VNAV" ties the FMC to control the autopilot/flt dir to give automatic "pitch" guidance during the climb, cruise and descent.
The "LNAV" and "VNAV" buttons tie the aircraft's autoflight system together. They're the only two buttons that need the FMC to guide them. Basically, the FMC's databases automatically controls the autopilot and autothrottles along the entire 3 dimensional flight.
Using vertical speed or heading select removes the FMC from directly controlling the aircraft's vertical and lateral direction. The pilot then becomes the "middleman" between the FMC and the aircraft's autoflight system. Using LNAV and VNAV lets the FMC to talk directly to the autoflight system.
THERE! Clear as mud, huh? /ubbthreads/images/graemlins/cwm27.gif /ubbthreads/images/graemlins/insane.gif
