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  Thursday 29 June 2017 09:04 GMT  

Aviation Theory

VOR Navigation part 3

Previous Chapter VOR Navigation part 2 -- Table of Contents -- ILS Navigation part 1: The Localizer Next Chapter

(Continued from VOR Navigation : part 2)

TRACKING TO A VOR

One of the most often used procedures with a VOR is tracking. ATC clear you direct XYZ VOR via 090 radial. Okay, great! We have a clearance, now how do we get there? Here's the outline of the process:

  • Select the proper frequency
  • Identify the station using the Morse Code identifier
  • Make sure you are receiving a usable signal (no Off flag)
  • Select the omni bearing of the desired course.

Once these steps are taken, orient the aircraft with respect to the desired course. In other words, if you are planning to track To the VOR on a given course, orient yourself to that heading either mentally or by turning to that approximate heading. If the aircraft heading is approximately the same as the desired course, the donut (center) of the indicator represents the aircraft, and the CDI is the desired course. To intercept the radial, turn toward the CDI. An intercept angle of 30 or less is a good one to use provided you are not too far off course. For greater distances off course, an intercept in the 60-90 range may be necessary.

Using the VOR indicator in this manner uses the indicator as a command instrument, i.e. commanding a turn toward the CDI to get on course. This only applies when the aircraft heading is in the same approximate direction as the desired heading.

Once the course is intercepted, you must maintain a heading that will keep you on the desired course. In a no wind situation, the MH on the HI (Magnetic Heading on the Heading Indicator) and the OBS will be the same. Let's look at a graphical example:

Illustration showing an aircraft intercepting a VOR radial

Aircraft 1 is on a heading of 060 and has set the desired course of 030 (R 210) into the OBS. The CDI indicates that the desired radial is right of the current position. The difference between the course and the OBS gives an intercept angle of 30.

The pilot elects to continue on the present course, as indicated by Aircraft 2. The CDI has started moving toward center, so we know that we are getting close to the desired course.

In Aircraft 3, the CDI is centered and a left turn to 030 is made. This puts the aircraft on a course of 030 to the VOR and on the 210 Radial.

WIND CORRECTION ANGLE DETERMINATION

If there is a crosswind, a wind correction angle (WCA) will be required. For example, if you are headed north on a course of 360 and there is a west wind, the aircraft will drift east. A good rule of thumb to follow in this situation is to double the number of degrees you are off course and turn that amount toward the desired course. For example, if you notice a drift to the east of two dots (4) you would turn west (left) 8 or a heading of 292. When you are again on course, turn right by half of the correction (4) to a course of 296. This process is known as bracketing the course. Since the wind may change in direction or strength as you continue along your course, small corrections may be required periodically. For a wind from the east, just reverse the process. The key to tracking a VOR course is small corrections before the error becomes too great.

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TRACKING FROM A VOR

The process of tracking from a VOR is essentially the same as tracking To a VOR. The main difference is that the heading and OBS settings will be the same, rather than the reciprocal as in tracking To the VOR.

REVERSE SENSING

A complication arises when tracking a VOR and then turning 180. The CDI in this situation will not be a command instrument and in order to get on course, you would steer away from the CDI rather than toward the CDI. The best course of action in this case is to reset the OBS so that proper indication is given. For example, if tracking From the VOR on the 180 radial and then turning to track inbound To the same VOR, as done in a course reversal or procedure turn, reset the OBS to 360 after the turn.

INTERCEPTING A COURSE

The key to intercepting a course is visualization. The pilot needs to ask several questions:

  • Where am I?
  • Where do I want to go?
  • What is the best route to get there?

The easiest method to orient yourself is to center the CDI. This will occur with two different (reciprocal) headings. Choose the indication that is closest to the aircraft heading. If the aircraft is heading toward the VOR, this will be a To indication and if headed away from the VOR a From indication. This answers the "Where am I?" question.

Let's assume you have been instructed to track outbound on the 130 radial from XYZ VOR. You are flying a heading of 030 and you tune in the XYZ VOR. As you turn the OBS, you get a From indication on 210 and a To indication on the reciprocal, 030. This tells you that you are tracking To the VOR on the 210 radial. Now You know where you are (Aircraft 1). Next, we determine where we want to go. Since you are heading northeast, you know that the 130 radial lies to your right, or southeast from the VOR.

Illustration showing an aircraft changing course to intercept a VOR radial

To intercept the 130 radial, you would set the OBS to 130 and make a right turn to 070 for a 60 intercept (Aircraft 2). You would then maintain a heading of 070 until the CDI starts to move from a full left deflection toward the center. Just prior to the CDI centering, you would "lead the turn" by starting your turn to 130 (Aircraft 3). Leading the turn helps prevent overshooting the desired course and the inefficient procedure of "chasing the needle". You can calculate the amount of time it will take to make the turn using a standard rate turn (3 per second). Since you are turning 60, this would require 20 seconds. When you roll out on 130, you should be on course, tracking outbound on the 130 radial.

Intercepting a course to a VOR uses the same process, except you would set the OBS for the course that gives a To indication, the reciprocal of the radial you will be tracking inbound. A simple way to figure reciprocals in your head, since some people have trouble adding or subtracting 180 is to use an addition and a subtraction. Either add 200 and subtract 20, or subtract 200 and add 20. Another way to figure the reciprocal is to visualize the head of the needle on the course and read the reciprocal under the tail of the needle.

OTHER VOR INDICATORS

The one type of VOR indicator we have not discussed is the RMI (Radio Magnetic Indicator). This is the type indicator used in the FS98 Lear 45 for VOR 2. The RMI needle always points To the VOR, therefore the tail of the needle indicates the radial. The RMI when used for a VOR gives the same indications as when used for ADF or NDB tracking. There is no OBS since the needle always points to the station. Using an RMI (Radio Magnetic Indicator) will covered more completely in the lesson on ADF/NDB.

The HSI, which was briefly discussed earlier has the advantage of always being a command instrument, since the needle will point toward the stations and will rotate as the aircraft turns. In other words, there is no reverse sensing with the HSI.

This concludes Part 3 of the lessons on VOR navigation. We will cover VOR approaches in later lessons.

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