|This article does not cite any references or sources. (July 2014)|
If used in the cruise, VNAV causes an aircraft to climb or descend according to a vertical elements of a pre-programmed FMS (flight management system) flight plan. The first aircraft to feature this use were Boeing 757 and 767 airliners in 1982.
When used on approach to landing, VNAV follows a calculated approach path from a Final Approach Fix or Waypoint to the runway, ie waypoints within the FMS navigation database. The path can either be based on stored database altitudes as displayed on the altimetry system ("Baro VNAV"), or as corrected within more advanced FMS equipment for temperature errors: significantly low temperature otherwise causing a shallow approach. During final landing approach, altitude must be controlled more accurately than heights referenced to Mean Sea Level (MSL) as barometric altimetry provides. An ILS Glide Slope signal originating from the landing point provides needed vertical alignment down to the final 100 ft. Commercial pilots rely on visual cues, and a down-looking-radar (lidar) annunciator that calls out Above Ground Level (AGL) height ("50", "40", "30", "20", "10"), in order to time the flare seconds before touchdown.
Future systems will derive altitudes from GPS, however, raw GPS elevations are referenced to worldwide average sea-level. Local sea-levels depart from this average by up to +- 200 meters, obviously too much error to use for auto-takeoff/landing. The GEOID99 software tables correct raw GPS elevation so that it is referenced to local sea-level for any location. A down-looking, terrain-reflecting sensor will take over below 100' as the primary AGL indicator.
Completely automating the final descent/touchdown under all flying conditions is unlikely, and thus pilot-in-the-loop will remain the primary means of vertical navigation low to the ground.