Pilot Courses of Instruction

Navigation

Steve Sconfienza, Ph.D.

Airline Transport Pilot

Flight Instructor: Airplane Single and Multiengine; Instrument Airplane

cell: 518.366.3957

e-mail: docsteve@localnet.com

Automatic Direction Finder

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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).

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Tracking To a Station

Orienting

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
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Tracking From a Station

Orienting

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
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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.

  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.

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.

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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