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ADF Automatic Direction Finder

Nondirectional Radio Beacon
Touted as being the simplest radio aid
To be clear the NDB is on the ground, the ADF is in the aircraft
Also referred to as a radio compass back in the day
When associated with an instrument final approach fix it may be referred to as a outer compass locator or LOM
Relative Bearing:
Bearing measured clockwise from the nose of the aircraft
Magnetic Bearing:
Bearing measured clockwise from magnetic north
MH + RB = MB to the station

NDB Components
Frequency range
190-535 kHz
AM radios 550-1650 kHz
The AM radio is not approved for navigation
Identification is usually by a 3 letter system 2 if it’s a compass locator
Pelly is MW
Can have voice including AWOS
Coverage depends on the strength of the transmitter

ADF Components
2 antennas
The loop
Receives signal better from 2 directions
2 perpendicular windings on a ferrite core
The sense
Nondirectional – receives signals equally from all directions
The long wire running from just above the cockpit to the tail on the C-23
Newer installations have the loop and sense combined like on the Bonanza

The NDB Signal
It has 2 electromagnetic components
The E field, vertical and electric
The H field, horizontal and magnetic
These are perpendicular in space
The H field induces a voltage in the windings on the loop antenna
There is one point where the signal on one side cancels the signal on the other
Referred to as a null point
Only one of these nulls points to the NDB
The sense antenna receives the electric portion of the signal
The voltage from the sense is compared to the voltage of the loop
It then measures the phase difference between the 2 windings to determine direction
The pole that has the matching phase is the direction of the station

The NDB Signal
Back in the day, the nav had to turn the loop by hand to find the null point
Now the loop doesn’t have to be rotated at all using a special loop antenna called a goniometer
This also made it very compact like the ones on the Bonanza

ADF Modes
Direction finding is active
Shuts off the loop
Provides the clearest audio reception for ID
Beat Frequency Oscillator
Emits a steady tone to help ID the station when an unmodulated carrier wave is present (no Morse code)

ADF Card
Most are manual, some are slaved
Allows the pilot to determine MB easily by setting the heading at the top
Slaved systems are much easier to fly approaches with

NDB Service Volumes

NDB Class Codes

Thunderstorms may pull the needle off the bearing momentarily cause it to hunt
Night effect
NDB’s are easily refracted by the ionsphere and able to travel 1000’s of miles
Yes 1000’s, in fact there is a whole community of people called DX’ers that spend their spare time listening to these
Other NDB’s can overlap frequencies especially at night
Mountain effect
Due to reflection of the signal by mountainous terrain
Coastal effect
NDB signal is bent slightly toward coastlines when crossing at an angle

Tracking involves applying wind correction so that a straight line to or away is executed
The head of the needle will move into the wind with no wind correction
If the needle moves away from the wind, to much correction has been applied
If the needle moves into the wind not enough
If the needle is steady then you have the right amount

Homing involves keeping the needle straight up the entire time
This will require a change in heading over time to do so
The result is a curved path to the station

Bearing Intercepts
Steps are similar to VOR tracking
1. parallel the course (bearing)
2. using the head of the needle determine how far off 360 if inbound, 180 if outbound
3. double the difference
4. turn toward the head of the needle
5. wait until the needle deflects off the 360 or 180 cardinal the same amount as your correction
6. turn back to the original course (bearing)

NDB Rules
Remember the needle always falls to the tail while intercepting
The NDB is only as accurate as you can hold your heading
Make sure HI is properly set before and during an approach
Monitor the Morse code during usage
The needle may point momentarily towards a lightning strike
Sometimes called “poor man’s radar” but not reliable for this purpose

Quadrantal Error
The angular error in a measured bearing that is due to the presence of metal in the vicinity of the direction-finding antenna, such as the metal structure and engines of an airplane or the hull of a ship.
This shows up as an error of as much as 20 degrees on the 32R approach
Large airplanes have systems built in to account for this error

Operational Errors
1. Failure to keep the heading indicator set so that it agrees with the corrected magnetic compass reading. Initiating an ADF approach without verifying that the heading indicator agrees with the corrected compass indicator reading may cause the pilot to believe that he is on course but still impact the terrain (CFIT).
2. Improper tuning and station identification.
3. Positively identifying any malfunctions of the RMI slaving system or ignoring the warning flag.
4. Dependence on homing rather than proper tracking. This commonly results from sole reliance on the ADF indications rather than correlating them with heading indications.
5. Poor orientation due to failure to follow proper steps in orientation and tracking.
6. Careless interception angles, very likely to happen if you rush the initial orientation procedure.
7. Overshooting and undershooting predetermined MBs, often due to forgetting the course interception angles used.
8. Failure to maintain selected headings. Any heading change is accompanied by an ADF needle change. The instruments must be read in combination before any interpretation is made.
9. Failure to understand the limitations of the ADF and the factors that affect its use.
10. Overcontrolling track corrections close to the station (chasing the ADF needle) due to failure to understand or recognize station approach.

Mag bearing to?

Mag bearing to?
Mag bearing from?

Mag bearing to?
Mag bearing from?

Convert to MB to a TO
Find RB
Aircraft 7 heading 270 which adf indicates the 120 bearing from the station?
120+180=300 MB To
-270 -270
030 RB
Answer 5

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