VERTICAL NAVIGATION (VNAV)

VNAV or Vertical Navigation, is used in sophisticated Aircrafts like the A320 and B737, whose main purpose is to predict and optimize the vertical path including control of the pitch axis and control of the throttle.

In order to have the information necessary to accomplish this, the FMS must have a detailed flight and engine model building a predicted vertical path along the lateral flight plan. This detailed flight model is generally only available from the aircraft manufacturer.

Implementation of an accurate VNAV is difficult and expensive, but it pays off in fuel savings primarily in cruise and descent. In cruise, where most of the fuel is burned, there are multiple methods for fuel savings.

As an aircraft burns fuel it gets lighter and can cruise higher where it is generally more efficient. Step climbs or cruise climbs facilitate this. VNAV can determine where the step or cruise climbs should occur to minimize fuel consumption.

Performance optimization allows the FMS to determine the best or most economical speed to fly in level flight. This is often called the ECON speed.

This is based on the cost index, which is entered to give a weighting between speed and fuel efficiency. The cost index is calculated by dividing the per-hour cost of operating the plane by the cost of fuel.

Generally a cost index of 999 gives ECON speeds as fast as possible without consideration of fuel and a cost index of zero gives maximum efficiency.

ECON mode is the VNAV speed used by most airliners in cruise.

The first thing the VNAV calculates for the descent is the top of descent point (TOD), where an efficient and comfortable descent begins. Normally this will involve an idle descent, but for some aircraft an idle descent is too steep and uncomfortable.

The FMS calculates the TOD by “flying” the descent backwards from touchdown through the approach and up to cruise, using the flight plan, the aircraft flight model and descent winds.

For airline FMS, this is a very sophisticated and accurate prediction, for simple FMS (on smaller aircraft) it can be determined by a “rule of thumb” such as a 3 degree descent path.

From the TOD, the VNAV determines a four-dimensional predicted path. As the VNAV commands the throttles to idle, the aircraft begins its descent along the VNAV path. If either the predicted path is incorrect or the downpath winds different from the predictions, then the aircraft will not perfectly follow the path. The aircraft varies the pitch in order to maintain the path. Since the throttles are at idle this will modulate the speed.

Normally the FMS allows the speed to vary within a small band. After this, either the throttles advance (if the aircraft is below path) or the FMS requests speed brakes with a message such as “ADD DRAG” (if the aircraft is above path).

An ideal idle descent, also known as a “green descent” uses the minimum fuel, minimizes pollution (both at high altitude and local to the airport) and minimizes local noise.

While most modern FMS of large airliners are capable of idle descents, most air traffic control systems cannot handle multiple aircraft each using its own optimum descent path to the airport, at this time. Thus the use of idle descents is minimized by Air Traffic Control.