Area Navigation Systems (RNAV) — Page 270, Lesson 261

Let’s start with the heart of this chapter: a simple 2D RNAV system. I want you to picture the aircraft flying from waypoint 1 to waypoint 2. Waypoint 1 is defined by the DTY VOR/DME, and waypoint 2 is defined by the POL VOR/DME. Look at Figure 16.3 with me. As the aircraft arrives at WP1, the pilot selects POL, and the system measures the range and bearing from the aircraft to POL — that comes out as 145 degrees magnetic, 104 nautical miles. The RNAV computer now knows its own position relative to POL. The pilot has already typed in waypoint 2’s position relative to POL. So the computer has two sides of a triangle, the included angle between them, and the orientation of magnetic north. From that, it computes the track and distance from WP1 to WP2 — 340 degrees magnetic, 102 nautical miles. Now here’s the key operating principle. The RNAV continually recomputes the aircraft’s position relative to POL, and it compares that live position against the pre-computed track. That comparison gives two outputs: the cross-track error, which is how far left or right of the desired track you are, and the distance to go to WP2. The steering demands are fed to a CDI or an HSI — course deviation indicator or horizontal situation indicator — so the pilot can keep the aircraft on track, and the system gives a continuous range read-out to WP2. One critical note: on this simple system, the deviation-from-track indications are in nautical miles, not in degrees or dots. That’s a fundamental difference from a raw VOR needle. Now, the limitations and accuracy of these simple systems. The beacons are selected by the pilot during pre-flight planning. The pilot must ensure each waypoint is within the DOC — that’s the Documented Operational Coverage — of the VOR/DME that designates that waypoint, and also within the DOC of the VOR/DME that designates the next waypoint. So you’re checking coverage for both the current and the next leg. Next, slant range error in DME. DME measures slant range, which is the straight-line distance through the air, not the ground distance. When you’re close to the facility, that slant range error becomes significant, so you must consider it when selecting facilities that are close to the track. Then there’s the human factor. The pilot must ensure the information is correctly input into the CDU — the Control Display Unit — because the computer cannot recognize or rectify mistakes. Garbage in, garbage out. The computer will happily navigate on a typo. To avoid positional errors, the aircraft must at all times be within the DOC of the in-use facility. And the accuracy of the fixing information depends on two things: the range from the facility, and whether the VOR or the DME element is predominant. Here’s the geometry. If the VOR/DME is close to the planned track to or from the waypoint, then the along-track element will be most accurate. If the VOR/DME designating the waypoint is perpendicular to the track, then the across-track element will be most accurate. So the orientation of the facility relative to your track determines which axis of your position fix you can trust. Now let’s move up to Level 4 RNAV systems, which is what modern passenger aircraft use. The area navigation function is carried out by a flight management computer — the FMC — which also provides guidance and performance functions. The system I’m about to describe is specific to the Boeing 737-800, but the principle holds for all aircraft. Look at Figure 16.4, the FMS schematic. You’ll see there are two FMCs — an offside FMC and a primary FMC — they work together for redundancy. The FMC receives inputs from a whole family of sensors. Let me walk you through them. From the navigation side: DME, VOR, ILS/MLS, and ADF — those are your radio navaids. Then the Inertial Reference System, which gives you accelerometer and gyro data. The Air Data Computer gives you pressure altitude and airspeed. The Fuel Quantity Indicating System and the Weight and Balance Computer feed in mass and fuel data. The Electronic Engine Control gives engine parameters. The Digital Clock gives time. And the Central Maintenance Computer monitors system health. On the output side, the FMC drives the Autopilot Flight Director System, the Mode Control Panel, the Auto Throttle Servo, the Flight Control Computer, and the Electronic Interface Unit, which feeds the Integrated Display System — that’s your ND and PFD, the navigation display and primary flight display. The pilot interacts through the MCDU — the Multipurpose Control and Display Unit — and there’s a Database Loader to load navigation data. The Electronic Interface Unit also connects to the Central Maintenance Computer. So the FMC is the brain. It takes all those sensor inputs, computes your position, your track, your performance, and it drives the displays and the autopilot. That’s the difference between the simple 2D system we started with and a Level 4 system — the FMC integrates navigation, guidance, and performance into one computer, and it has the redundancy of a second offside FMC. Let me make sure you’ve got the key contrasts. Simple 2D RNAV: pilot selects beacons, deviation is in nautical miles, accuracy depends on DOC and geometry. Level 4: FMC integrates everything, with dual FMCs and a full sensor suite. Both share the same fundamental principle — compute position relative to a known point, compare to the desired track, and steer to correct the error.