VNAV

In aviation, Vertical NAVigation is an autoflight function which directs the vertical movement of an aircraft (i.e. gains or losses in its altitude).

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 altimeter annunciator that calls out Above Ground Level (AGL) height ("50", "40", "30", "20", "10"), in order to time the flare seconds before touchdown.