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

We're starting a new topic now: Area Navigation Systems, or RNAV. Let's begin with the very heart of it — the waypoint table and the principle of a simple 2D RNAV system. First, let's define what a waypoint is. A waypoint is simply a geographical position you define, not necessarily by a radio beacon, but by a radial and distance from a known station. Look at this table. Each waypoint is defined by a station, its frequency, a radial, and a distance. For example, waypoint 1 is defined by DTY VOR/DME, on frequency 116.4 MHz, at radial 067, distance 42 nautical miles. Its application is en route navigation. Waypoint 2 is POL, 112.1 MHz, radial 066, distance 29. Waypoint 3 is NEW, 114.25 MHz, radial 218, distance 26. Waypoint 4 is also NEW, same frequency, but radial 251, distance 4 — and its application is holding LOM, the locator outer marker. Waypoint 5 is I-NC on 111.5 MHz, with no radial or distance — it's the ILS itself. Now, the principle of operation of a simple 2D RNAV system. Let's walk through Figure 16.3. The aircraft is flying from waypoint 1, defined by DTY VOR/DME, to waypoint 2, defined by POL VOR/DME. As the aircraft arrives at WP1, the pilot selects POL. The system measures the range and bearing from the aircraft to POL — in this case, 145 degrees magnetic and 104 nautical miles. So the RNAV computer now knows its position with respect to POL. The pilot has already input waypoint 2 with respect to POL. So the computer has two sides — the aircraft-to-POL vector and the WP2-to-POL vector — plus the included angle and the orientation of magnetic north. From that, it can compute the track and distance from WP1 to WP2: 340 degrees magnetic, 102 nautical miles. From that point on, the RNAV continually computes the aircraft's position with respect to POL, and compares that position with the computed track. That comparison gives two things: the cross-track error — how far left or right of the desired track you are — and the distance to go to WP2. Steering demands are fed to a CDI, the course deviation indicator, or an HSI, the horizontal situation indicator, so the pilot can keep the aircraft on track, and you get a continuous range readout to WP2. One critical note: on this simple system, the indications of deviation from track are in nautical miles, not in degrees. Now let's talk about the limitations and accuracy of these simple RNAV systems. The beacons are selected by the pilot during pre-flight planning. The pilot must ensure that each waypoint is within DOC — that's the declared operational coverage — of the VOR/DME designating that waypoint, and also within DOC of the VOR/DME designating the next waypoint. You also have to consider slant range error in DME when selecting facilities close to the track. Slant range error is the difference between the slant distance and the ground distance — it matters when the facility is near your track. The pilot must ensure the information is correctly input into the CDU, the control display unit, because the computer cannot recognize or rectify mistakes. To avoid positional errors, the aircraft must at all times be within the DOC of the in-use facility. The accuracy of the fixing information depends on range and on whether the VOR or DME element is predominant. Here's the key relationship: if the VOR/DME is close to the planned track to or from the waypoint, 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. Now let's move to Level 4 RNAV systems. In modern passenger aircraft, the area navigation function is carried out by a flight management computer — the FMC — which also provides guidance and performance functions. The system I'll describe is specific to the Boeing 737-800, but the principle holds for all aircraft. Look at Figure 16.4, the FMS schematic. You have two FMCs — one on each side, the offside FMC. The FMC receives inputs from a whole array of systems: the air data computer, the inertial reference system, the VOR, DME, ILS/MLS, and ADF receivers, the fuel quantity indicating system, the weight and balance computer, the digital clock, and the mode control panel. It also talks to the autopilot flight director system, the flight control computer, the electronic engine control, the central maintenance computer, and the electronic interface unit, which drives the integrated display system — the ND and PFD, the navigation display and primary flight display. There's also the pilot's MCDU — the multi-function control display unit — which is how the pilot interacts with the FMC. And there's a database loader to load navigation data, and the auto throttle servo. The FMC is the central brain that integrates all of this for area navigation, guidance, and performance. So to tie it together: the simple 2D system uses one VOR/DME and computes track and distance between waypoints, with deviation in nautical miles. The Level 4 system, the FMC, integrates multiple sensors and databases to do the same job far more comprehensively. That's the foundation of RNAV.