Vertical navigation

A flight management system (FMS) uses either a performance or a geometric VNAV path. The performance path is computed from the top of the descent to the first constrained waypoint, using idle or near idle power. This is referred to as an idle descent path at ECON speed. The geometric path is shallower descent and typically not at idle. The geometric path uses an assigned vertical angle or the computed point to point path between constrained waypoints.[1]^{: 3–5, 3–6}

RNAV approaches combine VNAV navigation equipment with LNAV navigation equipment to provide both lateral and vertical approach guidance. Vertical guidance comes from WAAS GPS or a barometric VNAV (Baro-VNAV) system. The FMS provides flight control steering and thrust guidance along the VNAV path.[1]^: 3–6

VNAV information on an approach plate includes the Final Approach Fix (FAF), the FAF crossing altitude, a Vertical Descent Angle (VDA), the landing runway threshold as a second fix, the Threshold Crossing Height (TCH), and perhaps a Visual Descent Point (VDP). A pilot uses the VDA, and ground speed, to compute a rate of descent (from a table found in the U.S. Terminal Procedures Publication), which is flown using the Vertical velocity indicator.[1]^{: 4–22, 4–23}

Aircraft approved for LNAV/VNAV minimums include the Boeing 737NG, 767, 777, the Airbus A300 and some ATRs.[1]^: 4–26[2]

A Baro-VNAV is an RNAV system which uses the aircraft altimeter to compute and display a vertical guidance path. The path is either geometric between two waypoints, or based on an angle from a single waypoint. Baro-VNAV procedures include a minimum and maximum temperature limitation. Otherwise, temperature compensations must be used.[1]^: 4–31

VNAV is also the name of autopilot vertical modes in several aircraft.[3] Sometimes there are two modes, VNAV Speed and VNAV Path. In VNAV Speed mode, the autopilot sets the aircraft pitch to achieve a desired speed, and then adjusts thrust to control the rate of climb or descent. In VNAV Path, the aircraft adjusts the pitch to achieve the desired vertical profile, and then adjusts thrust to control the speed.

It is possible to remain in VNAV mode for almost the entire flight. The aircraft will typically climb in VNAV Speed and descend in VNAV Path.

In some Boeing aircraft, there is a single VNAV selector button, and the autopilot will switch between VNAV Speed and VNAV Path automatically.[4] This is known as "Common VNAV".[5]

VNAV mode offers advantages over other autopilot modes. Use of "Vertical Speed" mode to achieve a given rate of climb at high altitudes can risk a decrease of airspeed below safe values. "VNAV Speed" will sacrifice rate of climb to protect the aircraft's airspeed.

• Sam Miller (April 2006). "Flight Management Computer System Vertical Navigation aka VNAV" (PDF). Archived from the original (PDF) on 2011-07-18. Retrieved 2015-10-18.