Quick Diversion Technique
Thread Starter
Joined: Dec 2003
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From: Cardiff, UK
Quick Diversion Technique
Hello I'm a PPL student and I was wondering if anyone could explain the inverse fraction technique for diversions, when track markings of a quarter, half and three quarters are drawn along your route.
Many thanks
Many thanks
Guest
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inverse fraction technique for diversions
im struggling with that, never mind the inverse fraction thingy.
Please advise.....
Speedy
Joined: Dec 2003
Posts: 59
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From: UK
''Draw a rough line''
Is that a fixed wing luxury? (wouldn't want to try and do that whilst flying an R22!) - Although a little tip for guesstimating hdg is to use the nearest VOR compass as ref - gives something to compare line to
Is that a fixed wing luxury? (wouldn't want to try and do that whilst flying an R22!) - Although a little tip for guesstimating hdg is to use the nearest VOR compass as ref - gives something to compare line to
Joined: Sep 2002
Posts: 218
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From: Pfffft
Having a wild stab I presume you are referring to proportional ETA corrections based on quarter/half/third/etc markings on your track.
You make appropriate markings on your chart at equal intervals on the leg, i.e. a halfway mark or at thirds, quarters, etc. Ideally look for a good feature that is as close as possible to a proportional point. You then work out what time you would arrive over those points in still air.
When you are flying your route take note of the time you actually pass over the point, from that work out the difference between the still air figure and the actual time you flew over it. This is where the inverse proportion bit comes in.... you multiply the difference by the inverse of the distance of the point over the whole leg. So a halfway point is 1/2 of the leg, the inverse of a 1/2 is 2 therefore you would multiply the still air/actual difference by 2. If a quarter of the way in you would multiply it by 4 and so on. This gives you how early/late you should be at the end of the leg assuming a constant wind velocity.
A quick example; you are flying a leg on a nav route, in still air figures you expect to fly over your halfway point at 5 minutes. You pass over the halfway feature at 6 minutes, so you were 1 minute late. Working on the idea that the error will be proportional you will be 2 minutes late at the end of the leg (1 minute late halfway, inverse of a half x one minute == 2 mins).
I use this method along with the Max Drift rule for wind correction and it all works very well. It is always accurate assuming a constant wind velocity. Be careful about any errors you induce through starting your stopwatch late and such like, one off errors are not proportional
I also apologise for a rather butchered explanation, best I could do at this time in the morning!
ASI
You make appropriate markings on your chart at equal intervals on the leg, i.e. a halfway mark or at thirds, quarters, etc. Ideally look for a good feature that is as close as possible to a proportional point. You then work out what time you would arrive over those points in still air.
When you are flying your route take note of the time you actually pass over the point, from that work out the difference between the still air figure and the actual time you flew over it. This is where the inverse proportion bit comes in.... you multiply the difference by the inverse of the distance of the point over the whole leg. So a halfway point is 1/2 of the leg, the inverse of a 1/2 is 2 therefore you would multiply the still air/actual difference by 2. If a quarter of the way in you would multiply it by 4 and so on. This gives you how early/late you should be at the end of the leg assuming a constant wind velocity.
A quick example; you are flying a leg on a nav route, in still air figures you expect to fly over your halfway point at 5 minutes. You pass over the halfway feature at 6 minutes, so you were 1 minute late. Working on the idea that the error will be proportional you will be 2 minutes late at the end of the leg (1 minute late halfway, inverse of a half x one minute == 2 mins).
I use this method along with the Max Drift rule for wind correction and it all works very well. It is always accurate assuming a constant wind velocity. Be careful about any errors you induce through starting your stopwatch late and such like, one off errors are not proportional
I also apologise for a rather butchered explanation, best I could do at this time in the morning!
ASI
Joined: Dec 2002
Posts: 205
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From: London
There's a brilliant tool called the Knightston diversion plotter, basically a little plastic disk that works like a mini flight computer and very easy to use in flight. It will give you very accurate headings to fly and accurate 6 minute markings along the chart. Sadly they're very difficult to come across. If you'd like on send me a PM and I'll disclose the location of where they come from (from memory somewhere like coventry! 
)
)
Thread Starter
Joined: Dec 2003
Posts: 3
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From: Cardiff, UK
Off track heading correction technique
Many thanks for the replies.
I should have made the title of the question as above.
Currently for nav I use 10 degree fan lines and quarter etc. fixed waypoints along my route. If a correction os required I estimate the angle and double it to give heading correction required (if at quarter I'll be back on track at half).
I may be mistaken but I heard the inverse fraction method referred to hdg correction (I use it for revised ETAs). So at the quarter waypoint if I was 5 degrees off-track I'd apply 4x5 20 degs of hdg correction. This can't be correct as it would imply less of a correction at the 3/4 point.
Another method in a textbook confirms my thoughts as it applies factors of 1.5, 2, & 3 and the 1/4,1/2, & 3/4 points respectively.
I was just wondering if anyone used it the inverse fractrion method for hdg correction, as I can't see how it will work.
I should have made the title of the question as above.
Currently for nav I use 10 degree fan lines and quarter etc. fixed waypoints along my route. If a correction os required I estimate the angle and double it to give heading correction required (if at quarter I'll be back on track at half).
I may be mistaken but I heard the inverse fraction method referred to hdg correction (I use it for revised ETAs). So at the quarter waypoint if I was 5 degrees off-track I'd apply 4x5 20 degs of hdg correction. This can't be correct as it would imply less of a correction at the 3/4 point.
Another method in a textbook confirms my thoughts as it applies factors of 1.5, 2, & 3 and the 1/4,1/2, & 3/4 points respectively.
I was just wondering if anyone used it the inverse fractrion method for hdg correction, as I can't see how it will work.
Joined: Dec 1999
Posts: 3,077
Likes: 1
From: Oop North, UK
The reason it is called INVERSE fraction is because you do just that, but you base it on DTG NOT distance gone ie. at 1/4 way along track you use 3/4 inverted x track error thus 5 x4/3= 7deg. (or near enough) and at 3/4 dist gone it then is 5x4/1=20deg.
another way of doing it is to use the fractions as you have been doing, but multiply by CLOSING ANGLE instead of track error.
another way of doing it is to use the fractions as you have been doing, but multiply by CLOSING ANGLE instead of track error.
Joined: Oct 2001
Posts: 2,517
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From: Vancouver Island
Just a couple of comments.
I map read when VFR and if I am off track I correct based on physical features identifiable on the map. ( Oh, I also use GPS )
I do not recall ever flying any trip of any length where the wind remained constant in direction and or velocity.
Also I am barely literate and attempting to do such complicated math would completely overwhelm my capabilities and I would get lost for sure.
The best advise I can give is when VFR keep it simple and enjoy your flight without cluttering up your mind with needless garbage.
And Merry Christmas to all, even those of you who are lost...
Chuck
I map read when VFR and if I am off track I correct based on physical features identifiable on the map. ( Oh, I also use GPS )
I do not recall ever flying any trip of any length where the wind remained constant in direction and or velocity.
Also I am barely literate and attempting to do such complicated math would completely overwhelm my capabilities and I would get lost for sure.
The best advise I can give is when VFR keep it simple and enjoy your flight without cluttering up your mind with needless garbage.
And Merry Christmas to all, even those of you who are lost...
Chuck
