Area Navigation Systems (RNAV) — Page 262, Lesson 260

I want to walk you through how a simple two-dimensional RNAV system actually works, because this is the foundation for everything else in area navigation. First, the name. RNAV stands for Area Navigation. The key idea is that you are no longer forced to fly directly to or from a ground station. Instead, you can fly any path you want, as long as the system can compute where you are relative to that path. Now, a simple RNAV system uses what we call rho/theta to define position. Rho is the Greek letter for range, and theta is the Greek letter for bearing. So rho/theta simply means range and bearing. The system derives your position from range and bearing information coming from VOR/DME stations. VOR gives you the bearing, and DME gives you the slant range distance. Here is the clever part. The pilot defines waypoints along the route to be flown, and each waypoint is defined as a range and bearing from a suitably located VOR/DME station. So instead of saying "fly to the VOR," you say "fly to a point that is 29 nautical miles from that VOR, on a bearing of 066 degrees from it." Then the equipment takes the VOR/DME bearing and range it is receiving, and it computes the QDM and distance to your waypoint. QDM is the magnetic bearing you would steer to fly directly to that waypoint. So the system is doing the trigonometry to convert "where the station is" into "where the waypoint is relative to you." And here is the beautiful trick: it presents this information to you on a CDI or an HSI exactly as if the waypoint itself were a VOR/DME station. CDI is the Course Deviation Indicator, and HSI is the Horizontal Situation Indicator. Because the waypoint behaves like a station that does not physically exist, we call these waypoints phantom stations. That is the term you need to remember — phantom stations. Let me give you the concrete example from the diagram. The pilot has defined waypoints along a planned route from SND to NEW, using available and sensibly placed VOR/DME stations. Look at the waypoint definitions: 066R/29 NM means a waypoint at a radial of 066 degrees from the station, at a range of 29 nautical miles. Then 218R/26 NM, and 067R/42 NM. Each waypoint is locked to a specific station by that radial and distance pair. Now, why do this? Because waypoints may be selected and programmed for several distinct purposes. First, en route navigation — flying your planned route between waypoints. Second, initial approach fixes — the point where you begin an instrument approach procedure. Third, locator outer markers — those are the non-directional beacon markers on the approach. And fourth, ILS frequencies. This is important: when you select an ILS frequency, the instrumentation automatically reverts to ILS mode. So the RNAV system hands control back to the ILS receiver for the final approach. So the whole system is built around you defining phantom waypoints as range and bearing from real VOR/DME stations, and the equipment then presenting you with steering guidance as if those phantom stations were real. That is the essence of a simple two-dimensional RNAV system.