Area Navigation Systems (RNAV) — Page 262, Lesson 257

Let’s start with the definition that everything else hangs on. RNAV, Area Navigation, is defined as a method of navigation which permits aircraft operations on any desired track within the coverage of station-referenced navigation signals, or within the limits of a self-contained navigation system. I want to unpack that. "Any desired track" means you are not locked onto a radial to or from a ground beacon. You can fly a straight line between any two points you choose, as long as you are within range of the ground-based signals, or within the limits of a self-contained system like an inertial system. The second sentence sharpens it: an area navigation system is any system that lets you navigate to the required level of accuracy without having to fly directly over ground-based facilities. So the whole point is you don't need to overfly the VOR or the DME station. You can be offset from it and still navigate precisely. Now, how does the system achieve that required accuracy? It processes inputs. The excerpt lists the possible inputs: VOR/DME, ILS/MLS, GNSS, INS/IRS, ADC, and Time. Let me name each one properly. VOR/DME is VHF Omni-directional Range with Distance Measuring Equipment. ILS/MLS is Instrument Landing System and Microwave Landing System. GNSS is Global Navigation Satellite System. INS/IRS is Inertial Navigation System and Inertial Reference System. ADC is the Air Data Computer, which gives you things like airspeed and altitude. And Time is simply the time input, often from the clock or the GPS. The system takes some or all of these inputs, processes them, and gives you the most accurate, continuously updated position, and then outputs to the pilot such as course and ETA, estimated time of arrival. Now, why do we bother? The benefits. RNAV lets the aircraft take a more direct flight path appropriate to the route, which improves operating efficiency and relieves congestion on the overcrowded airway system. Air traffic control centres have established RNAV routes that are more direct than the traditional airways, and they do not require aircraft to regularly fly to the overhead of beacons. So the benefits are threefold. First, a reduction in distance, flight time, and fuel, and hence costs, by giving airlines and pilots greater flexibility and choice of routes. Second, an increase in route capacity by making full use of available airspace, providing more direct routes, parallel or dual routes, and bypass routes for overflying aircraft in high-density terminal areas. Third, a reduction in vertical and horizontal separation criteria. That third one is important operationally: because the navigation is more accurate, you can safely put aircraft closer together. Now let's look at the types and levels. There are two types of RNAV. Basic RNAV, B-RNAV, is required to give a position accuracy to within 5 nautical miles on at least 95% of occasions. And it is now mandatory for all aircraft carrying 30 passengers or more to have B-RNAV capability within Eurocontrol airspace. So that's a regulatory requirement. Precision RNAV, P-RNAV, must be accurate to within 1.0 nautical mile on at least 95% of occasions. P-RNAV routes are now being established in terminal airspace. So the key contrast: B-RNAV is 5 NM accuracy, P-RNAV is 1.0 NM accuracy, both on at least 95% of occasions. Then there are three levels of RNAV capability. 2D RNAV relates to capabilities in the horizontal plane only. That's just lateral navigation. 3D RNAV indicates the addition of a guidance capability in the vertical plane. So now you have vertical guidance as well, which gives you a three-dimensional path. 4D RNAV indicates the addition to 3D RNAV of a timing function. So 4D adds time as the fourth dimension, which allows for things like time-of-arrival control. Now let's look at a simple 2D RNAV system, the flight deck components. The flight deck of a simple 2D RNAV system includes the following. A Navigation Computer Unit, which is the brain that does the processing. A Control and Display Unit, the CDU, which is how the pilot enters data and reads information. And an indicator, which is in the form of either a Course Deviation Indicator, the CDI, or a Horizontal Situation Indicator, the HSI. The CDI shows lateral deviation from course, and the HSI combines that with a compass display to give you a horizontal picture of your situation. That figure shows the VOR/DME RNAV integrated navigation system, which ties all these components together. So to summarise the whole picture: RNAV frees you from overflying beacons, uses multiple navigation inputs to compute an accurate position, and gives you direct routing with reduced separation. The two types, B-RNAV and P-RNAV, differ in accuracy, and the three levels, 2D, 3D, and 4D, add vertical guidance and then timing.