ADF Based Interception Problems

CPL Assist
The process of intercepting tracks to and from an NDB is a simple task in the air, so long as a good instructor has given you a few pointers. Why then do students have so much difficulty doing interceptions on paper?

I believe the first difficulty stems from the use of terminology. Most books and some instructors talk of intercepting inbound and outbound tracks. Many instructors talk of intercepting QDMs and QDRs. This must be confusing to a new pilot. When I teach a student Comm. pilot, I differentiate between where the aircraft is at the start of the manoeuvre; which can be defined by its current QDM/QDR, and the end position of the manoeuvre, which will be on the track (inbound or outbound). Once the manoeuvre is complete, the magnetic bearing of the inbound track will be equal to the aircrafts QDM, but not until the aircraft is on-track.

I say; lets all intercept ’tracks’ and it will make good sense to the student.

Lets examine the type of interception question that you can expect in the exam:

You will always be given the Magnetic Heading and the current RBI reading. The heading might be hidden among terms such as true, drift and variation but it will be there. Remember that MHdg + RBI = QDM and that QDM +- 180 = QDR. With these you can sketch the position of the aircraft at the start of the manoeuvre. A line sketched from the beacon at the bearing of the QDR will take you to the aircraft’s current position. Mark it with a cross.

You will also be given the track to intercept (sometimes, unfortunately, as a QDM or QDR). This represents where the aircraft has to get to. An outbound track can be sketched from the beacon outwards. A trick with inbound tracks is to get the reciprocal and sketch the track from the beacon outwards (but don’t forget that it is an inbound track)

You will then be given an intercept angle. This is angle between your required track to get to the interception point (Intercept Heading), and the track that you must intercept (inbound or outbound). On the diagram you will see it linking those two tracks. It is the key to working out the ‘Intercept Heading’.

The question will ask for the ‘Intercept Heading’ and the ‘RBI at Intercept’. On the diagram you will see that the ‘Intercept Heading’ = the ‘Track to Intercept’ + or - the ‘Intercept Angle’ and the ‘RBI at Intercept’ = 0 + or – the ‘Intercept Angle’ for inbound tracks and 180 + or – the ‘Intercept Angle’ for outbound tracks.