ppl_03_e2

QDM is the Q-code

designation of the

magnetic heading to steer to reach the ground station, assuming no wind.

VDF SERVICES.

The bearing information given by VDF equipment at a ground station can be provided to pilots in three ways: magnetic heading to steer to the station, true bearing from the station and magnetic bearing from the station.

The Magnetic Track to Steer to the Station (QDM).

An air traffic controller can pass to a pilot the magnetic track that he must steer to arrive overhead the ground station (assuming no wind). In the Q code, which was developed almost 100 years ago to facilitate communication between morse-code operators, the magnetic bearing required to reach a station is known as the QDM. A few of these Q codes are still used today.

ArequestfromapilottoacontrollerforaQDM,or‘steer’,isprobablythemostcommon use of VDF. By steering the QDM, a pilot is said to “home” to the ground station, i.e. to head towards it. Formerly, VDF-equipped ground stations were identified by the term “Homer”. Today, the expression “homing” usually refers to the phenomenon of an aircraft being given repeated QDMs by a controller which enable the pilot to arrive at the ground station, in a prevailing cross wind, by continually changing heading to match successive QDMs, all of which are different because the aircraft drifts with the wind. Homing is regarded as an inefficient way to fly to the ground station. (See Figure 15.3.)

Figure 15.3 Homing to a VDF-equipped aerodrome.

The correct way to fly to a ground station on a QDM is to track to the station, compensating for any drift by adopting a suitable wind correction angle, just as you would do to maintain a desired ground track in a cross wind, when navigating visually.

When tracking correctly to a VDF station, the pilot would continue to request a QDM, regularly, in order to check that the QDM was remaining constant. Of course, when tracking in this way, the aircraft’s heading would be different from the QDM read out from the ground station by an amount equal to the wind correction angle. (See Figure 15.4.) Requests for a QDM are normally made by the pilot every few minutes,

QDR is the Q-code

designation of the magnetic

bearing of the aircraft from the ground station.

QTE is the Q-code

designation of the

true bearing of the aircraft from the ground station.

Figure 15.5 A VDF radiotelephony exchange between pilot and VDF station.

During the pilot’s series of requests for QDMs, he places his call sign at the beginning and end of his transmission.

When an aircraft arrives overhead the VDF station, the controller’s VDF equipment will be unable to determine the direction from which the pilot’s transmissions are coming and will reply, “No Bearing”. Of course, when VDF is being used to back up a visual navigation flight, the pilot will have made visual contact with the aerodrome before arriving overhead and will be able to join the aerodrome circuit in an approved manner.

If, however, a pilot were tracking to the aerodrome on VDF QDMs in Instrument

Meteorological Conditions (IMC), and flying in accordance with the Instrument Flight

Rules (IFR), the receipt of the “No Bearing” message would be the indication that he was, in fact, overhead the aerodrome, and that he must now begin to fly the VDF approach procedure. You will learn about instrument approach procedures if you go on to train for an Instrument Rating or, perhaps, in the United Kingdom, an IMC Rating.

Magnetic Bearing from the Station (QDR).

An air traffic controller can pass to a pilot the magnetic bearing of his aircraft from the ground station. In Q code, this is known as the QDR. The QDR gives the pilot similar information to that which he would read from the tail of the needle on a cockpit ADF display, (see Chapter 16). The QDR is, therefore, useful to the pilot in determining a position line along which his aircraft must be located.

True Bearing from the Station (QTE).

A controller can also pass to a pilot the true bearing of his aircraft from the ground station. In Q code, this is known as the QTE. As aerodromes providing a VDF service have no Magnetic North reference, the QTE is more accurate than the QDR in enabling a pilot to plot his position on his chart, as the line may be drawn on the

chart, using a navigational protractor, referenced to the True North indication of a meridian of longitude, in a similar manner to the way in which a pilot measures a track-line during flight planning.

By obtaining a QTE from two VDF stations, and plotting them on a chart, a pilot may obtain an accurate position fix.

A request from a pilot for a QTE is made in the following manner:

Pilot: “True Bearing, True Bearing, Walsham Approach, G-ABCD, Request True Bearing, G-ABCD.”

Controller: “G-ABCD, Walsham Approach, True Bearing 246 degrees True, I say again, 246 degrees True, Class Bravo.”

The pilot pronounces the words “True Bearing, True Bearing” at the beginning of his transmission in order to give the controller time to select QTE on his VDF equipment

before the transmission comes to an end. (See Figure 15.1.)

Limitations to the Provision of a VDF Service.

You will read in the UK AIP that some aerodromes which indicate that they provide a VDF service also stipulate that the service is not available for the purposes of en-route navigation, unless the pilot is in difficulty. This stipulation is probably made because, invariably, the VDF service is provided on the same frequency as the aerodrome’s main approach or tower frequency, so providing en-route VDF information would overcrowdanalreadybusyradiofrequency. Duringtheflightplanningstage,therefore, you should check in the AIP (AD Section) whether an aerodrome from which you plan to request an en-route VDF service applies the aforementioned restriction.

Aerodromes which do apply the restriction may wish to limit the VDF service to an instrument approach service.

THE ACCURACY OF VDF BEARINGS.

VDF bearing information will be given to a pilot only when the bearings that a VDF station is able to give fall within prescribed limits of accuracy. Normally, if the provision of a VDF service is not possible, the pilot will be told the reason.

The accuracy of VDF bearing information is classified as follows. The class of any bearing passed to a pilot will normally be included in the initial transmission of the bearing from the VDF station.

The accuracy of a Class Bravo bearing is +/- 5°.

An emergency

VDF service is provided in the

United Kingdom on the international urgency frequency of 121.5 MHz.

FACTORS AFFECTING ACCURACY.

The accuracy of VDF equipment may be affected by the following phenomena.

Site Error.

Site error may be caused by transmissions being reflected from nearby high ground, or from buildings or vehicles at the site.

Propagation Error.

Propagation error is error in the diffusion of radio waves over different types of terrain, especially when the distance between the aircraft and the VDF station is great.

Aircraft Attitude.

As the VDF system and VHF communications are vertically polarised, the attitude of the aircraft can affect accuracy. The best reception and results will be obtained if the aircraft is flying straight and level.

Direct and Ground Reflected Waves.

The reception of both direct waves and ground reflected waves can cause signal fading although this phenomenon is usually short lived. Multi-path signals can also give rise to bearing errors.

Range.

VDF operates in the VHF radio wave band, and so its range is limited by the line-of- sight principle.

Intervening high ground, between an aircraft and the VDF transmitter, will limit range, especially for low flying aircraft in hilly terrain. The power of the airborne and ground transmitters also limits VDF range.

VHF DIRECTION FINDING IN AN EMERGENCY.

In the United Kingdom, a pilot who is lost or uncertain of his position may be able to obtain direct help from the ground station with which he is in contact, or he may seek assistance, or be advised to seek assistance, from a Distress and Diversion (D&D) Cell on the international aeronautical VHF distress frequency of 121.5 MHz.

There are two D&D Cells; one at Prestwick to serve the region North of Latitude 55° N, and one at West Drayton to serve the area South of 55° N. Although both D&D Cells are manned by Royal Air Force personnel, they provide a service to civil aircraft in an emergency, in addition to the service they provide to military aircraft. This includes the provision of a position fixing service.

The D&D Cells obtain information on the position of an aircraft in distress from VDF equipment, and are able to fix with good accuracy the position of aircraft transmitting on 121.5 MHz at altitudes of 3 000 feet, and above, over the United Kingdom land area and coastal waters. However, the ability to locate aircraft below 3 000 feet is poor and will probably be severely inhibited over the mountainous areas of Scotland, Wales and North-West England.

The D&D service is available around the clock to pilots flying within United Kingdom airspace.