NAVIGATION AIDS: VOR

Navigation Aids: VOR Lesson by wifiCFI


VOR (PHAK C16) (AIM 1-1-3)

VORs are a means of ground based navigation for pilots in flight.

VOR stands for Very High Frequency Omnidirectional Range. 

For civilian flight operations (non-military) there are 3 types of VORs:

VOR

VOR/DME

A VOR co-located with DME (Distance Measuring Equipment)

VORTAC

TAC stands for TACAN and is for military use only. 

To civilian pilots, VORTAC and VOR/DME mean the same thing.

VOR Basics

Very High Frequency Omnidirectional Range

Very High Frequency means that VORs operate on a frequencey range between 108.0 and 117.9 MHZ.

Omnidirection Range means that VORs provide pilots with 360 degrees of Navigational Coverage.

VOR Information Box

Next, we need to discuss the information given, regarding VORs, on the aeronautical charts.

Each VOR will be accompanied by an information box seen on the following slide.

VOR Service Volumes

Before we get into the inner/mechanical workings of the VOR, we need to understand it’s service volumes.

Service Volume = the ranges at which they can be safely and reliably used by pilots.

Line of Sight

First, VORs operate on a line of sight principle. 

High, Low, and Terminal Service Volumes

Secondly, VORs will have different ranges based on type (high VOR, low VOR, or terminal VOR)

These are discussed in more detail during the Instrument Rating Course but the basics are shown below:

Inner Workings

Now let’s jump in to a VORs Inner Workings and all of the information we can get from our VOR Instruments in the cockpit.

Timing

A VOR works based on timing between 2 radio signals.

One of the radio signals a VOR sends out is a Uni-Directional (sweeping) signal.

The other signal is Omnidirectional (all-directions).

Everytime the sweeping, uni-directional signal points due North, the omnidirectional signal flashes.

Let’s create a visual for better understanding.

The Lighthouse Example

To start, let’s first visualize a lighthouse.

As we all know, a lighthouse has a uni-directional sweeping signal.

This signal rotates 360 degrees around the lighthouse.

Now, let’s say we install an omnidirectional light on top of the lighthouse.

The omnidirectional light can be seen from all directions.

We then rig the omnidirectional light to flash everytime the sweeping light is pointing due north.

At this point, we could use the timing between the omnidirectional signal and the sweeping signal to assign “radials”.

The timing difference between when a ship would see (receive) the sweeping light signal and when they would see (receive) the omnidirectional signal would be assigned a “radial”.

The ships captain could then know where he is in relation to the lighthouse:

North, East, South, West

Let’s assume the sweeping signal takes 12 seconds to make a full rotation.

We can now create the radials and timing.

Difference between when we received the directional and omnidirectional signals.

Example:

If we were due West of the Lighthouse:

We would first see the omnidirectional flash,

Then, 9 seconds later, we would see the directional flash.

We could then reference our chart and know:

That we were due West of the Lighthouse and

That we were on the 270 Radial.

This is exactly how a VOR works.

The VOR receiver in the airplane is equipped with the timing chart necessary.

Except, instead of lights, they utilize radio signals.

VOR Cockpit Instruments

Now that we know the inner workings of a VOR, we can discuss cockpit instrumentation.

VOR information can be relayed to pilots by use of 1 of 2 different instruments.

VOR OBS

OBS = Omnibearing Selector

A VOR OBS can relay the following information to a pilot:

Which radial the plane is on (in relation to the VOR)

Amount of deviation from that radial

And, if the selected radial will take the pilot closer TO or further FROM the VOR.

CDI Needle

CDI = Course Deflection Indicator

The CDI tells a pilot:

Which radial the plane is on (in relation to the VOR) or

Amount of deviation from that radial

If the CDI Needle is Centered:

The pilot is “on course” with the selected radial.

Any radial can be selected by twisting the OBS Knob.

If the CDI Needle is Deflected:

The pilot is “off course” with the selected radial.

The needle indicates direction of course.

The pilot must turn toward the needle to get back “on course.”

Amount of deviation (in degrees) is also noted.

Each tick mark = 2 degrees of deviation.

In this example we have 7 degrees of deviation.

TO/FROM Indicator

The TO/FROM Indicator is a simple instrument.

It tells the pilot if:

The selected radial will take the pilot closer TO

Or further FROM the VOR

When will the TO/FROM Indicator Switch?

If crossing the VOR:

At station passage.

When will the TO/FROM Indicator Switch?

If not crossing over the VOR:

90 degrees from tuned radial.

The Cone of Confusion

The closer a pilot gets to the VOR the smaller the distance between the different radials.

The area close to the VOR is referred to as the “Cone of Confusion.”

In this area the CDI Needle Indications will become extremely sensitive.

When approaching the Cone of Confusion, be sure not to chase the needle.

Flying with the OBS

Let’s start by using the OBS example pictured.

The Course

As can be seen, we have selected a course of due North (360) with the OBS Knob.

We can also see that our CDI Needle is deflected to the West.

This means we need to make a turn correction to the West.

Remember, chase the needle!

The Direction

We can see that if we continue North that we will get further FROM the VOR.

In this example, if we turned to the West we could center our CDI Needle.

At that point, we would be travelling away FROM the VOR on the 360 Radial.

What is Reverse Sensing?

Reverse Sensing is a Pilot Induced Error that can confuse the Pilot into making incorrect course corrections.

This occurs when a pilot’s heading indicator is not aligned properly with their VOR OBS.

How to Avoid Reverse Sensing?

ALWAYS speak in Cardinal Directions (N, E, S, W) when announcing course deviations.

NEVER speak in Left or Right!

Time and Distance from The Station (VOR)

A pilot can also calculate their time and distance to the VOR with the following equation:

Time in seconds between bearings/Degrees of bearing change = Minutes to the station.

VOR Trick

The CDI Needle and TO/FROM Indicator tell you where you are NOT.

Simple cover up the half of the OBS that the CDI Needle is on and cover up the half of the OBS the TO/FROM Indicator is on.

The remaining uncovered section is where you are in relation to the VOR.

Here’s another VOR Trick:

With the needle centered, the TO/FROM Indicator always points at the Radial you are NOT on.

VOR Checks:

VOR Receivers are required to be checked every 30 days for IFR Flight Operations.

However, it is also important for VFR Pilot’s to check their aircraft’s VOR Receivers.

What to Write (SLED)

Signature (of pilot performing the check)

Location (of the check)

Error (amount of error detected during check)

Date (of the check)

VOT (VOR Test Facility)

A VOT is coded to emit the 360 Radial in all directions around the facility.

This means the airplane’s VOR Receiver should read either:

360 FROM or…

180 TO

Regardless of the aircraft’s location in relation to the VOR.

How the check is done:

Tune and Identify the VOT.

Twist the OBS Knob to center the CDI Needle.

Check for proper TO/FROM Indication.

The radial selected must be within:

+/- 4 degrees of 360 or 180.

Ground Check

With a VOR ground check:

The Pilot must park the airplane in the designated ground spot.

The Pilot must tune and identify the correct VOR.

The Pilot must use the ground check sign to know:

Which radial he/she should be on.

Whether he/she should have a TO or a FROM Indication.

How the check is done:

Park aircraft in designated check spot.

Tune and Identify the Correct VOR.

Twist the OBS Knob to center the CDI Needle.

Check for proper TO/FROM Indication.

The radial selected must be within:

+/- 4 degrees of Designated Radial.

Airborne Check

With an Airborne VOR check:

The Pilot must position the airplane over the designated location.

The Pilot must tune and identify the correct VOR.

The Pilot must use the information in the Chart Supplement to know:

Which radial he/she should be on.

Whether he/she should have a TO or a FROM Indication.

How the check is done:

Position aircraft over designated check spot.

Tune and Identify the Correct VOR.

Twist the OBS Knob to center the CDI Needle.

Check for proper TO/FROM Indication.

The radial selected must be within:

+/- 6 degrees of Designated Radial.

Dual VOR Check

With a Dual VOR check:

The airplane must be equipped with 2 VOR Receivers.

How the check is done:

The pilot tunes both VOR Receivers to the same VOR.

The pilot centers both CDI Needles.

Check for proper TO/FROM Indications.

With both CDI Needles Centered:

The Selected Radials should be within 4 degrees of each other.

VOR Check Summary:

VOT = +/- 4

Ground Check = +/- 4

Airborne Check = +/- 6

Dual Check = within 4 degrees of each other

FAA Sources Used in this Lesson

Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 16

Aeronautical Information Manual (AIM) Chapter 1

FAA Aeronautical Chart User Guide


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