Area Navigation Systems (RNAV) — Page 270, Lesson 264

We're starting a new topic now: Area Navigation Systems, or RNAV. This is the chapter where we move from ground-based navigation aids to the aircraft's own computer managing the whole flight path. And I want to begin with the heart of it all, the Flight Management System, or FMS, as fitted to a Boeing 737-800. Let me break down what the FMS actually is. It's not one single box. On the 737-800, the FMS comprises four independent systems. First, the Flight Management Computer System, which we call the FMCS. Second, the Autopilot/Flight Director System, the AFDS. Third, the Autothrottle, abbreviated A/T. And fourth, two Inertial Reference Systems, the IRS. Now, here's the key point. Each of those four components is an independent system. That means each one can be used on its own, or they can be used in various combinations. But the term FMS, the Flight Management System, implies something more. It implies the joining of all these systems into one integrated system. And that integrated system provides three things: automatic navigation, guidance, and performance management. So what does that mean in practice? The FMS provides what we call 4D area navigation. The four dimensions are latitude, longitude, altitude, and time. So it's not just where you are in three-dimensional space; it's also when you'll be there. And it optimizes performance to achieve the most economical flight possible. It gives the crew centralized cockpit control of the aircraft's flight path and its performance parameters. Now, let's look inside. The Flight Management Computer, the FMC, is the heart of the system. It performs all the navigational and performance calculations, and it provides the control and guidance commands. So it's the brain. To talk to that brain, the crew uses a Control and Display Unit, the CDU. The CDU allows the crew to input the flight details and the performance parameters into the FMC. And then the navigation and performance computations are displayed back on that same CDU for reference and monitoring. The FMC produces commands for two types of navigation. Lateral navigation, which we call LNAV, and vertical navigation, VNAV. These commands may be coupled to the AFDS and the A/T. So the FMC decides the path, and it can hand those commands to the autopilot and autothrottle to actually fly it. Now, how does the FMC know where it is? In its navigation functions, it receives inputs of position and heading from the IRS. And it also receives fixing information using twin DME, that is, two DME ground stations. The FMC compares these inputs, and by a process known as Kalman filtering, it produces a system position. Let me explain that. Kalman filtering is a mathematical process that blends the different inputs, weighting them by their accuracy, to produce a single best-estimate position. Here's the clever part. In operation with radio position updating, the FMC is combining the short-term accuracy of the IRS with the long-term accuracy of the external reference, the DME. The IRS is very accurate over a short time, but it drifts. The radio fix is accurate over the long term. Together, they give you the best of both. But if the FMS is using just the IRS information to derive position, with no radio updating, then a warning is displayed to the crew. That warning indicates that the positional information is downgraded. So the crew always knows the quality of the navigation they're working with. Now, the crew has a choice. They may select the level of automation required. At one end, they can simply use the data displays to fly the aircraft manually, for example using heading or TAS, true airspeed, or Mach number. At the other end, they can have fully automatic flight path guidance and performance control. But even with full FMS operation, the crew have absolute control of the management and operation of the aircraft. And there are certain functions that can only be implemented by the crew. These include thrust initiation, take-off, altitude selection, ILS tuning, aircraft configuration, and landing rollout. The FMS will not do those for you. And finally, the crew should always monitor the FMC navigation throughout the flight, to ensure the flight plan is being accurately followed by the automatic systems. The automation is a tool, not a replacement for the pilot's oversight. Let me show you the schematic of how all this fits together. That's the FMS architecture on the 737-800. Now let's look at how it actually navigates between waypoints in practice.