How to intercept a NDB radial for NDB approach

[737Andi 6]

Hi,

I was desperately looking for a good tutorial explaining the interception of an NDB radial to fly a NDB approach on a runway.
For example. I'm flying heading 360°. The NDB needle of my RMI is pointing 90° to the right at that moment. The runway is not aligned parallel to my flight course (360°). How do I find the correct interception angle flying to the NDB to continue my approach to the runway? I would like to do this with the instruments of an airbus a320 or 738.

Hopefully my explanation was understandable.

Thx!

Best regards Andi

[lidorx7 8]

Hi Andi

An NDB or Non-Directional Beacon is a ground-based, low frequency radio transmitter used as an instrument approach for airports and offshore platforms.
 
The NDB transmits an omni-directional signal that is received by the ADF or Automatic Direction Finder, a standard instrument onboard aircraft.  The pilot uses the ADF to determine the direction to the NDB relative to the aircraft.  To navigate using the ADF, the pilot enters the frequency of the NDB and the compass card (or arrow) on the ADF will indicate the heading to the station.  The signal is transmitted on an uninterrupted 24/7 basis.  An audible Morse Code call sign of one or more letters or numbers is used to identify the NDB being received. its basically just give you the direction to the NDB station, not to the runway.

VOR is different thing and can give you radial to the runway.

hope i explained it good..

 

[skelsey 765]

Hi Andi,

My next article about instrument flying was/is going to cover this.... I'll give you a summarised version here but I haven't got the pictures to hand which will make it much easier to understand!

First, to clear up some terminology:

QDM = the magnetic bearing of the NDB from the aircraft

QDR = The magnetic bearing of the aircraft from the NDB (the reciprocal of QDM).

The RMI is a lovely instrument because it always indicates the current QDM! Less well-equipped aircraft may be fitted with a fixed-card ADF (also known as a Relative Bearing Indicator, or RBI) which would require you to calculate the QDM/QDR based on the relative bearing indication, or if you are slightly posher a rotatable-card ADF (the 'poor man's RMI') may be installed which allows you to manually set the current heading at the top of the ADF instrument and thus get a direct reading of QDM.

In your example, you need to intercept QDM 360 to the NDB. This means that you want to be tracking to the NDB with the RMI needle indicating 360.

The first thing to to is to orientate yourself with respect to the NDB and the easiest way to do this is to visualise the aircraft on the 'tail' of the ADF needle. This should make the turns required to place yourself on an intercept heading apparent.

In your case if the ADF needle is pointing over the right wing then you are abeam the NDB  (visualise that aeroplane on the tail of the needle on heading 360). So in this instance the first thing you are going to need to do is head south and give yourself some space! 

Once you have a reasonable amount of space to intercept the inbound course, again visualise the aircraft on the trail of the needle. If we assume we are heading 180 at this point, we now need to pick an intercept angle - in a light aircraft a 90 degree intercept angle would be a reasonable start but at higher speeds you might want to select a slightly lower angle.

In our case, turning to heading 090 would be a good start. The RMI needle now will be pointing say at about 10 o'clock; as you continue on the intercept heading the needle will start to fall toward QDM 360. As the needle approaches QDM 360 you will need to start turning left toward heading 360 to establish - the amount of 'lead' on the turn will need to be judged depending on your speed and the rate at which the needle is falling. 

Once you are rolled out, you will need to adjust your tracking to ensure that you maintain the head of the ADF needle on 360. The way to do this is to remember to push the head, pull the tail.

If the needle starts to fall away to the right, then we need to adjust our heading to 'push' the head of the needle back round to 360 (like kicking a football) - in other words, if the needle has fallen to 005 then you would need to adjust your heading to, say, 010 or 015 to 'push' the head back around to 360. Again, as the needle approaches 360 you will need to adjust heading left again to maintain. Hopefully the corrections required will be smaller and smaller as you refine your tracking... obviously any wind will require the drift to be 'laid off'.

I would probably recommend trying this in a slower, smaller aircraft (and in still air) first before attempting it in a fast jet like 737 or A320 as everything will happen extremely quickly at 200+ kts!