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Question 51 in practice test (RPL/PPL Vol 2, p404)
Margetts1976 created the topic: Question 51 in practice test (RPL/PPL Vol 2, p404)
Hi,
I can't seem to get this basic question correct, I am using the Jeppesen wiz wheel to answer it.
I can get the heading calculation correct, however I calculate a head wind of 15kts making GS = 130kts.
I convert the TR(T) to magnetic, which gives 70 degrees which I use in the wiz wheel. The only way I can get a headwind of 22kts and the correct answer is if I use a track of 80 degrees, but that is T.
I've done these calcs heaps of times in the past, what I am I doing wrong?
Margetts1976 replied the topic: Question 51 in practice test (RPL/PPL Vol 2, p404)
I believed I have solved my own problem after consulting the Jeppesen manual. Something that I was never shown in flight school!! If the crab angle exceeds 10 degrees, for GS calcs you use the subscale to the right of the TAS. Rather embarrassing after 3 years of flying. I must be a fair wind flyer only lol.
John.Heddles replied the topic: Question 51 in practice test (RPL/PPL Vol 2, p404)
you use the subscale to the right of the TAS.
Perhaps you intended "to the left" ?
Either way you have fallen trap to a very strange idea which has been around for many years and probably dates back to the development of the CR by Lahr in the 1950s. Indeed, it is in the user guide and most pilot textbooks.
It appears that the idea was to make the G/S somehow be equivalent to the sum of the TAS and the wind component, which is only correct for nil drift. This idea is somewhere between stupid and very fanciful, particularly when you consider that the wind bit is considered correctly but the TAS bit incorrectly (exits stage left shaking head in confusion). The 10 degrees thing acknowledges the reality that the error starts to get a bit ridiculous once you get to 10-15 degrees of drift.
The mathematically correct technique is to run the calculations so that you figure the vector components for both wind and TAS along the track vector regardless of the drift angle. Now, that probably doesn't mean much to most pilot folk so you might like to have a run through a recent thread in which the story is explained.
