ADF Based Interception Problems
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
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.