Aviation Theory

INTRODUCTION

VORs and VORTACs are the two most commonly used navigational aids. The advantage of the VOR over the NDB is the fact that it directly supplies an indication of the proper course to reach the VOR or to track a particular course away from the VOR. The VOR system is used in enroute navigation as well as being a part of most precision instrument approaches and many non-precision approaches. While often confusing at first, once you have mastered the use of the VOR, you are prepared to fly a precise course regardless of the wind direction. As a matter of fact, by knowing your TAS (True Air Speed), ground speed and the WCA (Wind Correction Angle), you can calculate wind direction and speed.

The information you can obtain from a VOR will also allow you to calculate your ground speed.

From AIM 1-1-3. VHF OMNIDIRECTIONAL RANGE (VOR)

1. VORs operate within the 108.0 to 117.95 MHz frequency band and have a power output necessary to provide coverage within their assigned operational service volume. They are subject to line-of-sight restrictions, and the range varies proportionally to the altitude of the receiving equipment. The normal service ranges for the various classes of VORs are given in AIM, VOR/DME/TACAN STANDARD SERVICE VOLUMES (SSV) paragraph 1-1-8d.
    
2. Most VORs are equipped for voice transmission on the VOR frequency. VORs without voice capability are indicated by the letter "W" (without voice) included in the class designator (VORW).
    
3. The only positive method of identifying a VOR is by its Morse Code identification or by the recorded automatic voice identification which is always indicated by use of the word "VOR" following the range's name. Reliance on determining the identification of the omnirange should never by placed on listening to voice transmissions by the Flight Service Station (FSS) (or approach control facility) involved. Many FSSs remotely operate several omniranges with different names. In some cases, none of the VORs have the name of the "parent" FSS. During periods of maintenance, the facility may radiate a T E S T code (- . ... -) or the code may be removed.
    
4. Voice identification has been added to numerous VORs. The transmission consists of a voice announcement, "AIRVILLE VOR" alternating with the usual Morse Code identification.
    
5. The effectiveness of the VOR depends upon proper use and adjustment of both ground and airborne equipment.
    
   
1. Accuracy: The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree.
    
2. Roughness: On some VORs, minor course roughness may be observed, evidenced by course needle or brief flag alarm activity (some receivers are more susceptible to these irregularities than others). At a few stations, usually in mountainous terrain, the pilot may occasionally observe a brief course needle oscillation, similar to the indication of "approaching station." Pilots flying over unfamiliar routes are cautioned to be on the alert for these vagaries, and in particular, to use the "to/from" indicator to determine positive station passage.
    
1. Certain propeller RPM settings or helicopter rotor speeds can cause the VOR Course Deviation Indicator to fluctuate as much as plus or minus six degrees. Slight changes to the RPM setting will normally smooth out this roughness. Pilots are urged to check for this modulation phenomenon prior to reporting a VOR station or aircraft equipment for unsatisfactory operation.

1-1-6. VHF OMNIDIRECTIONAL RANGE / TACTICAL AIR NAVIGATION (VORTAC)

1. A VORTAC is a facility consisting of two components, VOR and TACAN, which provides three individual services: VOR azimuth, TACAN azimuth and TACAN distance (DME) at one site. Although consisting of more than one component, incorporating more than one operating frequency, and using more than one antenna system, a VORTAC is considered to be a unified navigational aid. Both components of a VORTAC are envisioned as operating simultaneously and providing the three services at all times.
    
2. Transmitted signals of VOR and TACAN are each identified by three letter code transmissions and are interlocked so the pilots using VOR azimuth with TACAN distance can be assured that both signals being received are definitely from the same ground station. The frequency channels of the VOR and the TACAN at each VORTAC facility are "paired" in accordance with a national plan to simplify airborne operation.

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