Originally Posted by
whoswho
Pretty sure this is the way to do it - I'm sitting my Nav exam on Sunday - if I'm wrong....oh b****cks.
Sorry to pee on your fire, but yes, you are wrong. To me this shows two things:
- Using the wind-down method for problems like this one (at least at PPL level) means asking for more complication.
- Using any method without having first a clear picture of the problem in mind means potentially getting it wrong without even realising it.
Think of the three vectors and draw them on paper, before doing anything on the wind computer. If you are tracking 180° with a wind from 090°, your heading will be a few degrees to the left of track, and in this particular case, with track and wind at a right angle, you are always going to have a (small) headwind component: how can your TAS be less than your GS
It has to be slightly higher!
So, of the three vectors, here we have TR/GS and W/V, and need to find HDG/TAS. The way I'd do it is:
- Lay down the TR/GS vector on the wheel's centre line. i.e. set 180° on the index and 80 kt under the centre dot. That's it, one down.
- Next, the W/V vector will always blow us from the HDG/TAS towards the TR/GS vector, i.e. it will come from the left, from 90°, and point to the centre dot. To draw it, turn for a moment the wheel to get 90° under the index and mark 20 kt up from the centre dot. Put back the wheel to index 180°: two down.
- That's it, you have your triangle of velocities: the HDG/TAS vector goes from the base of the wheel to the wind mark, which tells you that your Wind Correction Angle is 14° to the left (i.e. HDG = 166°, which you can read off the drift scale) and your TAS is 83 kt.
whoswho, what you got wrong is that you stopped too early: at point 8, where your read 78 kt of TAS, your wind mark indicates 75 kt, while it should always stay at 80 kt, because that mark represents the arrowhead of the TR/GS vector (which is 80 kt-long of course). Basically you have to go back to step 5 and repeat the steps until the solution doesn't change (have I already said that the wind-up method is quicker and easier?
)
Hope that helps.
Deeday