Vertical Navigation
VNAV is the function in the FMS that sends commands to the flight guidance computer for vertical control of the aircraft. VNAV can be used to climb, cruise, and descend the aircraft on the pilot selected speed schedules. These speed schedules can be defined to meet the operational objective of the flight. For example, to reduce cost, the cruise speed schedule chosen could be Long Range Cruise (LRC) or to reduce time enroute, maximum speed may be chosen.
On more sophisticated aircraft, the vertical descent path can be defined as a geometric angle as opposed to a vertical descent rate.
VNAV is not a required minimum function for navigators today
Performance
The performance function of the navigator varies greatly between manufactures. At a minimum, navigators provide ETA to waypoints in the flight plan using current groundspeed. More sophisticated FMS provide fuel management estimates for the flight plan, estimate optimum altitude, cruise modes (LRC, maximum speed, maximum endurance), and step climbs. Step climbs are enroute climbs made to higher altitudes by the aircraft to reduce fuel consumption. These climbs are necessary enroute because of aircraft weight, it is not possible to climb directly to the higher altitude. As fuel is burned, the altitude is achievable for the aircraft.
What-if situations is the capability of the pilot to determine the consequences of the flight if a performance parameter is changed. For example, the pilot may ask “what if I fly at maximum speed instead of long range cruise at this altitude”. The FMS allows the pilot to enter the necessary data and provide the new fuel requirements and ETA.
Thrust Management (Autothrottle)
Some FMS are equipped with a full flight regime autothrottle. These FMS mange the vertical and speed of the aircraft. Full flight regime means the autothrottle can be used from takeoff to landing.
When combined with vertical navigation, autothrottle provides the ability of the pilot to fly complex vertical flight plans involving descent and speed requirements.
Navigation display
Navigation displays vary with each aircraft. The low end general aviation aircraft
consisting of a “panel mount” navigator, has a built in CDI and does not interface to
displays on the flight deck. The CDI provides crosstrack information.
The more complex navigators interface to electro-mechanical displays, and EFIS
displays. EFIS integrates other situational information such as waypoints, NAVAIDs,
airports, and weather radar displays.
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