AOPA - Career Pilot

Turbine Aircraft Technology

Flight Management Systems

The brains of the outfit


Flight management systems, once found only in air carrier aircraft, are now finding their way into the general aviation fleet. Although many pilots are familiar with the term flight management system, or FMS, many of us also are a bit unsure just what that term means.

What it isn't

First of all, let's define what an FMS isn't. It isn't a glass cockpit. The glass cockpit, substituting cathode-ray tubes (CRTs) or liquid crystal display (LCD) screens for conventional flight instruments, is one way of interfacing with an FMS, but it is not the FMS itself. An FMS is not simply a global navigation system, although such a system is one of its parts.

What it does

Figure 1 (see page 104) is a graphical representation of a typical flight using an FMS interfaced with an autopilot and autothrust (also called autothrottles) system. Let's follow along and see what a typical flight with an FMS is like.

The first action by the pilot is to enter the flight plan for the trip using the control display unit (CDU). (See Figure 2.) This may involve entering each leg of the flight plan or, in the case of the airlines, selecting from several preplanned routes. After takeoff, the FMS commands the autopilot, or the pilot through the flight director, to capture and track the preassigned route. The FMS, through the autothrottles, maintains speed and thrust for optimal fuel economy. When initial cruise altitude is reached, the FMS can command the other systems to level off at the most fuel-economical speed. As aircraft weight is reduced through fuel burn, the FMS can command a climb to a higher cruise altitude.

Throughout the flight the FMS evaluates and revises fuel consumption .and provides alerts when predetermined reserves are in danger of being consumed. The progress along the flight-plan route is displayed for the pilot on the moving map, which also can contain weather data provided by the on-board weather avoidance radar.

The descent is planned to provide an idle-thrust descent to the destination airport while complying with speed and altitude restrictions found in arrival routes or federal aviation regulations. The airplane is transitioned to the instrument landing system and delivered at the appropriate approach speed. The pilot is advised of the proper landing speed.

How does it know?

In order to provide flight management, the system must have several inputs from other aircraft systems. (See Figure 3.) The flight management computer at the heart of the system receives pilot inputs through the control display unit. This is where the pilot enters and revises flight plans, and controls the radar display and other displays in the cockpit.

The air data computer (ADC) measures ambient pressure and temperature and monitors pitot pressure to provide indicated and true airspeed. Since the various navigation systems on the aircraft are available to calculate groundspeed and drift, input to the FMS allows calculation of actual winds at altitude.

The attitude heading reference system (AHRS) replaces the attitude gyros of years past. These computers, using inertial reference and/or laser reference equipment, are much more stable than mechanical instruments and do not suffer from the precession and comparatively high failure rates associated with mechanical gyros.

Fuel flow and fuel computers calculate and continually update the fuel state of the aircraft, providing the pilot with a constant fuel-and-time-remaining readout. Alerts also can be sent when the planned reserves are being depleted. The thrust management computer (also called autothrottles or ATS) communicates with the fuel computers to provide the most economic cruise power settings. FMS can incorporate various navigation receivers. The most common are VOR and GPS, but DME and ADF receivers are also commonly used. The instrument landing system components, which present localizer and glideslope information, are available for precision approaches at the destination.

A digital clock provides the time for time/speed/distance calculations — when needed — and a reference for the pilots during nonprecision approaches.

What you see

All of this information is manipulated, stored and computed, and displayed to the pilot as necessary on the EFIS — electronic flight information system — displays in the cockpit. Each pilot station typically has a primary flight display (PFD) with one or more multifunction displays (MFD) shared by both pilots. Typically, attitude, heading, course reference, airspeed, altitude, and vertical speed are shown on the PFD — along with navigation and performance information. Traffic advisories also can be displayed from traffic alert collision avoidance system (TCAS) and other collision-avoidance devices. The MFD displays the moving map and other navigation information, aircraft system information, radar overlays, and various pilot messages and alerts. At a glance, the pilot is able to see progress along the planned route, fuel available, actual winds aloft, heading and drift correction, and many of the other bits and pieces of information that go into conducting a safe flight.

Is it all good?

Too much information can be counterproductive, and the big risk with the new-generation flight management systems and EFIS displays is the notion that because a piece of information can be displayed it should be displayed. The potential for information overload is high, in which case any additional information provided is lost in the clutter and becomes meaningless. It is possible to have too much information in front of the pilot, but luckily most displays allow for "declutter" or deselecting certain classes of information from the display to make the important messages more prominent and easier to interpret and act upon.