C16 Automatic Direction Finding (ADF)

Question 1

What type of beacon is used for an Automatic Direction Finder (ADF)?

A VOR, DME or NDB

Answer 1

NDB (Non directional Beacon)

Question 2

Why would an NDB be used over a VOR?

Answer 2

VORs use VHF which are limited to the range of sight principle.

NDBs can follow the curvature of the earth so aircraft tracking NDBs can fly lower and can be tracked from a much greater distance than VORs

Question 3

Are NDB frequencies in kHz or mHz?

Answer 3

kHz

Question 4

What is the cone of silence and what is the relationship between the size of the cone of silence and how high you are above the ground?

Answer 4

The cone of silence is directly above the NDB, where no signal is received by the aircraft.

The higher you are, the greater the size of the cone of silence.

Question 5

What is the difference between an RBI (Relative Bearing Indicator) and an RBS (Relative Bearing Selector)

Answer 5

The RBI is used when the compass card of the ADF is fixed, with the nose of the aircraft always pointing to 000 degrees at the 12 o’clock position. The RBI will tell you how many degrees to steer to reach the NDB

The RBS is used when the compass card of the ADF is not fixed and can be turned manually using a knob. The RBS will tell you exactly the heading you need to reach the NDB

Question 6

With an RBI, the ADF needle will indicate a relative bearing to the NDB, that is relative to the nose of the aircraft (000 degrees).

This means that the relative bearings indicated by the ADF needle are not the same as compass headings.

So, if you are heading 270 degrees, and your ADF shows you to turn 150 degrees left (so the RBI needle is pointing to 210 degrees on your ADF) then your new heading is 110 degrees to track towards the NDB.

Answer 6

Understood

Question 7

What do the following mean on your RBI reading?

000

090

180

270

Answer 7

000 = NDB is directly ahead

090 = NDB is 90 degrees to the right

180 = NDB is directly behind

270 = NDB is 90 degrees to the left

Question 8

You are flying on a heading of 150 degrees and you RBI indicates a relative bearing of 330 degrees to the NDB. What is your QDM? (Magnetic heading to the station)

Answer 8

RBI is telling you to turn 30 degrees left (360-330).

So the QDM is 120 degrees (M) to fly directly towards the NDB

Question 9

Explain how the method of tracking an NDB is different between an RBI and RBS

Answer 9

With the RBI:

1. The ADF compass card always indicates 000 with respect to the aircraft nose and is fixed
2. The RBI shows you how many degrees you need to turn to reach the NDB

With the RBS:

1. The ADF compass card is moveable, so you rotate the card so that the red arrow is aligned with current aircraft heading
2. The RBS shows you the exact heading you need to reach the NDB

Question 10

You are on a magnetic heading of 340 degrees. Your NDB is 135 degrees to your right. Explain how you would track the NDB using an RBI vs an RBS

Answer 10

Using the RBI:

1. Compass card of ADF reads 000 degrees with respect to aircraft nose and does not change
2. The RBI will have an arrow pointing to 135 degrees, telling you to turn 135 degrees to the right
3. As a result, to track the NDB your new heading is 115 (M)

Using the RBS:

1. Compass card of ADF is set so that the red marker is aligned with your current heading 340 (M)
2. The RBS shows exactly the heading you need to fly to the NDB. In this case, the RBS will have an arrow pointing towards 115 degrees (M)

Question 11

What is the Radio Magnetic Indicator (RMI)

Answer 11

A type of ADF that allows the compass card of the ADF to move with the aircraft and to display a heading to the NDB.

With an RMI, the heading indicated by the arrow is pointing towards the NDB

With RMIs can can fly TO and FROM an NDB

Question 12

What is the Twin Needle Radio Magnetic Indicator (RMI) for?

Answer 12

It does what a regular RMI does, but can also be used to track a VOR at the same time.

So a twin needle RMI can track 2 stations (an NDB and a VOR) at the same time!

Question 13

What is the best guaranteed accuracy of an NDB?

Answer 13

+/- 5 degrees

Question 14

What is the difference between an RBI and RMI?

Answer 14

The RBI has a compass rose which is fixed in the ADF instrument, with 000 degrees at the 12 o’clock position.

The RMI always shows the aircraft’s current magnetic heading.

The RMI will show a QDM to the NDB, whereas the RBI will show you a relative bearing to the NDB

In both the RBI and RMI, the ADF needle points towards the NDB

Question 15

An aircraft is heading 085 (M) as indicated by the DI. The ADF’s RBI indicates a relative bearing of 350 degrees to an NDB to which the pilot has tuned his ADF receiver. if the local magnetic variation is 4 degrees West, what are the QDM and QTE respectively?

Answer 15

1. QDM = Magnetic heading + relative bearing

So QDM is 085 - 10 = 075 degrees (M)

2. QTE = QDM + 180 +/- magnetic variation

As the QTE is a True heading relative from the station:

QTE without magnetic variation = 255 degrees

Going from True to Magnetic: West is Best, East is Least

But since we’re going Magnetic to True: the opposite applies

So the QTE = 251 degrees (T)

QDM = 075 (M)
QTE = 251 (T)

Question 16

What are the 5 components needed for ADF?

Answer 16

1. Loop aerial
2. Sense aerial
3. Control Unit
4. ADF receiver
5. Instrument display