Just a question regarding the exercises covering calculating TMG and Drift using the ADF (Page 5.20 of Nav CPL Book).

In the first example when FPT is given as 090 degrees M, HDG of 085 degrees M and ADF bearing of 176 degrees relative, what is confusing me is why when the heading correction of 4 degrees is made to the left the TMG becomes 081 degrees?

If you were flying with a heading of 085 to maintain a track of 090 and then apply the correction of 4 degrees to the left, your new heading would be 081 but wouldn't the new track be 086 degrees rather than 081? Thence a track error of 4 degrees rather than 9 degrees.

Would someone be able to explain to me why the TMG in the questions has been calculated from the HDG rather than the original specified FPT?

Thanks

Hi there,

The thing to remember about Drift is its definition.
Drift is the angular difference between the heading that is held and the TMG.

I like to start by thinking what should it look like if I was on track. In this particular question, the FPT was 090°M and HDG was 085°M. All being well, we should see the ADF indicating 185°R if we were on track.

Now I look at what we are actually tracking (TMG) from ADF indication given to us in the question. ADF - 176°R.

Using 'HATs', I get my TMG. 085°M + 176°R = 261°M to the station. I'm flying away from the station, so knock 180degrees off 261°M and my TMG is 081°M.

Now here's the important bit that had to teach myself because I used to struggle with this concept as well.

Now that I have the TMG of 081°M, I say to myself "STOP!! What is the definition of Drift, again? Drift is the angular difference between the heading that is held and the TMG.
In this case, HDGis 085°M and the TMG is 081°M and we have a difference of 4° which gives the answer of 4° drift. Do my little drawing like Bob's (or visualize it if I'm flying) and we find left wing forward. We are experiencing 4° Left drift!

Doing it this way has served me well all throughout my IFR flying and IFR teaching days.

In summarising.
- Revisit the definition of Drift often.
- Once you have found TMG, forget about FPT for the moment. It's now of little use to us when we are trying to answer these types of questions and can be distracting. You think about FPT again when you go to re-establish it via an intercept of some sort, then you can apply that drift you just worked out.

Hope this helps.

Cheers,

Mister W.