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

I want to walk you through the heart of modern airliner navigation — the Flight Management System, or FMS, as fitted to the Boeing 737-800. This is where all the separate radio and inertial systems we've been discussing get fused into one integrated brain. Let's start with the architecture. The 737-800 FMS is not a single box. It comprises four independent systems. First, the Flight Management Computer System — that's the FMCS. Second, the Autopilot/Flight Director System, which we call the AFDS. Third, the Autothrottle, abbreviated A/T. And fourth, two Inertial Reference Systems — the IRS. Here's the key point: each of these components is an independent system. That means each one can be used individually, or in various combinations with the others. The term "FMS" — Flight Management System — implies the joining of all these systems into one integrated system. And that integration gives you three things: automatic navigation, guidance, and performance management. Now, what does that actually mean in flight? The FMS provides what we call 4D area navigation. The four dimensions are latitude, longitude, altitude, and time. So it's not just positioning you in space — it's also managing when you arrive. And it optimizes performance to achieve the most economical flight possible. It gives you centralized cockpit control of the aircraft's flight path and performance parameters. Let me introduce the brain. 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. Think of it as the processor that does all the thinking. To talk to that computer, the crew uses a Control and Display Unit — the CDU. This is the crew's interface. Through the CDU, the crew inputs the flight details and the performance parameters into the FMC. And the navigation and performance computations are displayed back on the CDU for reference and monitoring. So it's a two-way street — input in, display out. Now, the FMC produces commands for two specific types of navigation. Lateral navigation — that's LNAV — and vertical navigation — VNAV. These commands may be coupled to the AFDS and the A/T. So the autopilot and autothrottle can fly those commands automatically. Let's look at how the FMC actually determines where the aircraft is. In the navigation functions, the FMC receives inputs of position and heading from the IRS — the Inertial Reference System. It also receives fixing information using twin DME — that's two Distance Measuring Equipment stations. The FMC compares these inputs, and by a process known as Kalman filtering, it produces a system position. I want to pause on that, because Kalman filtering is a critical concept. It's a mathematical process that blends multiple, imperfect sources of information into a single best estimate. In this case, the FMC is combining the short-term accuracy of the IRS with the long-term accuracy of the external reference — the DME fixes. The IRS is very accurate over a short period but drifts over time. The DME fixes are accurate in the long term but may have momentary errors. Kalman filtering merges them so you get the best of both. Now, there's an important failure condition here. If the FMS is using just the IRS information to derive position — meaning it has no radio updating — then a warning is displayed to the crew. That warning indicates that the positional information is downgraded. So the crew is always told when the navigation quality has degraded. Let me also mention the level of automation. The crew may select the level of automation required. At one extreme, you simply use the data displays to fly the aircraft manually — for example, using heading or TAS/Mach number. TAS is true airspeed, and Mach number is the ratio of the aircraft's speed to the speed of sound. At the other extreme, you have fully automatic flight path guidance and performance control. But here's a crucial safety principle, and I want you to remember this for your professional career. Even with full FMS operation, the crew have absolute control of the management and operation of the aircraft. The automation is never in charge — you are. Furthermore, certain functions can only be implemented by the crew. These include thrust initiation, take-off, altitude selection, ILS tuning, aircraft configuration, and landing rollout. The automation cannot do these for you. And finally, the monitoring responsibility. 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 flies, but you verify. Let me show you the schematic of how all this fits together. And here's the practical picture — the aircraft flying from waypoint 1, defined by a DTY VOR/DME, along its RNAV route. So to summarize what we've covered: the FMS integrates four independent systems — FMCS, AFDS, A/T, and two IRS units. The FMC is the computing heart, the CDU is the crew interface, and together they provide 4D navigation — latitude, longitude, altitude, and time. Position is derived by Kalman filtering, blending IRS short-term accuracy with DME long-term accuracy, and if radio updating is lost, you get a downgrade warning. The crew selects the automation level, retains absolute control, and must always monitor the system.