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7.6.6 Radio Navigation Aids

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NDB Navigation

We will take you through the uses of using a NDB and VOR. Even though you are unlikely to use the instrument for plotting position lines the need to understand this procedure will be of help to your General Navigation training. The process of homing and understanding the Jeppesen plate is essential with instrument flying and acquiring an Instrument Rating.

ADF Bearing

Procedure for obtaining an ADF bearing:

  • Determine the frequency, identification and modulation of the required beacon and ensure that your aircraft is within the published (promulgated) range.
  • Switch on the ADF and adjust volume.
  • Tune the frequency and identify the station using ANT and BFO as necessary.
  • Select ADF on the control panel and note the bearing on the indicator.

Line of Position (LOP) using the RBI

With the help of the information we get from our instruments, we are now able to determine the line of position along which our aircraft is positioned. To draw this LOP on the chart we need the QDR (Magnetic bearing from a station) or the QTE (True bearing from a station). Assume the aircraft is on a heading of 015ºM:

  • The relative bearing from the indicator is 340°.
  • The QDM (Magnetic heading to the station) is the relative bearing plus the heading, 340 + 015 = 355°
  • The QDR is the reciprocal, 175°

Line of Position (LOP) using the RMI

An RMI solves the bearing automatically. The RMI provides continuously QDMs and QDRs. Magnetic Bearings can only be used on charts that are oriented to Magnetic North. The beacons on most instrument charts have the direction of magnetic north attached with an arrow.

Assuming that the single pointer is the ADF:

  • The QDM is 017.
  • The QDR is 197.

Homing

The ADF needle always points towards the station, and the easiest way to reach the beacon is to constantly fly with the needle pointing to the top of the indicator. This procedure is known as homing.

The easiest way to home to a station is to turn the aircraft in the direction of the needle until the needle points to the top of the indicator. This points the nose of the aircraft directly towards the station.

Once aimed at the station, any crosswind component will displace the aircraft to either side of the straight track to the station and the ADF needle will swing away from the top of the indicator.

The pilot will then have to make a correction of the heading towards the needle in order to continue heading to the station.

This process will have to be repeated again and again since the crosswind pushes the aircraft away from the straight track. The resulting path to the station will thus be a curved one.

The crosswind component requires the aircraft to turn further and further into the wind in order to continue toward the station. The aircraft must turn until a point is eventually reached where the aircraft is headed directly into the wind. At that point, the aircraft will no longer drift off the direct track and is now heading straight to the station. The actual curved path that results will be different for each combination of crosswind and TAS.

A strong crosswind component and low TAS will result in a large deviation. A weak crosswind component and a high TAS will result in a small deviation. Since the actual track over the ground will vary with every wind and airspeed combination, there is no way to ensure that any given aircraft will stay within the boundaries of an airway or approach path when homing. Homing is a very simple but extremely inefficient procedure, because of the uncertain demands on airspace, it is not commonly used.

VOR Navigation

This chapter will take you through the uses of VOR. Three indicators of the VOR will be discussed:

  • Radio Magnetic Indicator (RMI)
  • Omni-Bearing Selector (OBS)
  • Horizontal Situation Indicator (HSI)

Radio Magnetic Indicator

The RMI, combines the information from the radio navigation instruments with the directional information from the directional gyro. The RMI has two needles, which can indicate both ADF and VOR information.

The two needles are usually marked with single and double lines to make it easier for the pilot to identify the stations.

There are two small buttons on the bottom of the instrument. These enable the pilot to select either VOR or ADF to be displayed by the needles. The indicator needles indicate exactly the same information whether determined from the VOR or the ADF and they will constantly point towards the tuned station. The RMI card is slaved to the directional gyro, so that the heading of the aircraft can be read directly off the lubber line at the top. In this way, the needles will show the bearing to the ground stations continuously:

  • The tip of the needles will indicate a magnetic bearing to the ground station QDM
  • The tail indicates magnetic bearing from the ground station to the aircraft QDR

When tuned to a VOR, the tail of the needle indicates the VOR radial. In our example, the single needle points to a VOR station, indicating that the aircraft is on radial 195º. The needle marked with a double line indicates QDM 314º (QDR 134º).

You should be aware that the bearing registered from the ADF is a magnetic bearing against the magnetic meridian passing through the aeroplane. If there is a significant variation change or meridian convergence between the station and the aeroplane the bearing indicated will not be the same as a QDM.

If you fly at high latitudes, find a station with both a VOR and an NDB where there is a marked difference of variation and longitude between the station and your aeroplane. Select both transmitters and feed one to the single needle and the other to the twin needle of the RMI. Will they have the same bearing indications?

Omni-Bearing Selector

The indicator shown below has three components:

  • The Omni Bearing Selector
  • A TO/FROM Indicator
  • A LEFT/RIGHT Course Deviation Indicator

When using this instrument the pilot has certain selections that he can make:

  • OBS Selector: The control knob selects the desired magnetic track that a pilot wishes to fly TO or FROM a VOR beacon. In the case above the pilot has selected a track of 100°M.
  • TO/FROM Indicator: When the required magnetic track has been selected the TO or FROM arrow will appear showing where the aircraft is relative to the beacon. In the case above the TO arrow is showing. The indication will change as the aircraft passes through the beacon.
  • Course Deviation Indicator: The indicator has 4 small dots and one large central dot. Each of these dots represent 2°, with a full-scale deflection of the needle being 10°. The vertical bar moves left or right according to the relative position of the aircraft to the magnetic track selected. With the vertical bar central the aircraft is on the magnetic track selected. In the instrument above the bar shows 3½ dots left. This means that the aircraft has a deviation of 7° from the selected course. To get back to track the aircraft should always be flown towards the needle, in this case left.
  • Warning Flag: A warning flag will appear when:
    • There is a failure of the aircraft’s receiving equipment.
    • There is a failure of the ground station.
    • There is a failure of the indicator.
    • Signals received are too weak or the aircraft is out of range of the beacon.

The indications are totally independent of aircraft heading. The instrument shows the aircraft position in relation to the course selected.

  • TO Flag: If the TO flag is showing the number shown by the arrow at the bottom of the instrument (if the vertical bar is central) is the radial of the aircraft.
  • FROM Flag: If the FROM flag is showing the number selected at the top of the instrument (if the vertical bar is central) is the radial the of the aircraft.

Using The OBS

The diagram below shows three aircraft at different positions, each is discussed in turn (Note that the angles shown are exaggerated):

The OBS is set to 100° as a magnetic track.

Aircraft 1

The aircraft in position 1 would have the indications shown above. The aircraft is flying TO the beacon and must fly left to regain the track. Note that when centralising the needle the aircraft will home to the beacon. When airways flying an aircraft must regain track as quickly as possible.

With TO showing, if the aircraft is on track (vertical bar central) it will be on the 280° radial. The lubber line shows this at the bottom of the OBS indicator.

We know that the vertical bar shows a fly left situation of 3½ dots, which is the deviation from track selected. 3½ dots are equal to 7°.

If we take the radial minus the deviation because the aircraft is right of track we can work out the radial of the aircraft (280 –7) = 273°

With TO in the window the deviation shown by the vertical bar should be:

  • ADDED to the OBS radial if the aircraft is left of track.
  • SUBTRACTED if the aircraft is right of track.

Aircraft 2

Where an aircraft is within the area of ambiguity, 10° either side of the perpendicular cutting track, no positive indications will be given.

Aircraft 3

The aircraft in position 3 would have the indications shown above. The aircraft is flying FROM the beacon and must fly left to regain the track

With FROM showing, if the aircraft is on track (vertical bar central) it will be on the 100° radial. The top of the OBS indicator shows the radial.

We know that the vertical bar shows a fly left situation of 3½ dots, which is the deviation from track selected. 3½ dots are equal to 7°.

If we take the radial plus the deviation because the aircraft is right of track we can work out the radial of the aircraft (100 + 7) = 107°

With FROM in the window the deviation shown by the vertical bar should be:

  • ADDED to the OBS radial if the aircraft is right of track.
  • SUBTRACTED if the aircraft is left of track.

Using VOR Navigation

The VOR is a very versatile navigational aid, which forms the basis of the Airway routes structure. It can be used to assist VFR pilots, as the main navigational aid for en-route navigation, a holding aid or also as an approach to landing aid.

Let us have a look at the different ways of using the VOR, but first we point out a few important things that we have to do before using the information indicated by the instruments:

  • Always make sure you are within the coverage area of the VOR stations you plan to use. Checking the official AIP (Aeronautical information publication) or other published en-route manuals can do this.
  • After having turned the receiver on, dial the frequency of the aid, then listen to the identification signal to make sure you are receiving the correct and desired station and that it is “on the air”.
  • Make sure that the warning flag (NAV or OFF) is not visible, indicating that a satisfactory signal is being received and that the aircraft installation is working properly.

Establishing position

Using the VOR to find our present position, we need either a VOR in combination with DME, or we can use two VOR stations. By turning the OBS to centre the needle with a FROM indication, we determine the radials on which the aircraft is located. This procedure gives us two crossing position lines, good enough to determine a fix position.

Tracking a radial inbound from a present position

If we have to fly towards a VOR station from our present position, all we have to do is to turn the OBS to centre the CDI needle with a TO indication and fly a heading equal to the indicated value in the selected course window. The inbound track will be the reciprocal of the radial on which you are positioned. In a no wind condition, a heading equal to the inbound track will take you to the VOR with the CDI needle centred.

If there is any crosswind, heading corrections have to be made in order to keep the CDI needle centred. Initially make a small heading correction, if the needle drifts to one side, turn towards the needle, since the needle actually indicates the position of the desired track. Use only small changes in heading at any one time and wait for the needle to move back to centre position. This procedure of changing the heading to stabilise the needle in centre is called “bracketing”. Use small changes of heading and keep the new heading for a while to await needle movement. If we see that the needle remains still at a position off centre, we have found the correct WCA, but a correction will still be needed in order to regain track.