Flight Management System — Page 270, Lesson 321

Let’s start with the big picture of what a Flight Management System actually is, because the name tells you a lot. A flight management system is designed to do three things: improve navigation, aid fuel efficiency, and reduce crew workload. Those are the three goals written into its purpose. So when you think about an FMS, think of it as a computerised tool that helps the crew fly complex routes more precisely, more economically, and with less manual effort. Now, the system achieves those goals through two main guidance functions. The first is Lateral Guidance, abbreviated LNAV. Lateral guidance is about steering the aircraft along the horizontal plane — that is, following the planned route across the ground, turning at the right waypoints, staying on the correct track. The computers use LNAV to fly the aircraft along complex routes. The second function is Vertical Guidance, abbreviated VNAV. Vertical guidance deals with the vertical plane — altitude and speed. VNAV enables the system to calculate optimum cruise altitudes, and to determine 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. Here’s a key operational point: at all times when the crew are not actually controlling the aircraft by hand, they use the FMS controls to “fly” the aircraft. That’s a very important concept. The controls of an FMS are, in effect, a miniature flight deck with fingertip control. So instead of physically moving the control column, the crew commands the aircraft through the FMS, and the system translates those commands into actual control inputs. Let me now introduce the main components, because the system is built from several distinct units. I’ll go through the legend of a typical schematic layout. First, the CDU — that stands for Control and Display Unit. The FGS is the Flight Guidance System. The FMC is the Flight Management Computer. The ADC is the Air Data Computer. The IRS is the Inertial Reference System. And GPS is the Global Positioning System. Each of these has a specific role, and I’ll explain the CDU and FMC in detail now, since they’re the heart of the system. Let’s look at the Control and Display Unit, the CDU. Its primary function is to act as the interface between the aircraft and the crew. That means it’s the crew’s window into the system — the way they talk to the FMS and the way the FMS talks back. The CDU can be used to command completely automatic control of the aircraft, or semi-automatic with varying degrees of pilot involvement, including full manual control. So the CDU gives you a spectrum of control authority, from fully hands-off automatic flight down to full manual control. In terms of physical layout, two CDUs are usually fitted, one either side of the centre console, with the left CDU normally being the master. In the B747-400, they are joined by a third CDU placed upon the centre console, used primarily by engineering staff. So on that aircraft you have three CDUs — two for the pilots and one for engineering. Each CDU comprises a monochrome or coloured cathode ray tube display — that’s the CRT — on which different “pages” of selected data can be shown, and a selector key panel. So the display shows you pages of information, and the key panel lets you select which page you want and enter data. Now let’s talk about the Flight Management Computer, the FMC, and the different modes of operation. The FMCs may be decoupled to provide fully Independent Mode operation. That means each FMC works completely on its own, with no cross-checking. This is not usual, because there will be no safety or cross-check between the two FMCs. So in Independent Mode, you lose the redundancy that protects you from errors. The normal mode for ordinary flight profiles is Dual Mode. In Dual Mode, both FMCs independently process pilot entries on both MCDUs — note that’s MCDU, the Multi-purpose Control and Display Unit — and they compare the results to ensure that crucial information is consistent on both systems. The same output is then passed to both FMCs. So in Dual Mode, you have two computers doing the same job, cross-checking each other, and both receiving the same output. That’s your safety net. If there is a failure of one FMC, the second system can be expected to operate the aircraft successfully on its own. This is known as Single Mode. The failed FMC may be selected out of the system to allow single mode operation of the “surviving” FMC, if required by the crew. So Single Mode is your degraded but still fully functional state — one computer flying the aircraft alone. Finally, let’s talk about the data base. The information stored in the FMC is called its data base. This data base is divided into two major sections. One section contains performance related information, and the other contains information dealing with navigation. So the FMC holds two distinct sets of data: performance data, which relates to how the aircraft performs — things like engine and speed parameters — and navigation data, which relates to the route and position information. Both are essential for the FMS to do its job of navigating and optimising fuel efficiency. So to tie it all together: the FMS uses LNAV for horizontal guidance and VNAV for vertical guidance, the crew interacts through the CDU, the FMC processes the data and runs the modes — Independent, Dual, or Single — and the whole thing is powered by a data base split into performance and navigation sections. That’s the core architecture of the system.