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Practice Exam

VHF Direction Finding

Some Air Traffic Control Units have VHF Direction Finding (VDF) equipment which shows the air traffic controller the direction a radio transmission is coming from.

An example VDF display available to some air traffic controllers

Using VDF

A pilot can make use of this VDF equipment by making a normal radio transmission on the aircraft’s VHF radio and asking for one of the following codes:

QDM – the code for a magnetic bearing to the station
QDR – the code for a magnetic bearing from the station
QTE – the code for a true bearing from the station
In this example, the true bearing is 5° less than the magnetic bearing, so we can say the Magnetic Variation is 5°W

Most commonly, a pilot will want a QDM – the magnetic track to the VDF station. Once given a QDM, the pilot can either home or track to the station:

Homing to the station
Homing is when the pilot turns onto the QDM and makes no correction for any drift from the wind.
Tracking to the station
Tracking is when the pilot compensates for the wind by flying a wind corrected heading. This is more accurate and the pilot should arrive overhead the VDF station.

VDF Accuracy

VDF accuracy is divided into 4 classes:

Class A
Accuracy within +/- 2°
Class B
Accuracy within +/- 5°
Class C
Accuracy within +/- 10°
Class D
Accuracy worse than Class C

Since VHF radio waves only travel in straight lines, you need to be in line of sight of the VDF station for it to work. Any terrain or obstacles in the way will block your VHF transmissions.

Aerodromes with VDF

Full details of Air Traffic Control Units with VDF services, their radio frequencies, and hours of operation are found in the AIP.

Aerodromes with VDF services also have “VDF” and the appropriate frequency marked on the 1:500 000 aeronautical chart.

Here we can see the Cranfield VDF details on the UK 1:500000 aeronautical chart. EGTC is Cranfield’s ICAO code and their VDF frequency is 122.855