Flight Management System — Page 270, Lesson 321

Let’s start with the big picture. A Flight Management System, or FMS, is built to do three things: improve navigation, aid fuel efficiency, and reduce crew workload. The way it does that is by using computers to fly the aircraft along complex routes using what we call Lateral Guidance, or LNAV. That’s the side-to-side steering along the planned track. Then there’s Vertical Guidance, or VNAV. That’s the up-and-down control. VNAV lets the system calculate the optimum cruise altitudes and work out the best combination of autothrottle control and speed during climb and descent. So LNAV handles the horizontal path, VNAV handles the vertical profile, and together they manage the whole flight path. Now, here’s the key idea about how the crew interacts with it. At all times when the crew are not actually controlling the aircraft by hand, they use the FMS controls to “fly” the aircraft. The controls of an FMS are, in effect, a miniature flight deck with fingertip control. So instead of moving the control column, the pilots are commanding the system through the FMS. Let me introduce the main components, because the schematic in Figure 21.1 shows how they fit together. We have the CDU, which is the Control and Display Unit. Then the FGS, the Flight Guidance System. The FMC, the Flight Management Computer. The ADC, the Air Data Computer. The IRS, the Inertial Reference System. And the GPS, the Global Positioning System. Let’s focus on the CDU first, because that’s the crew’s main interface. The primary function of the CDU is to act as the interface between the aircraft and the crew. It can command completely automatic control of the aircraft, or semi-automatic with varying degrees of pilot involvement, right down to full manual control. So the CDU is the box the pilots type into and read from. Typically, two CDUs are fitted, one either side of the centre console. The left CDU is normally the master. In the B747-400, they’re joined by a third CDU placed on the centre console, used primarily by engineering staff. Each CDU comprises a monochrome or coloured cathode ray tube display, a CRT, on which different “pages” of selected data can be shown, plus a selector key panel. So you have the screen and the keypad. Now, about the FMCs and how they work together. The FMCs may be decoupled to provide fully Independent Mode operation. That’s not usual, because in Independent Mode there’s no safety or cross-check between the two FMCs. So each one is working on its own without verifying against the other. The normal mode for ordinary flight profiles is Dual Mode. In Dual Mode, both FMCs independently process pilot entries on both MCDUs and compare the results, to ensure that crucial information is consistent on both systems. The same output is then passed to both FMCs. So they’re both doing the same calculation and checking each other. If there’s a failure of one FMC, the second system can be expected to operate the aircraft successfully on its own. That’s called Single Mode. The failed FMC may be selected out of the system to allow single mode operation of the “surviving” FMC, if the crew requires it. So you can isolate the failed unit and let the good one run the show. Finally, let’s talk about the data base. The information stored in the FMC is called its data base. It’s divided into two major sections. One contains performance related information, and the other contains information dealing with navigation. So the FMC holds two big libraries: one for how the aircraft performs, and one for where the aircraft is and where it’s going. That’s the core structure of the FMS. We’ve got the CDU as the interface, the FMC as the brain with its data base split into performance and navigation, and the modes—Independent, Dual, and Single—that govern how the two computers cooperate.