123s of Radio range

First of all, I want to wish you all a Happy New 2009! Do you have a New Year's resolution? I know I do.

All the best in your aviation studies and training. Remember to have fun and to stay safe!

Here is a short post I think will prove useful.

Here is a sample exam question:

You are flying at 8,500 feet and you are wondering for how many more miles you can expect to receive a readable VOR signal from the station.

A. 100 nm
B. 92 nm
C. 184 nm
D. 113 nm

What is the answer?

First thing to understand is that VOR uses VHF radio signal which stands for Very High Frequency. VHF is mainly used for FM radio, TV and two way radio communications. Unlike low, medium frequency radio signal, very or ultra high frequencies do not 'bend' much around the surface of the earth, Very High Frequencies can only be received by the receiver that is in line-of-sight to the sending station.


In order to calculate the max line-of-sight distance in nautical miles from the transmitter (ATC, VOR, DME) and the airplane, you use the following simple calculation:

1.23 * square root of airplane's altitude

Let's take the exam question and calculate the distance.

1.23 * square root of 8,500=113.4

You can also do an easy mental check. You know that if the altitude is 10,000, the answer will be 123 nm. Generally, you can assume an altitude lower than 10,000 will generate a shorter distance. On a practical level, remember that if you are out of comm range, one way to get into range is to climb. However it is also reasonable to expect that at higher altitudes you can encounter more interference from other transmitters on similar frequencies hence weaker signal etc.

Now that you know all that...

The answer to the exam question above is D.