Pilot Courses of Instruction

Navigation

Automatic Direction Finder

Introduction

Something of a lost art these days, twenty years ago using the automatic direction finder [ADF] was one of the hallmarks of the instrument pilot. Of the three different approaches required for the instrument flight test, one was the non-directional beacon [NDB] approach. Today, with GPS and glass panels, the ADF is scarcely used; but it would not hurt to set the GPS to ADF mode and practice just in case one finds oneself someplace where NDBs are the rule (there still are a few).

Tracking To a Station

Orienting

• Tune in and identify

  1. Obtain the correct frequency
  2. Tune-in ADF receiver
  3. Listen to morse code or other station ID to confirm correct station
     • It may be necessary to set the receiver to "BFO" or "receive" in order to identify the station

• To fly directly to the station

  1. Insure ADF function switch is set to "ADF"
  2. Calculate relative bearing (i.e., degrees the needle is offset from straight ahead) to the station
  3. Turn towards the station approximate number of degrees of relative bearing; adjust for relative bearing of 0 degrees
  4. Record heading on relative bearing of 0 degrees: this is the bearing to the station [QDM]

• To intercept a bearing to the station

  • When the location of the aircraft with respect to the bearing is known

    This is the procedure used when on a heading toward intercepting a bearing, such as on a radar vector to an airway or approach course or for coming out of an instrument approach procedure turn. A radar vector to an airway could be at any intercept angle, while a vector to a final approach course should be within 30 degrees, and a procedure turn would normally be about 45 degrees.

    1. Insure ADF function switch is set to "ADF"
    2. Calculate relative bearing (i.e., degrees the needle is offset from straight ahead) to the station
       • If the needle is to the right of the nose, the bearing is on the left: when on the airway, the needle will be pointing the number of degrees of the intercept angle to the right of the nose (i.e., an angle between 0 and 90 degrees).
       • If the needle is to the left of the nose, the bearing is on the right: when on the airway, the needle will be pointing the number of degrees of the intercept angle to the left of the nose (i.e., an angle between 270 and 360 degrees).
       • The intercept angle will be the difference between the aircraft heading and the relative bearing
         • if the bearing is 350 deg. and the heading is 020 deg., the intercept angle is [020 + 360] minus 350, 380 minus 350, or 30 degrees (remember, 380 degrees is identical to 020 degrees, so adjust as required to get a positive number).
         • The needle will be pointing in the direction of the turn.
    3. Turn inbound toward the beacon the number of degrees of the intercept angle

  • When the location of the aircraft with respect to the bearing is not known

    This is a somewhat odd procedure that actually involves maneuvering the aircraft, but there is not a suitable alternative to "do in one's head"; and has anybody actually had to do this lately? Alaska or Canada maybe, or maybe the Caribbean?

    1. Insure ADF function switch is set to "ADF"
    2. Calculate the aircraft's position relative to the relative bearing as follows:
       1. Steer parallel to the bearing to be intercepted
          • Remember that bearings are "to" the beacon, unlike VOR radials, which are "from" the station
          • Generally, parallel the bearing on the bearing heading, more-or-less towards the beacon (i.e., if the bearing were 150 degrees then you should turn to 150 degrees)
          • If circumstances require (e.g., if the bearing intercept is happening very close to the beacon or if you are not sure of your location with respect to the beacon), you might have to parallel the bearing on the reciprocal heading, more or less away from the beacon (i.e., if the bearing were 150 degrees then you would turn to 330 degrees)
       2. Note the direction of deflection of the needle: that is the direction of the bearing
          • momentarily hold the heading to determine the needle's movement
            • When flying parallel to the bearing on the bearing heading, make sure the needle is moving towards the wing: if it is moving towards the tail you are already past the beacon!
            • When flying parallel to the bearing on the reciprocal heading, make sure the needle is moving towards the tail: if it is moving towards the wing you have not yet reached the beacon!
       3. Turn 90 degrees toward the beacon (i.e., the bearing)
       4. The beacon is directly abeam when it moves to 90� either side of display: turn inbound (or outbound, as the case may be) the number of degrees of relative bearing (90 degrees) and immediately begin to correct for drift (begin the turn about five degrees prior to reaching the bearing heading, more if the needle is coming-in rapidly)

Tracking the bearing

1. Do not alter heading until some persistent wind drift becomes apparent.
2. If a persistent wind drift becomes apparent, take a 30 degree cut to the needle (i.e., into the wind)
3. Hold heading until aircraft recrosses original bearing
4. Take-out 1/2 of the correction (15 degrees): hold 15 degree wind correction angle (pointer of needle points 15 degrees away from the wind off nose of aircraft)
5. Adjust wind correction angle as necessary to to hold original bearing

Tracking From a Station

Orienting

• Tune in and identify

  1. Obtain the correct frequency
  2. Tune-in ADF receiver
  3. Listen to morse code or other station ID to confirm correct station

• To fly directly from the station

  1. Insure ADF function switch is set to "ADF"
  2. Calculate relative bearing (i.e., degrees the needle is offset from straight ahead) to the station
  3. Turn away from the station approximate number of degrees of relative bearing; adjust for relative bearing of 180 degrees
  4. Record heading on relative bearing of 180 degrees: this is the bearing from the station [QDR]

• To intercept a bearing away from the station

  This is basically the same as above.

  • When the location of the aircraft with respect to the bearing is known

    This is the procedure used when on a heading away from a station intercepting a bearing. As with above, this could be a radar vector to an airway. IAPs where the FAF is between the beacon and the airport are exceedingly rare, if any at all exist (generally the FAF is the beacon, or there is no FAF and the beacon is the MAP). Likewise, is is extremely unlikely to have the procedure turn between a beacon and an airport.

    1. If the intercept angle is not otherwise known, it is the angle between the aircraft heading and the bearing
    2. The aircraft is on the bearing when the needle is at the intercept angle by the degrees off the tail, left or right, on the display.
    3. Turn outbound to the bearing-from heading (begin the turn about five degrees prior to reaching the bearing heading, more if the needle is coming-in rapidly)

  • When the location of the aircraft with respect to the bearing is not known

    1. Insure ADF function switch is set to "ADF"
    2. Calculate the aircraft's position relative to the relative bearing as follows:
       1. Steer parallel to the bearing to be intercepted
          • Generally, parallel the bearing on the bearing heading, more or less away from the beacon (i.e., if the bearing were 150 degrees then you would turn to 150 degrees, paralleling the bearing away from the station)
          • It is difficult to imagine a scenario where it would be necessary to parallel on the reciprocal bearing heading
       2. Note the direction of deflection of the needle: that is the direction of the bearing
          • momentarily hold the heading to determine the needle's movement: flying parallel to the bearing on the bearing heading,
            • if the needle is moving towards the wing then you have not yet reached the beacon: use an intercept angle of 45 degrees
            • if the needle is moving towards the tail, then you are past the beacon: use an intercept angle of 90 degrees
    3. Turn toward the bearing
    4. The aircraft is on the bearing when
       • if using a 45 degree intercept, the needle is at 135 degrees (i.e., 45 degrees off the tail), left or right, on the display
       • if using a 90 degree intercept, the needle is at 90 degrees, left or right, on the display
    5. Turn outbound to the bearing-from heading (begin the turn about five degrees prior to reaching the bearing heading, more if the needle is coming-in rapidly)

Tracking the bearing

1. Do not alter heading until some persistent wind drift becomes apparent.
2. If a persistent wind drift becomes apparent, take a 30 degree cut to the needle (i.e., into the wind)
3. Hold heading until aircraft recrosses original bearing
4. Take-out 1/2 of the correction (15 degrees): hold 15 degree wind correction angle (tail of needle points 15 degrees away from the wind off nose of aircraft)
5. Adjust wind correction angle as necessary to to hold original bearing

NDB/ADF behaviour: action and errors

Non-Directional Beacon NDB navigation has several advantages that are often overlooked. The NDBs transmit in the frequency band of 200 to 415 kHz. Therefore, the signal is not transmitted in a line of sight as VHF or UHF, but rather follows the curvature of the earth; this permits reception at low altitudes over great distances (see NC2S Amateur Radio [nc2s.docsteve.com] for notes on propagation).

NDB Errors

Aircraft electrical interference

Alternator noise

Radio waves are emitted by the aircraft alternator in the frequency band of the ADF. An alternator suppressor is fitted to contain those emissions but this component does not have a long life and it is wise to test the ADF for correct operation during pre-flight checks.

• The tests are made by selecting a station (an NDB/compass locator or AM radio station), which must be some reasonable distance away, not so close so as to have a signal that overwhelms the receiver regardless of noise but not so far as to have an untrackable signal: about 10 to 30 miles should do, depending on the type of station:

  Nondirectional Radiobeacon (NDB) usable service range

  Class	Power in Watts	Distance in miles
  Compass locator	Under 25	15
  MH	Under 50	25
  H	50 to 1999	50(*)
  HH	2000 or more	75
  (*) Note: Service range of individual facilities may be less than 50 miles

  Broadcast stations transmit power ranges from several hundred to 50,000 watts: the strongest stations may be too strong to make a reliable test.

1. One test is to listen to a station prior to engine start and then again after engine start: any added noise comes from the engine.
2. The second test is to watch the ADF needle during the engine run up. If the needle moves as rpm increase there is electrical interference and probably the alternator suppressor should be replaced.

Other aircraft components

The other aircraft component most likely to generate noise in the NDB band are the magnetos. It may be difficult to distinguish alternator noise from magneto noise, but if replacing the alternator suppressor fails to solve the problem then the magnetos may need to be serviced or replaced.

Weather and other atmospheric affects

Weather

Thunderstorms emit electrical energy in the NDB band and will deflect the ADF needle towards the storm. There will also be static bursts if the volume is up.

Twilight and other night-time effect

Radio waves arriving at a receiver come both directly from the transmitter — the ground wave — and indirectly as a wave reflected from the ionosphere — the sky wave. The sky wave is affected by the daily changes in the ionosphere (see NC2S above). When the ground wave and a sky wave, or two sky waves, arrive at different times, there will be interaction between the two. The signal may rapidly increase or decrease or disappear altogether. Generally, this is referred-to as "fade," or QSB.

• This is frequently quite pronounced at twilight (morning or evening), and as a distinct event is called "gray line."
• Fading is unlikely to occur during the day as the sky wave is generally totally absorbed.

Terrain and coastal effects

In mountainous areas NDB signals may be reflected by the terrain, which can cause the bearing indications to fluctuate. Some NDBs located in conditions where mountain effect is troublesome transmit at the higher frequency of 1655 kHz (the top-end of the AM radio band). Ground waves are refracted when passing across coast lines at low angles and this will affect the indicated bearing for an aircraft tracking to seaward and following the shore line.

Aircraft attitude effects

The indicated bearing will not be accurate while the aircraft is banked as the antennas will be off-line. Thus, all turns should be conducted to specific headings, not bearings, and the ADF should be consulted only after the turn is completed.

NDB Glossary

QDM: Magnetic bearing to a beacon

QDR: Magnetic bearing from a beacon

QTE: True bearing from a beacon

QUJ: True bearing to a beacon

|