Aviation Theory

(Continued from VOR Navigation : part 1)

The VOR can be used for primary navigation, non precision approaches, and orientation (answering the question, "where am I?"). Before we get too far into the details, let's look at an example of some VOR indications in relation to a VOR station.

The gauge with the black frame is the HSI from the Lear 45. The two gauge group is the HI (Heading Indicator) and the VOR from the Cessna. It is difficult to see in the diagram, but each indicator is showing "From".

Aircraft 1 is on the 270° Radial From the station (the compass rose symbol). On the HSI, this is indicated by the green needle being centered and the head of the arrow pointing to West (270°). On the Cessna gauges, you can see that the HI indicates 270° and the CDI is centered.

Aircraft 2 is right of the desired course, indicated by a left deflection of the CDIs. To get back on course, fly toward the needle (in this case, left turn).

Aircraft 3 is left of course as indicated by the right deflection. Again, fly to the needle to get back on course.

Aircraft 4 is flying a heading of South (180°) and is crossing the 270° Radial. This shows that the centered CDI is NOT related to aircraft heading.

ORIENTATION

The art of orientation is an essential skill for any pilot. In the barnstorming days, it was not uncommon for a lost pilot to land in a hayfield and ask directions, fill up the tank from the local gas station and be on his way. This practice is now heavily frowned upon by the FAA and leads to reams of paperwork.

The best choice today is to orient yourself by using known locations, in this case, VORs. The first step is to establish a position line. To do this, first tune in and identify a VOR. Then center the CDI by turning the OBS. Note whether you have a To or From indication. This does not give you an exact location. It only tells you that you are somewhere on given radial (your position line).

In order to positively "fix" your position, you need at least one more point of reference. This could be another VOR, a DME fix, or a NDB. If you have two NAV radios, tune the second one to another VOR and establish a second position line. To use a NDB, you would tune the ADF to the station and get a bearing (NDB navigation is covered in a later lesson). If you have DME and are tuned to a VORTAC, you can tell exactly where along that line you are and plot your position (DME is also covered in a later lesson).

The following diagram shows a position fix using two VORs.

The first VOR, on the left, has been tuned in and indicates a reading of 030 From (or 210 To). This places the aircraft somewhere along the 030 Radial.

Then the second VOR, on the right, is then tuned in and indicates 290 From (or 110 To). This places the aircraft on R 290. Where the two radials cross is the position of the aircraft.

For the most accurate fix, the two position lines should be as close to 90° as possible. Using this method, provided two VORs are in range, you should be able to quickly determine your position.

If you are flying an aircraft with only a single VOR, you can still use this method. It is a little more work, since you have to retune your NAV radio to get the second position line.

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FIXING A POSITION OVER A VOR

As the aircraft approaches a VOR, the CDI will become more sensitive. Directly above a VOR is an area known as the "zone of confusion" or "zone of ambiguity". The width of the zone depends on your altitude. The higher you are, the wider the zone. It can extend in an arc of 70°, so it may last for a few minutes. During the time in the zone, the To/From indicator may flick back and for and the Off flag may pop up temporarily. Once through the zone, the To/From indicator will switch and become steady.

Station passage is the point where the first positive complete reversal of the To/From indicator occurs.

A note to autopilot users: If you are trying to use NAV hold, when you get near a VOR, the AP is going to "chase the needle" and make a series of S turns to try and track the radial. The best solution is to switch to HDG hold until passing the VOR and a steady signal is received.

FIXING A POSITION ABEAM A VOR

If your course does not take you directly over a VOR, you can establish the point where you are abeam, or to one side of, the VOR. To do this, select and identify the VOR. Set the OBS to a radial perpendicular (90°) to your course. It is best to set the OBS to indicate the radial From the VOR. This causes the CDI to be deflected to the side of the indicator toward the VOR. As you near the point abeam the VOR, the CDI will start to center. When the CDI is centered, you are abeam the station.

Fixing your position in this manner can also be used to calculate your distance from the VOR by using the 1-in-60 rule. In a no wind situation, you can estimate the time it would take to fly directly to the station by measuring the time for a bearing change as you pass the station. The formula is:

minutes to station = seconds between bearings/degress of bearing change

For example, if you measure the time for a 10° bearing change as being 5 minutes, then the calculation would be:

minutes to VOR = 300 seconds/10° = 30 minutes

If your groundspeed was 240 kts (4 nm/min) that would mean your are 120 miles from the VOR (4 nm/min x 30 minutes).

IDENTIFYING A RADIAL CROSSING

There are times when you may want to identify your position along an airway. In fact, many times in IFR, this is required and is one basis for identifying an intersection. This method of using a VOR can also be used to check groundspeed and make revisions in estimated times for flight planning.

In the following illustration of identifying an intersection, we will assume that you are using dual Nav radios with NAV 1 tuned to the ILS for Rwy 12R (IANT on 110.9) at San Antonio, Texas and NAV 2 tuned to the SAT VOR (116.8) used to establish the RAIME intersection.

The course is 124° as indicated by the OBS on the top gauge. The lower gauge is tuned to SAT and the OBS is set to 265° From.

The numbers above the gauges correspond to numbered positions on the flight path. In position 1, you can see that we have not reached the intersection because the CDI is to the left.

At position 2, we are precisely at the RAIME intersection as indicated by the centered CDIs on both VOR indicators.

In position 3, we have passed RAIME, as shown by the right deflection of the CDI, and are inbound on a heading of 124°.

Identifying intersections is used heavily in both instrument approaches and enroute operations. You are encouraged to increase your proficiency in quickly identifying fixes using the VORs and in navigating (tracking) to a VOR, which we will cover in Part 3.

This concludes Part 2 of the VOR Navigation Lesson.

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Read on : VOR Navigation : part 3