Cone of Confusion: When to Stop Chasing the Needle

F

FDX8891

Well-Known Member
Other than saying "Correct for needle deflection, but don't correct too much", is there a generally accepted distance from a VOR station to stop correcting for needle deflections before you pass the VOR station in a slow piston single at low altitudes (below 10000')?

I want to teach my students to correct for deflections, but don't want them making huge corrections and chasing the needle when nearing the cone of confusion. Right now I'm telling them that within 1/2 mile of the VOR to make less than 10 degree corrections - Does that seem like an acceptable practice? It works out OK most of the time, but one of my students has a tendency to make huge corrections which throws him off, especially when he tries to intercept a radial outbound (to enter a hold for instance).

Thanks in advance for any input.
 
Just depends on your altitude. If you are at FL300 it can be up to 3-5 miles depending on conditions, while down low at a few thousand, it will be a lot less (normally somewhere inside a mile). IIRC the ROT I have heard is within a mile of the station slant range, so that would pretty much support the examples above.
 
FDX8891 said:
Other than saying "Correct for needle deflection, but don't correct too much", is there a generally accepted distance from a VOR station to stop correcting for needle deflections before you pass the VOR station in a slow piston single at low altitudes (below 10000')?

I want to teach my students to correct for deflections, but don't want them making huge corrections and chasing the needle when nearing the cone of confusion. Right now I'm telling them that within 1/2 mile of the VOR to make less than 10 degree corrections - Does that seem like an acceptable practice? It works out OK most of the time, but one of my students has a tendency to make huge corrections which throws him off, especially when he tries to intercept a radial outbound (to enter a hold for instance).

Thanks in advance for any input.
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I used to tell my students when entering a hold to keep corrections from 15-30 degrees and see how it works. Once established in the hold use 15 or less. If they need more after a few seconds, then you know you have some nasty winds aloft, but for goodness sake, *be paitent*. It's not going to happen instantly. AFAIK, there is no one method that works for everyone, so your student is just going to have to learn to be paitent. Me thinks they know the procedure, but they don't have a good understanding of what to do yet.

On airplanes noe equiped with DME, I used to try and find another radial from a different VOR to see how close I was. Sometimes it worked, sometimes it didn't. It depends on how close the next VOR was, where it was, and how quick you can pull out a chart and look. But when the needle start moving fast, you know your close, so just fly a heading, and stick to it.
 
It all depends on your groundspeed and altitude.

In the T-38, where we are going between 250 and 300 knots and holding usually at a higher altitude where the cone is larger, I use what //AMG said, 1 NM of slant range when DME is available.

That's obviously too much when you're only flying 1/2 or 1/3 of that speed and at a lower altitude, so 1/2 NM probably will work for most piston/GA aircraft.
 
It should be less of an issue if the pilot has been paying attention to heading all along and knows the reference heading that has kept him on track up to now. I'm sure there are situations where this can vary but I don't know of too many where the winds substantially change velocity as you get closer to a VOR.

Less than 10° should work. More is a recipie for overcorrecting.

For that student that doesn't believe and insists on overcorrecting, try this: Ensure that he knows the refernce heading and is flying it. As you get closer to the VOR and the needle starts to move, cover the VOR. After enough time has passed that you have gone past it and beyond the cone of confusion on the other side, uncover the VOR. Let him see if that works better than his mehod.
 
For any CFII who was wondering, this works out very well for a slow piston single operating at relatively low altitudes:

Within .5NM of the VOR, make less than 10 degree corrections to correct for needle deflection.

My instrument student has aced his VOR tracking now that he has implemented this.

Thanks everyone who commented for their input.
 
The rule of thumb that I use is that if you convert your altitude (above the VOR, so AGL altitude, not MSL) from feet to miles, that is approximately the same size as the diameter of the cone of confusion. As an example, if you are flying at 6000 ft (AGL), then you are about 1 mile above the VOR. The cone of confusion at that altitude is pretty close to about 1 mile in diameter, (which makes the radius 1/2 mile). If you are at FL360, you are almost 6 miles above the VOR, so the diameter of the cone of confusion is about 6 miles, and the radius is about 3 miles. Therefore, at .5 DME at 6000' AGL, or 3.0 DME at 36000' AGL, you are probably in the cone of confusion.

What I teach students, however, is to just look at the needle, and expect the cone of confusion when you get "close." If you are pretty close to overhead the VOR, and you see the needle jumping or fluctuating in both directions, then simply hold the last known appropriate heading until you pass the station. I break out the above formula as an "extra," if the student needs or requests further elaboration.

Incidently, the theory above implies that the cone of confusion is almost 53 degrees wide (26.5 degrees either side of a line directly above the VOR). That seems a little large to me, but I can't find data out there to come up with a better rule of thumb... and hey, it's just a rule of thumb anyway. Even if the cone of confusion is actually a little smaller than that, this rule is relatively easy to exercise and gets you close. Probably good enough for most of us.
 
fish314 said:
What I teach students, however, is to just look at the needle, and expect the cone of confusion when you get "close." If you are pretty close to overhead the VOR, and you see the needle jumping or fluctuating in both directions, then simply hold the last known appropriate heading until you pass the station. I break out the above formula as an "extra," if the student needs or requests further elaboration.
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Good point, bottom line IMHO is fly the thing until it doesn't make sense anymore (its not hard to tell looking at the needle when you are in the cone of confusion), and just maintain inbound and outbound headings that make sense. If you are a little off course on the backside of the station, just correct....even in high winds, you are probably going to still be within reasonable parameters if you take into account your wind corrected headings that you were using coming inbound. If equipped with DME (or for folks with TACAN), when the DME stops ticking down and starts rising again, you have overflown the station. Pretty simple, don't overthink it using math :)
 
im with midlife. If the student has bneen tracking inbound and paying attention there shouldn't be much chasing at all. Now I know how students are, so you should be trying to focus their attention on centering the needle as far from the VOR as possible (in the training environment) - let's say 5 miles inbound. When They center the needle and take note of the difference between the inbound radial and heading and note if there is any deviation (bracketing). By the time they are within 3-5 miles there should be little if any corrections, since I assume you are not changing altitudes and therefore having different winds.

Make a pass by letting the needle deflect while flying the heading and passing the station. Then allow the student to make another inbound pass while chasing the needle. Ask them which is easier. They will figure it out.
 
MidlifeFlyer said:
For that student that doesn't believe and insists on overcorrecting, try this: Ensure that he knows the refernce heading and is flying it. As you get closer to the VOR and the needle starts to move, cover the VOR. After enough time has passed that you have gone past it and beyond the cone of confusion on the other side, uncover the VOR. Let him see if that works better than his mehod.
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On the flip side, you could just not tell him about the cone of confusion and insist the needle always be centered then take him over some VORs. After a half hour or so he's sure to give up and try something new, or so we'd hope.
 
shdw said:
On the flip side, you could just not tell him about the cone of confusion and insist the needle always be centered then take him over some VORs. After a half hour or so he's sure to give up and try something new, or so we'd hope.
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You sir, are evil! Hahahaha. It would be fun to watch, though, wouldn't it?

Actually, you know what that formula that I gave above is REALLY good for? Telling the student that his deviations off the needle 3 miles from the VOR at 10000' AGL were definitely NOT due to the cone of confusion when he tries to claim that "it wasn't him, it was the VOR."
 
http://marinegouge.com/~marin43/mediawiki-1.13.3/index.php?title=I4301

According to the site above, the cone of confusion on a VOR is about 40-50 degrees wide (which makes the approximation I gave above, at 53 degrees wide, actually a pretty good WAG).

On a TACAN, it is about 100 degrees wide, so you can still use the estimate above, except that converting your altitude above the station from feet to miles would give you a pretty close guess on the radius of the cone (no need to divide by 2, in other words). On a TACAN, the cone would be slightly bigger than the rule of thumb, whereas on a VOR it is slightly smaller than what you get from the Rule of Thumb.
 
fish314 said:
You sir, are evil! Hahahaha. It would be fun to watch, though, wouldn't it?

Actually, you know what that formula that I gave above is REALLY good for? Telling the student that his deviations off the needle 3 miles from the VOR at 10000' AGL were definitely NOT due to the cone of confusion when he tries to claim that "it wasn't him, it was the VOR."
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Both the Hornet as well as T-45C have wind corrected ground track markers on the HSI/moving map display, and using said crutch, even a monkey can be precise enough to only see the cone of confusion for maybe a mile or two (if high) or much shorter (if low). Contrast that with my personal experience on old school needles and wind corrected guesstimate headings, and it is easy to see that pilot technique (or lack thereof) contributes significantly to this phenomenon.

I will add that, though it has been quite a while since I have used a straight up VOR, I remember them being quite a bit less accurate and much more shifty than the TACAN. So that may be part of it....
 
Just look at this image, JC automatically destroys my white space that I use to draw it all out: Picture 2.png

That works out to a little more than .2NM for every 1000' gives you a buffer before you hit the zone.
 
fish314 said:
You sir, are evil! Hahahaha. It would be fun to watch, though, wouldn't it?

Actually, you know what that formula that I gave above is REALLY good for? Telling the student that his deviations off the needle 3 miles from the VOR at 10000' AGL were definitely NOT due to the cone of confusion when he tries to claim that "it wasn't him, it was the VOR."
Click to expand...

I just went back and looked that over now, that is some good stuff. Oh, and I'm not evil the FAA just told me that impressionable teachings are well remembered!
 
ppragman said:
Just look at this image, JC automatically destroys my white space that I use to draw it all out:View attachment 16051

That works out to a little more than .2NM for every 1000' gives you a buffer before you hit the zone.
Click to expand...

This just jogged my memory. Remember way back when the fed books used to say that crossing a VOR you had the CONE of ambiguity and the ZONE of confusion? At some point oh 5 or 10 years ago they decided to just call it the cone of confusion to simplify it I suppose. :dunno:

IIRC The old school books explained the ZONE of confusion as the highly sensitive area above the VOR where your needle sways around to full scale deflection while the CONE of ambiguity was the area (to the side if you didn't fly directly over the VOR) where the TO/FROM flag would flip back and forth before settling in to give an appropriate indication.:wtf?:

I guess it's a useless piece of aviation information that has nothing to do with the topic at hand. The "zone" comment just knocked away a bunch of cob webs in an old dusty and unused portion of my brain and it makes me feel old...:o
 
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