The Non-Directional Beacon (NDB) is a radio transmitter in a fixed location on the ground.
The Automatic Direction Finding (ADF) equipment in the aircraft will point a needle towards the NDB.
Cockpit Displays
There are three main types of ADF cockpit displays:
Fixed Card Relative Bearing Indicator (Fixed Card RBI) The ADF needle is overlaid on a fixed compass card that cannot move. The needle will indicate the relative bearing of the NDB – i.e. the position of the beacon relative to the nose of the aircraft.
Moving Card RBI The ADF needle is overlaid on a compass card that can be moved by the pilot. This allows the pilot to set the heading of the aircraft and the needle will show the magnetic direction to the beacon (QDM) and the magnetic direction from the beacon (QDR).
The head of the needle will show the QDM The tail of the needle will show the QDR
Radio Magnetic Indicator (RMI) The RMI overlays the ADF needle on a heading indicator. This heading indicator rotates as the aircraft turns so no manual resetting from the pilot is needed to read off a QDM or QDR.
Using the ADF
Once the correct NDB frequency has been entered into the ADF equipment, the morse code identifier should be confirmed. Each NDB transmits its unique identifier in morse code on its radio frequency so the pilot can confirm the correct frequency is entered and the radio signals are being received. The pilot can then either home or track to the beacon.
Homing to the NDB To home to the NDB, point the nose directly at the NDB by keeping the head of the ADF needle in the 12 o’clock position.
This is not the preferred method as it does not account for wind drift and you will not fly a straight line to the beacon.
Tracking to the NDB To track to an NDB, you need to fly a heading that compensates for any drift you are experiencing from the wind.
This is the preferred method as you will fly a straight line to the beacon.
The closer you get to the beacon, the more sensitive the ADF needle will become.
As you pass overhead the beacon, the needle will ‘drop’. It will move from roughly the 12 o’clock position to the 6 o’clock position. If you fly directly overhead the NDB, the needle will drop quickly. If you fly to one side of the NDB, the needle will be slower to drop.
ADF Limitations
Thunderstorms The ADF suffers from interference when in the vicinity of thunderstorms and the needle will often point towards a storm rather than the NDB
Terrain Terrain can reflect NDB signals, causing incorrect indications
Night Effects During dusk and dawn, the changing characteristics of the ionosphere can reflect NDB signals and cause interference from NDBs that are a long distance from your aircraft
Limited Range The low power of most NDBs limits their range to about 15-30nm
Coastal Refraction When an NDB signal crosses a coastline, the signal is often refracted (‘bent’) causing inaccuracies
No Failure Flag Typical ADF instrumentation in the cockpit does not have any failure indication, increasing the chances of accidentally using an unserviceable beacon
NDB Frequencies & Details
Maintenance & Testing Check NOTAMs to make sure an NDB hasn’t been notified as unserviceable. If maintenance is being carried out, the morse code ident ‘TST’ may be transmitted.
Designated Operational Coverage (DOC) The usable range (known as the Designated Operational Coverage, or DOC) of an NDB is stated in the AIP. Some airports have particularly low powered NDBs, known as Locators. Locators typically have a DOC of only 10nm
Locations, Frequencies & Idents NDB locations, frequencies, and identifiers (the 1 – 3 letter unique code) are marked on aeronautical charts, and further information about each one can be found in the AIP.
This NDB has the unique identifier MP and its frequency is 373
