ADF Navigation

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


Otto may be driven by

  • –Electrical
  • –Hydraulic
  • –Digital
  • –Vacuum
  • Otto may be
  • –Single axis (roll)
  • –2 axis (roll & pitch)
  • –3 axis (roll, pitch & yaw)
  • Most GA autopilots are 2 axis

Autopilot Servo


  • Most autopilot’s function two ways
  • –Position based
  • –Rate based
  • –Combination of the two
  • Position based
  • –An attitude indicator senses how far off level
  • Rate based
  • –Uses the turn and bank sensor to determine rate


  • Otto will control (if equipped)
  • –Heading
  • –Altitude
  • –Vertical speed for climbs and descents
  • –Approaches
  • Otto will lower your workload until he goes on strike then it’s circuit breaker pullin’ time
  • –Make sure you know where Otto’s cb is so you can find it in the dark
  • –Sometimes Otto gets scared in turbulence and will disengage
  • Throttles are your responsibility unless equipped with autothrottle
  • Otto can cause laziness, lack of flying skills, loss of situational awareness, communication problems with the other pilot, loss of bladder control, kidney malfunction, swelling of the hands and feet, lack of appetite and death. Only use Otto after consulting with your CFI, if symptoms worsen contact your CFI right away.

Flight Management System (FMS)

  • If Otto is the pilot, FMS is the navigator
  • It is a central computer that uses current position and waypoint database to navigate
  • –Independent of the autopilot
  • The Control Display Unit (CDU) is the pilot interface with the FMS system
  • –This is what is used to create a flight plan, load approaches, SIDs and Stars
  • Nav systems such as GPS, VOR, ILS, IRS and DME all feed data to the FMS

Distance Measuring Equipment
Developed in the 50’s in Australia
Before digital displays, analog gauges were used

DME Specs
Can have an interlock with ILS
When the ILS shuts down DME shuts down
Battery backup of 4 hours
Coverage volumes are the same as for VOR
1000 watts for the H class
100 watts for the L class
However they may reach out up to 199 NM
Line of sight limitations still apply
Accuracy limits are +or- 1200 feet
Better than ½ mile or 3% of the distance
DME can handle 200 interrogators at a time
If there are more than this, the DME automatically desensitizes the receiver to reply to the stronger signals
DME is required at and above FL240 if using VOR for Nav (91.205)

DME’s Are Co-located
DME may be co-located with VOR, LOC, ILS, and NDB’s
The exception may be military where they may not be co-located
If with an NDB, a VOR frequency will be published

Tuning is through the VOR or ILS frequency
Each DME frequency is paired to a specific VOR or ILS frequency
The radio is automatically set to the appropriate DME frequency
There are 200 such pairings
UHF frequency range 960-1215 MHz or L band (AIM reference)
Your transponder operates in the UHF band as well
The antenna therefore will be of approximately the same size
Look for the shark fin under the plane

How does it work?
When tuned, the aircraft transmits an interrogation signal to the DME facility
There is a 50 microsecond delay to eliminate uncoordinated operation when close to the station (see pic)
These are in form of a paired pulse RF signal
The ground receiver then sends a transponder signal back
The DME on board measures the time it takes for the signal to return
At a 186,000 fps it doesn’t take very long
In fact it’s about 12.359 microseconds per NM
After that it’s a rate x time = distance

How Does It Work?
So how does it tell your DME from others?
Your DME randomly jitters the spacing of its interrogations
This makes you unique
It looks for its signature in the replies that are being sent out by the ground station
When it finds a match it uses that data to do the calculation

DME Identification
The Morse code for the station in question is the same for the DME
The difference is it occurs every 30 seconds
It is also a higher pitch at 1350Hz
VOR is about every 10-12 seconds and is lower pitch at 1020Hz
So why bother with the ident on a DME if its paired to the VOR?
At high altitudes you may have the DME from a different station on the same freq
It happens
It is possible to have the VOR go down and not the DME

DME Identification
So how do you know if your ILS/LOC has DME?
DME may be used in lieu of the OM if approved

DME Errors
Groundspeed and time readouts are only accurate when going to or from the station
When doing an arc, it only measures your lateral movement on the arc
All distances are slant range
So if you cross the station at 6,000 feet your distance will be 1 NM
Negligible if 1 mile or more for each 1000 feet above the elevation of the station

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