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Old 1st Nov 2011, 00:11
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eninem
 
Join Date: Jun 2007
Location: Scotland
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Rough Formula

You ask for a mathematical method to get round the whizz wheel, but this involves working sin and cos values for angles in your head, which is not an easy thing to do if they are non standard angles. That is why the whizz wheel is so good for the exam but not necessary for real life. There is a simple way to do it in your head, but it will only be of limited help in your exam. The examiner no doubt knows that you can work the answer out approximately in your head and that is perhaps why the answers are so close, to force you to use the whizz wheel (and to do so properly.) Working it out in your head does at least help you to rule out the dross answers however. The formula to use is: Max drift = 60/TAS x wind velocity. From your example: TAS = 170 kt HDG (T) = 100° W/V = 350/30 kt Track (T) and GS? In your head you can work out your example by rounding TAS to 180 which gives - 60/180 x 30 = 1/3 x 30 = 10° max drift. Using the old rule of 60, your heading is 100° which gives a wind angle of 110° i.e. the wind is blowing from your left and slightly behind you. This gives you a tail wind. The rule of 60 says crosswind is max at 60° or more. Your crosswind is 70° so is at a max of 10°. Thus, your track will be 110°. If you deduct your crosswind component from 90° this gives you your head/tailwind component. 90° - 70° gives 20°. Using the rule of 60 (i.e. 20/60) this equates to 1/3 or one third of the wind speed which is 10 kts and we are dealing with a tail wind here. This gives a ground speed of 180kts. This all sounds complicated, but with practice it takes only ten or fifteen seconds to work out. The answer using this rough rule of 60 method is 110° at 180kts. However, this only narrows the answers down to two options. Unless you have a slide rule in your head or have memorised your sin/cos tables for every 5 to 10 degrees, you will have to turn to the whizz wheel. (You could use vector diagrams as an alternative...) Hope that is of some help.
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