VOR Intercepting and Tracking

Background for VOR navigation – these things you should know

  1. History of radio navigation

  2. VOR nav as the basis for the airway system

  3. VOR types: low altitude, high altitude, and terminal VORs

  4. VOR symbology on sectionals, IFR enroute charts, and IFR approach charts (IAPs). Sectionals and WAC charts. How to distinguish a VOR, a VOR/DME, and a VORTAC. Where to find the frequency and identifier.

  5. VOR frequencies: 108.00 – 117.95 Very high frequency range. Line of sight. Less susceptible to interference than lower frequencies.

  6. Identifying VOR signal and DME signal

  7. VOR terminology — Radial nomenclature – radials always expressed as FROM the VOR, despite your heading or course on that radial.

  8. Use and nomenclature of the OBS/VOR instrument in the airplane –omnibearing selector (OBS), course deviation indicator (CDI), angular displacement, 2 degrees per dot, TO/FROM indication, warning flag, glideslope needle and flag (on some instruments)

  9. How to determine if DME is available from each of the above types of charts.

  10. DME pairing with VOR frequency

  11. How DME works – system capacity, slant range, ground speed and ETA calculations. GPS, which is not the same distance, may be substituted for some enroute and terminal applications, such as DME holding pattern legs

  12. Concept of service area – what is the standard service volume

  13. Terminal VOR (25nm/12,000ft), Low altitude VOR (40nm between 1,000 – 18,000 ft), High altitude VOR (40nm to 14,500 and 100nm 14,500-18,000 and 130nm 18,000-FL450)

  14. VOR signal anomalies, specific to radial and altitude. A/F Directory. Airways reception guaranteed as per MEA, MOCA.

How to use the VOR for navigation – tune, identify, navigate

  1. How to find VOR on the chart and obtain the frequency and the identifier and read the compass rose.

  2. Tune and identify – how to identify using the aircraft nav radio and audio panel.

  3. To/From indication and concept of reverse sensing. (Which we believe to be a falsehood.) Obtain a To or From indication, depending on your route and heading and how to center the needle and track the radial

  4. Victor airways – defined by radials, changeover points (midway unless otherwise indicated). MOCA and MEA implications for VOR signals.

How to avoid confusion about orientation and intercept

  1. Select a reasonable intercept angle from the OBS, depending on distance from the VOR, the urgency of intercept and so on.

  2. Never use the OBS for heading information. Ascertain the proper heading for the intercept from the OBS, but then refer to the DG to turn the airplane to that heading.

Common Errors

  • Continuing to turn in the direction of the radial to intercept, rather than determining and maintaining an intercept heading. The result is a circle short of the radial.

How to track with a wind correction:

Concept of reference heading. Determine the reference heading before you obsess about exactly tracking the radial. Then intercept the radial and return to the reference heading. Refer to the OBS only occasionally, to determine the performance of the chosen heading. Do not fly by chasing the OBS needle.

Common Errors

  • Failing to ascertain and establish a reference heading – intercepting the radial, being blown off it by the wind, reintercepting, and then turning to the radial heading rather than a reference heading – which results in merely being blown off the radial again.

INTERPRETING THE VOR INDICATOR . . . A FOOLPROOF METHOD

The OBS (omni-bearing course selector), when tuned to a specific radial, divides the world into four quadrants. The TO/FROM flag divides the world into two hemispheres, courses in one of which will take you TO the VOR while courses in the other hemisphere will take you FROM or away from the VOR. (By TO the VOR, in this context, we mean “on a course that will intercept a radial on a heading that would be appropriate if your intention was to then follow that radial TO the VOR” – consider TO the VOR as a shorthand. The same applies to the expression “FROM the VOR.”) The deflection of the CDI (course deviation indicator) divides the world into two different hemispheres, which overlap the two created by the OBS TO/FROM flag. The two hemispheres created by the CDI are courses that will take you TO the radial and courses that will take you FROM or away from the radial. You can choose the hemisphere that will take you to the radial if you wish to intercept it, and the quadrant of that hemisphere that will take you either TO or FROM the VOR, depending on whether you wish to intercept the radial going to the VOR or away from it. The course exactly midway (ninety degrees from the OBS setting) of the hemisphere TO the radial will take you to the radial by the shortest route, with a right-angle intercept.

This means of determining an appropriate intercept course will work regardless of whether you have a TO or FROM indication and regardless of your airplane heading, so long as you determine the course from the OBS and then look to the DG to determine how to turn to that course.

In both of the following tunings of the OBS, we have tuned the 360 radial and one can see from both tunings of the OBS that any heading between 180 and 360 will intercept the radial. A heading of 270 (the course exactly midway (ninety degrees from the OBS setting) of the hemisphere TO the radial) will take you to the radial by the shortest route, with a right-angle intercept.

VOR Orientation

If we want to proceed to the VOR or away from the the VOR, we simply choose the appropriate quadrant of that hemisphere. From both tunings, we can see that a course between 270 and 360 will intercept the radial headed TO or toward the VOR. Such a course might require a right turn or might require a left turn, depending on the present heading of the airplane. That can be determined only from the DG. Similarly, a course between 180 and 270 will intercept the radial headed FROM or away from the VOR.

References:

  • Sollman & Harris, chapter 10 (VOR procedures)

  • Dogan, chapter 5 (VOR Equipment, Orientation and Tracking)

  • Kershner, chapter 5 (Navigational Aids and Instruments, p. 87-105, review of the VOR, distance measuring equipment)

  • FAA Instrument Flying Handbook, chapter VII (Electronic Aids to Instrument Flying) p. 111-123, chapter VIII (Using the Navigation Instruments) p 133-142 (VOR receiver, orientation, tracking a course, course interception, transition and localizer approach, VOR procedures using the RMI)

  • Thom, chapter 13 (The VOR)

  • Jeppesen Sanderson, INSTRUMENT COMMERCIAL MANUAL, chapter 1 (Principles of Instrument Flight) Section C (Instrument Navigation) p. 1/37-49 (VOR facilities, VOR accuracy checks, interpreting VOR indicators, distance measuring equipment, horizontal situation indicator)

This article was written by George Scheer (george.scheer@gmail.com) and is reproduced here with his permission.