Calculating Holding Pattern Joins & Outbound Legs
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Calculating Holding Pattern Joins & Outbound Legs
As I'm a relatively inexperienced IMC Rating holder, when I'm intending to do an instrument approach, I practice it first on FS2000. This also has the benefit that I can afterwards visualise the join and patterns I have actually flown. What I am finding is that the instructions given in text books and on my course don't seem to give the best methods, and I'd appreciate any informed comment or references.
There are 2 areas I'm concerned about:
1) Teardrop (Sector 2) joins. All the advice is to apply single Wind Correction Angle on the teardrop leg. However, I find that applying double WCA works better: I would put this down to allowing for drift during the teardrop turn as well as the straight leg.
2) Outbound leg calculation. There is a variety of methods given for this:
(Thom) 3 x WCA to a max of 30 degrees, unless this gives a direction within 30 degrees of wind, in which case use 2 x WCA. Add 1 second per knot of headwind.
(IMC Class) 3 x WCA unless this gives a direction within 30 degrees of wind, in which case use 2.5 x WCA. If this results in a direction within 45 degrees of wind, use 2 x WCA. Add 1 second per knot of headwind.
(US Web sites) 2 x WCA
(One US web site) 3 x WCA (claims this is now the accepted standard)
(VATSIM - http://home.t-online.de/home/n.vorst...dings.html#Fly) 3 x WCA. For a C172, add 2 seconds per knot of headwind (the usual 1 second/knot figure applies to a/c flying at 200 kts)
According to FS2000, the 3 x WCA rule seems to work best, but I'm concerned that this is too simple and under some circumstances could give bad results (e.g. if WCA >30 degrees, although this is probably academic)
There are 2 areas I'm concerned about:
1) Teardrop (Sector 2) joins. All the advice is to apply single Wind Correction Angle on the teardrop leg. However, I find that applying double WCA works better: I would put this down to allowing for drift during the teardrop turn as well as the straight leg.
2) Outbound leg calculation. There is a variety of methods given for this:
(Thom) 3 x WCA to a max of 30 degrees, unless this gives a direction within 30 degrees of wind, in which case use 2 x WCA. Add 1 second per knot of headwind.
(IMC Class) 3 x WCA unless this gives a direction within 30 degrees of wind, in which case use 2.5 x WCA. If this results in a direction within 45 degrees of wind, use 2 x WCA. Add 1 second per knot of headwind.
(US Web sites) 2 x WCA
(One US web site) 3 x WCA (claims this is now the accepted standard)
(VATSIM - http://home.t-online.de/home/n.vorst...dings.html#Fly) 3 x WCA. For a C172, add 2 seconds per knot of headwind (the usual 1 second/knot figure applies to a/c flying at 200 kts)
According to FS2000, the 3 x WCA rule seems to work best, but I'm concerned that this is too simple and under some circumstances could give bad results (e.g. if WCA >30 degrees, although this is probably academic)
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As far as timing is concerned, beware. The US hold differs from the European hold. In the US they try to achieve 1 minute on the inbound leg. The European way is to achieve 3 minutes from abeam the fix outbound. This will also affect drift calculations due to the differences in outbound timing. However, in the UK the 3 minutes is not some kind of holy grail and all examiners are looking for is a sensible correction outbound. At the end of the day though it's all pretty empirical and, in the air, it rarely works out as per the master plan due to the differences in actual wind and the accuracy of your flying. The most important bit really is working out what to do to sort it all out as you come to the end of the inbound turn.
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Can only comment on the method taught in my IR:
3 x WCA unless wind is within 30 degs of hold axis, in which case 2 x WCA. Outbound leg extended/decreased 1 second per knot of head/tail wind respectively.
Has always worked for me in European style holds. Stan Evil's comments hold good for the IR, initial and renewals.
3 x WCA unless wind is within 30 degs of hold axis, in which case 2 x WCA. Outbound leg extended/decreased 1 second per knot of head/tail wind respectively.
Has always worked for me in European style holds. Stan Evil's comments hold good for the IR, initial and renewals.
FC
Your insight in item 1) demonstrates that you already know more about item 2) than most of the people who make up the rules of thumb!
In the limit of small angles, 3xWCA is clearly correct. You have one minute to correct for the drift that you will experience over three minutes (outbound + two turns). But as the angle increases, nonlinearities start to creep in.
Provided you remember to increase the outbound timing appropriately, 3xWCA should work for WCA up to 20 degrees or more, but in the real world, strong winds tend to be unpredictable winds, and the error margins increase rapidly.
As for timing, I think Stan Evil makes an excellent point about the Europe vs US difference, but I don't think the "3 minutes from abeam the fix outbound" has much basis in regulation. PANS-OPS calls only for the outbound leg to be "corrected for wind". Taken simplistically, I would say this means adjust the distance flown to be the same as in nil wind.
Your insight in item 1) demonstrates that you already know more about item 2) than most of the people who make up the rules of thumb!
In the limit of small angles, 3xWCA is clearly correct. You have one minute to correct for the drift that you will experience over three minutes (outbound + two turns). But as the angle increases, nonlinearities start to creep in.
Provided you remember to increase the outbound timing appropriately, 3xWCA should work for WCA up to 20 degrees or more, but in the real world, strong winds tend to be unpredictable winds, and the error margins increase rapidly.
As for timing, I think Stan Evil makes an excellent point about the Europe vs US difference, but I don't think the "3 minutes from abeam the fix outbound" has much basis in regulation. PANS-OPS calls only for the outbound leg to be "corrected for wind". Taken simplistically, I would say this means adjust the distance flown to be the same as in nil wind.
