Crows are not the bird world’s fastest flyers, but in the process of exer- cising their legendary ability to get where they’re going in straight lines, they’re bound to save some time when flying from tree A to tree B. In- strument pilots anxious to maximize the efficiency of their airplanes should also think in terms of straight lines, but it seems impossible when our primary navigational system (the VOR, or “Victor” airways) is built on IFR routes that dogleg their way across the country. Every now and then you’ll come across a situation in which there’s a VOR at the departure airport and another at destination. Then an accurate, straight-line course is possible, but that’s the exception, hardly the rule. On the other hand, there are a thousand or so VORs scattered across the United States today, and almost anyplace you’d want to go in an airplane is on some radial of some VOR; however, you must be on that radial to know where you are. Plotting two radials that cross above the destination is a big help, but it’s still a blind navigational sit- uation until you locate yourself on one of the radials and fly until the other needle centers.
The advent of the VORTAC (TAC refers to the military’s Tactical Air Navigation system and indicates DME capability for civilian users) and the self-explanatory VOR-DME facility mean that any point within the service volume of that station can be defined in terms of a radial and distance. For example, you might be located at a point 15 miles from the VORTAC on the 225 degree radial and the place you’d like to go might be on the 315 degree radial, 15 miles out (Fig. 10 -1). Any stu- dent who managed to stay awake in Trigonometry 101 would recog- nize the triangle thus produced and could offer a solution. When the 127
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lengths and angular relationships of two sides of a triangle are known, a simple calculation can be used to figure out the length of the remaining side and, in this case, the direction of the third side with respect to magnetic north.
But few pilots have the time (or the inclination) to consult a table of trigonometric functions and come up with the answers, especially when the airplane has moved several miles during the calculations. Why not give the job to a computer that has been pro- grammed to figure out these triangles and come up with the right answers continuously and instantaneously? When the electronics people were able to reduce computers from the size of your living room to the size of a matchbox, area navigation (RNAV, for short) became possible. Now, instead of having to fly the dogleg airways and fumble with two-VOR fixes, you can navigate directly to or from any point within the area served by a VOR/DME facility. RNAV opens a whole new world of efficient, straight-line navigation. Look out, crows!
RNAV has introduced a new term — waypoint — to the aviator’s vocabulary. This is the proper way of referring to those locations de- fined by radial and distance from a particular VORTAC. Because a waypoint can be placed anywhere you want it — on an airport, a Final Approach Fix, a Missed Approach Point, or a mountain (so you can stay away from it) — RNAV makes possible a navigation system with complete flexibility that allows you to choose your own route.
A typical RNAV installation consists of an onboard computer, a means of entering the “waypoint address” (that’s RNAV talk for the radial-distance information that defines the waypoint), and a display 128
Fig. 10-1
The RNAV triangle.
that shows magnetic bearing and miles from the selected waypoint. An electronic interface sends the directional signal from the computer to the Course Deviation Indicator to provide steering information. From an operational standpoint, you fly RNAV exactly the same way you fly a normal VOR, but the LEFT-RIGHT and TO-FROM indica- tions, the setting of the OBS, and the computer-generated range are based on a waypoint instead of a VOR station.
Nearly all RNAV manufacturers have taken advantage of the mi- croprocessor revolution to provide additional navigational data at the touch of a button. For example, you can call up the radial and dis- tance from the VORTAC you’re using —“raw data” (uncomputed) — and that information comes in handy to satisfy your occasional curiosity as to your location, to say nothing of the easy, accurate re- ply to ATC when a controller asks, “Where are you?” Pushing another RNAV button will display groundspeed to or from the waypoint and the time required to fly there.
You’ll not fly with RNAV very long without noticing that the CDI has lost its “Nervous Nellie” characteristic close to the station and that RNAV station passage is accompanied by a “soft” change of the TO-FROM indicator. These welcome changes are due in part to elec- tronic filtering as the raw data are massaged by the computer, but more important, the steering display (CDI) is based on a totally different and more realistic concept than conventional VOR: RNAV displays linear deviation.
On course (CDI centered), RNAV and VOR presentations are identical, but the picture changes remarkably when your airplane moves to the left or right of a selected course. The VOR-only installa- tion shows how many degrees you are from the selected course; RNAV presentations indicate how many miles you are from the de- sired course. VOR indicators are designed with a scale of 10 degrees either side of center, whereas most RNAVs have a scale of 5 miles left and right in the enroute mode.
To illustrate the difference, imagine your airplane equipped with both standard VOR and RNAV, both set up on the same station/ waypoint and with the same course selected on the OBS. When the airplane is 2.5 degrees off course, the VOR indicator will show one- quarter of the 10-degree distance between center and full-scale de- flection; if that same location is also 2.5 miles off course, the RNAV indicator will show a displacement of one-half scale.
As you continue flying straight ahead in this situation, the VOR needle will show more and more deviation, and since the angle is constantly increasing, you are flying farther and farther from the
Chapter Ten
selected radial. On the other hand, the RNAV CDI sits there steady as a rock, telling you that the course you have selected is — and remains — 2.5 miles to one side. You are flying parallel to the desired course, something that’s impossible with ordinary VOR. In both cases, the TO-FROM indicator changes as you fly by the station/waypoint, but with RNAV, you’ll always know the displacement distance. This can be a very useful feature, such as when you’d like to arrive on a 2-mile downwind leg at a strange airport. Set up a waypoint on the airport, fly a parallel course on the desired side with the CDI 2 miles off center, and when the TO-FROM changes, you’ll be at midfield on a 2-mile downwind.
From the very beginning, RNAV units have incorporated a feature that lets you select a more sensitive CDI mode for RNAV approaches and other situations that require a finer display of off-course distance. Commonly called the “approach” mode, this selection changes the scale of the CDI; typically, the CDI now indicates only 2.5 miles either side of center, and small deviations from course will be easier to see. In general, the approach mode should be selected when you’re ap- proximately 20 miles from the waypoint and should remain in that configuration throughout an RNAV approach procedure.