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Wind Grid Vs the flight computer

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rodcarey created the topic: Wind Grid Vs the flight computer

Hi Guys, My question relates to Q6 on page 120 CPL performance, where there is a cross wind component for a PNR question. My understanding on cross winds is that if you are on the ground T/O or Landing to use the grid which will be close enough, but when planning aloft, to calculate the winds with a computer. This way magnetic variation can also be included in the details which was coincidentally included in a recent exam. Thanks in advance
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  • Posts: 1953
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bobtait replied the topic: Wind Grid Vs the flight computer

I recommend that you use the conversion table in ERSA-CON_7. That is CASA's own product for assessing head/tail wind components when the wind is at an angle to the intended track. It doesn't matter whether it's a runway or a flight planned track, the maths is the same. As for the magnetic variation, all you have to do is convert the given wind direction to magnetic in the first place, the answer is still valid as long as the flight planned track is also in degrees magnetic. I'm sure CASA would have to accept that approach.

The whole thing is a bit academic when you consider that the forecast wind is just some computer's guess in the first place, (if you can put any credit on the ridiculous WPWT table accompanying the GAF).
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rodcarey replied the topic: Wind Grid Vs the flight computer

Thanks Bob, without questioning your expertise on the subject, but to flesh out more detail, if i want to compare say 30 knots at 60 degree cross wind, the cross wind correction table has the HW component as 15, the flight computer (E6-B) has a reading of around 18 knots. Will that be enough to fail the exam question? I have calculated two scenario's for discussion below! I understand that CASA has provided some tolerance in answers for the questions, but having failed PNR on my KDR's twice in a row, i'm looking to split hairs to see where the issue may be. Thanks in advance and your consideration in the answer.

Example 500nm, Wind 30 knots at 60 degrees outbound, echoe 65%, isa conditions 5000ft ie TAS 189knts. 200 gallons usable fuel at take off. Distance from departure to PNR?
200-15(FR)/1.1 (VR) = 168.2 Gals. 168.2/28 (GPH)*60 = 360.4 Minutes (Safe endurance) GS speed outbound 174 (grid), 171 (FC), GS Home 204 (grid) 201 (FC)

Cross wind correction grid = 360.4 (SE)*204 (GS Home)/ 378 (2*TAS)= 194.28 Minutes. 194.28*174/60=563.41nm
Computer = 360.4 (SE) * 201 (GS Home) / 378 (2*TAS)= 191.64 minutes 191.64 * 171/60=546.17nm
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  • Posts: 1953
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bobtait replied the topic: Wind Grid Vs the flight computer

The grid in ERSA is not an approximation. The cosine of 60° is .5 (check on trig tables). Therefore, a 30kt wind at 60° to flight planned track produces a headwind component of 15kt.
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