Direction Finding (DF)

The aeronautical stations that offer a VHF Direction Finding (VDF) service are listed in the UK AIP AD. Some VDF stations stipulate that the service is not available for enroute navigation purposes (except in emergency). VDF bearing information will only be given when conditions are satisfactory and radio bearings fall within calibrated limits of the station. If the provision of a radio bearing is not possible the pilot will be told of the reason.

A pilot may request a bearing or heading using the appropriate phrase or Q code to specify the service required. Each aircraft transmission shall be ended by the aircraft call sign.

A VDF station will provide the following as requested:

1. QDR – Magnetic bearing of the aircraft from the station (i.e. ..... Approach G-ABCD request QDR G-ABCD).
2. QDM – Magnetic heading to be steered by the aircraft (assuming no wind) to reach the VDF station (i.e. ..... Approach G-ABCD request QDM G-ABCD).
3. QTE – True bearing of the aircraft from the station (i.e. ..... Approach G-ABCD request True Bearing (or QTE) G-ABCD).

The direction-finding station will reply in the following manner:

1. The appropriate phrase or Q code.
2. The bearing or heading in degrees in relation to the direction finding station.
3. The class of bearing.
4. The time of observation, if necessary.

The accuracy of the observation is classified as follows:

• Class A – Accurate within plus or minus 2 degrees
• Class B – Accurate within plus or minus 5 degrees
• Class C – Accurate within plus or minus 10 degrees
• Class D – Accuracy less than Class C

NOTE: Normally no better than Class B bearing will be available.

 

QGH Procedure

QGH letdowns may be provided, when requested by a pilot, at aerodromes where the procedure is approved. The procedure provides for control of an aircraft from its initial approach level to a position from which an approach can be completed visually, approach may not be aligned with a runway.

On receiving a request for a QGH the aircraft is to be homed overhead the VDF aerial at or descending to the lowest available flight level/altitude taking into account the minimum safe flight level or safety altitude as appropriate. During homing the following message will be passed to the pilot:

During the procedure aircraft replies are used to obtain D/F bearings. Pilots may be asked to make additional transmissions for D/F. With some equipment the full callsign is sufficient to obtain bearings.

Two D/F indications are required to confirm the aircraft is overhead the VDF aerial; then instructions are given to achieve the desired outbound track.

On completion of the overhead turn and when bearings indicate the aircraft is outbound the controller starts timing the outbound leg and provides heading corrections derived from a series of bearings to make good the desired outbound track. Descent instructions and the appropriate pressure setting are also given during this procedure.

On completion of the timed outbound leg (e.g. 3 min) the aircraft is instructed to turn onto a heading to achieve the final approach track.

When the aircraft reports steady on completion of the inbound turn, headings will continue to be given to achieve the inbound track. During the inbound leg the controller will pass instructions to be followed in the event of a missed approach (Note: At military airfields the weather and missed approach procedure may be passed at an earlier stage in the procedure). Descent clearance to minimum descent height and the QFE will be given on this leg.

If the pilot has not reported ‘visual’ by the time minimum descent height is reached he may be instructed to maintain level flight until he is overhead the VDF. If the aircraft is not ‘visual’ when overhead the VDF, the controller will give the instruction to carry out a missed approach procedure.

 

VDF Procedure

This is a procedure whereby a pilot requests a series of QDMs to home to a VDF station on or near an aerodrome and to carry out a prescribed VDF instrument approach procedure to the aerodrome. VDF procedures are notified in the AD section of the UK AIP.

Requests for QDMs are normally initiated by the pilot at intervals of about 1 minute during the initial stages of the homing, increasing in frequency as the VDF overhead is approached. During this procedure QDMs are requested as required to achieve and maintain the specified tracks. The VDF Procedure is totally pilot interpreted.

The pilot employs a series of QDMs to home to the VDF overhead positioning himself to arrive from a direction which will entail the minimum of manoeuvring in the overhead to proceed outbound on the specified track.

The pilot starts timing the outbound leg and, employing a series of QDMs to establish and maintain the prescribed track, descends as notified for the procedure. The timed outbound leg ends with a turn (normally level) onto the final approach QDM.

At the end of the outbound leg the pilot turns as prescribed onto the final approach QDM using a series of QDMs during the turn to achieve the final QDM.