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Confussion on NDB intercept calculations from the Text Book.

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P.Potter created the topic: Confussion on NDB intercept calculations from the Text Book.

Evening All,

A little help please. After Discussions with a few IREX instructors, all have a different answer to what is printed in the text book. I have looked up the amendments on the website and can't seem to find if there has been any corrections to the text book.

The problem is as follows:

Exercise 9 page 43.

Q1. Present Heading 175° M
Present ADF indic 190° M
FPT 200° M
Intercept Angle 30°

Find the Heading required to intercept
Find the ADF indication when the FPT is Intercepted.

BOB's Solution

H A T Then H A T
175° M +10° = 185° M Fr 230° M -30 Fr = 200° M

Heading to Intercept is 230° ADF Indication 150° R

The first HAT calculation is correct, it's the second that has me concerned.

Problem being is that the;

Heading should be HDG 175° + (15° + 30°) = 220° To intercept. (The 15° to bring Parallel the the 30° to intercept)

ADF Indication should be 220° - 20Fr = 200° which would be 160° R

Is there anyway this can be explained as i do not see why the drift hasn't been accounted for in the text book.

Thanks in advance.


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baddles replied the topic: Confussion on NDB intercept calculations from the Text Book.

My interpretation of the text on pages 41-43 is that drift is ignored, to simplify the calculation for teaching purposes.

There's not enough information to determine drift anyway, because Exercise 9 does not explicitly say that HDG and ADF were constant over a period of time. Rather, we are just given the HDG and ADF indications at some instant of time.

From the given HDG and ADF we can find the bearing to the beacon (that's the first H+A=T) and calculate what TRACK should be followed so as to meet the desired course at a 30 degree angle.
Assuming zero drift, the required heading is the same as the track.

Then the exercise asks what will be the ADF indication when we intercept the desired course. The ADF shows the angle between the bearing to the NDB and the aircraft heading. Again assuming zero drift, this is the same as the angle between the bearing to beacon and the calculated track, which is determined by the intercept angle.

I think this exercise is simplified because it is designed to help us grasp the overall idea of relative bearing and course intercepts.

A pilot could also do this simplified calculation in flight to determine roughly the heading to fly to intercept a course, ignoring crosswind.

Drift calculations would be a lot more complicated because the drift experienced on one heading is not the same as the drift on another heading (because the headings are so different). You would have to get out the trusty E6B and, given TAS and drift, calculate the wind speed and direction, then calculate WCA on the required track. In the cockpit.. :)

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  • John.Heddles
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John.Heddles replied the topic: Confussion on NDB intercept calculations from the Text Book.

First, a caveat. I don't know anything about HATs and the like. I was brought up on the ADF to orient, orient, orient ... and then do some more orienting. That was to keep focus on

(a) where am I ?

(b) (roughly) how far out from the aid am I ?

(c) where do I need to go (ie point initially to end up somewhere useful) ? This depends, quite strongly, on (b) and where it is that I might want to end up.

(d) what's the better direction to turn - left or right ?

(e) how far should I turn ?

(f) then how do I know when I am where I am trying to go, initially, to eventually get when I need to end up, finally ?

(g) what do I need to do to achieve whatever it is I am trying to do when I get to (f) ?

If you draw pictures, life gets ever so much easier when you are playing I/F. Doesn't matter whether you draw the pictures on a bit of paper or in your mind's eye as you are motoring along .. same, same, although the mind's eye makes for an easier life when pushing or pulling at the same time.

It helps, greatly, when asking this sort of question, if you just post a scan of the original so that you don't, inadvertently, misreport the story in the question. So, for instance, I suspect that your initial story intended the present ADF reading to be relative, not magnetic ?

Your discussion, apparently to do with a second HAT (?) makes very little sense to me, at all.

First, the question appears to be concerned principally with getting back onto the FPT so you can fix the present track error and continue on your planned, merry way. That's just a simple intercept consideration, made much easier as you are tracking generally away from the aid.

It is clear that Bob's preference is to use 30 degrees as a standard intercept angle. No problem with that, although, depending on where you are in terms of distance from the aid, that angle might not be very useful.

It doesn't matter what angle you use, so long as it is relevant and pertinent to the present situation - 20, 30, 45, 60, 90 degrees all have their uses from time to time.

When you have chosen whatever intercept angle looks to be particularly useful or convenient, drift doesn't come into the intercept problem, specifically, although consideration of the wind might well influence the particular intercept angle you select on the day. You just head off on the particular chosen heading and then, when you approach the desired inbound (or outbound) track, you lead onto the track and then set a best guess drift angle which you refine over the next few minutes without too much difficulty at all. Once you selected the heading to track to the intercept, then you have uniquely decided on what the intercept angle will show on the ADF when you get to the required track. The only point to finesse is to start your turn a little ahead of that track (lead) so that you end up on the track when you have completed the turn.

Once again, for the theory training bit, it is useful to emphasise one intercept angle at the start so that things aren't too complicated while you are getting your head around the stuff. However, once you get into the aircraft, you will (rapidly) find that one size certainly doesn't fit all situations.

However, first crawl, then walk, then run.

Engineering specialist in aircraft performance and weight control.

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