Aviation Theory

INTRODUCTION

The very high frequency omni-directional radio range. What a mouthful! This is one instance where acronyms are handy. All of that is condenses into VHF omni-range or VOR! If you need a basic explanation of what a VOR or VORTAC is, please see the Navaids lesson on VOR/VORTAC.

The VOR system was developed in the US in the late 1940s and adopted by the ICAO as the standard short range radio navigation aid in 1960. It had several advantages over the existing system of ADF/NDB, such as low interference from atmospheric conditions and no night effect.

USAGE

The VOR is used for: orientation and position determination navigation by tracking to or from a station as a holding fix as a non precision instrument approach

The main use is for following a specific course to or from a VOR. The VOR will indicate the desired course and the deviation from that course. The VOR display is NOT heading sensitive. The heading has nothing to do with the indication. More on this later.

In the VOR system, a ground station emits a signal in all directions. By convention, 360 different tracks, or radials, are used. Radials are always From the VOR and in 1° increments, related to magnetic north. The normal accuracy of a VOR is ± 2° or better.

The range of a VOR is limited by power and line of sight. There are several classes of VORs. For information concerning ranges, see the Navaid Service Volume lesson.

Most aeronautical charts show the VOR's position, frequency and ident, the three letter identifier for the VOR in Morse Code. This information is also found in the AF/D. The Victor Airways and Jet Routes both use the VOR system and the routes themselves are marked with the appropriate radial of each VOR forming the route. You should be aware that these course indications are in degrees magnetic. If you are plotting a course on a chart, you need to convert the degrees true to degrees magnetic. To convert true to magnetic, you add or subtract the magnetic variation for the area. An easy way to remember this is: "East is Least, Add Whiskey" or "East is Least; West is Best". In other words, if variation is East, subtract it from magnetic to get true and if it is west, add it.

COCKPIT INDICATORS

Although there are various types of indicators, they present the same information and are operated in similar manners. The two we are concerned with in FS are the standard VOR indicator, also called an OBI (Omni Bearing Indicator), and the HSI (Horizontal Situation Indicator). The differences in the two will be covered later in the lesson.

The components of the VOR indicator are:

1. Omni Bearing Selector (OBS) or course selector - turns the compass card
2. Course Deviation Indicator (CDI) - the VOR needle, centered when on the selected course and deflected left or right when off course
3. To/From flag - indicates whether the selected course would take the aircraft toward or away from the VOR
4. On/Off indicator or flag - indicates whether or not you are receiving a reliable signal - A red and white flag on the Cessna and a red X on the Lear HSI

A VOR should only be used for navigation if:

• the Off flag is hidden
• the correct Ident is heard
• the CDI is not moving erratically

The Off flag may show when the signal is not strong enough for reliable use, too low for reception, when directly over the VOR in the "zone of ambiguity", or the unit is switched off.

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COURSE DEVIATION

The CDI indicates an off course situation by showing the deviation as an angular deviation from the selected course. The reference is the course selected by the OBS and shown at the top of the gauge. The deviation is referred to in terms of "dots" off course. There are five dots to either side of center and each dot represents 2° of deviation. If the aircraft is on the selected radial, the CDI is centered. If the aircraft is 4° off course, the CDI will be deflected two dots from the center (also called the donut). If the aircraft is 10° or MORE off course, a full scale deflection will be shown (five dots).

Since the off course indication is angular, the closer to the station, the less the actual distance off course. The lateral deviation (distance off course) can be calculated or estimated using the following distances from the station: 

• at 1 nm, 1 dot = 200 ft.
• at 30 nm, 1 dot = 30 x 200 = 6000 ft = 1 nm
• at 60 nm, 1 dot = 60 x 200 = 12000 ft = 2 nm

This matches with the 1-in-60 rule which states: 1 nm off course in 60 nm = 1° course error.

TO OR FROM

Each radial has a reciprocal radial. In other words, the 090 radial, a bearing of 090 From the station, is the same line as the 270 To the station. If the aircraft is on this line, the CDI will be centered when 090 or the 270 is selected with the OBS. The To/From indicator shows where you are in relation to the station.

The presentation of the CDI differs depending on the instrument. It may pivot on the top of the needle, the entire needle may move left or right, or in the case of some HSIs, the center portion of the needle will move left or right.

USING THE VOR

• tune to the proper frequency
• identify the VOR using the Morse Code identifier
• make sure the signal strength is sufficient (no Off flag)
• turn the OBS to the desired course

This concludes Part 1 of the VOR Navigation Lesson.

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Read on : VOR Navigation - Part 2