Accuracy of whiz wheel

[CharlieDeltaUK]

I'm practicing using the whiz wheel - I've got an AFE 'ARC 1' Flight Computer. It doesn't seem very accurate - I often find that answers are astray by 1 degree or 1 knot. I know that is irrelevant in the real world but it seems that accuracy is essential for the PPL ground exams.

On my whiz wheel, the slide has some lateral movement, and I guess there is inaccuracy in the printing of the card and scale as well. And, some of the fine scales aren't exactly easy on the eye to read either.

So, my question is this: will the inaccuracy be enough to cause me grief in the ground exam? Is there a more accurate make of whiz wheel ie something better engineered than the one I have.

[MIKECR]

Ive witnessed build quality issues with CRP5 whizz wheels before. Saw a whole box of them thrown in the bin on one occasion.

[IO540]

Assuming you are talking about wind triangle calculations, note that these involve iteration. I don't think it is possible to solve the wind triangle geometrically (to get the heading to fly and the GS) in just one step. Sure you can get close enough in just one step if the wind is say only 1/10 of your TAS but a lot of exam questions use stronger winds and you need to do the second step to get the error below 1 degree.

(Obviously the winds aloft forecast is too far off for this to mean anything, but that's not the point :) )

The other side of the slide rule is just a standard slide rule, bent around in a circle. It is a generic multiply/divide device, nothing to do with aviation except that the aviation slide rules have some conversion ratios marked for convenience. It should be accurate to nearly 3 significant digits if you are really careful and have good eyes.

I don't know how old you are but if you are in your 40s or older it is worth getting some cheap supermarket +2 reading glasses for the exam. I don't recall the PPL exams but in the IR exams they ban magnifiers so powerful reading glasses are the way to go.

[Deeday]

I don't think it is possible to solve the wind triangle geometrically in just one step.It is, by using the wind mark-up method, which is the one I used to apply (when I was still bothering with the whiz wheel) precisely because it's not iterative, unlike the -down method.

[Gertrude the Wombat]

I often find that answers are astray by 1 degree or 1 knot. I know that is irrelevant in the real world but it seems that accuracy is essential for the PPL ground exams.
You got it.

It's perfectly accurate enough for actual flying, but you can lose exam questions due to reading one degree or one knot one side or the other. Or measuring one degree off with the protractor, or drawing a line one degree out, or whatever.

That's the way the world is I'm afraid. The foolproof answer is to aim for 100% in the exam, there aren't enough of these trick questions to fail you from that position.

[rans6andrew]

my first CRP-1 was visibly "off" I noticed not long after I bought it. When rotating the inner it could be seen running eccentric against the outer scale. Just as a demo I figured out what position it was worst in and set it to multiply by an obvious whole number. At 3 times that number (almost directly opposite the index) the answer was not an exact whole number????? Clearly the rivet in the centre wasn't. I took it back to the supplier and they swapped it.

When in the exam situation it is easy enough to mis-use without having the accuracy of the unit playing on your mind. Take it back to where you bought it from.

Rans6....

[CharlieDeltaUK]

I recall the old engineers' slide rules were quality tools with accurate scales. Isn't there any equivalent for the whiz wheel? I see Jep make a metal one - is that likely to be better?

[Genghis the Engineer]

I recall the old engineers' slide rules were quality tools with accurate scales. Isn't there any equivalent for the whiz wheel? I see Jep make a metal one - is that likely to be better?

I used a Pooleys/Aviat 617 for my CPL exams, which is comfortably good for better than a half degree of drift, and two, normally three significant figures on other calculations.

Hellishly expensive, but it does work very well.


That said, I have to confess, I use it mostly for engineering and flight planning calculations at work. I have a Pooleys CRP-1W in my flight bag.

G

[moreflaps]

"I often find that answers are astray by 1 degree or 1 knot. I know that is irrelevant in the real world but it seems that accuracy is essential for the PPL ground exams"

For my exam I think there was more leeway than that. After all the wind direction and speed aloft is not accurate to that degree is it? If it's multiple choice I expect you will see the answers differing by more than 1knot/1 degree. I suggest you strive for reasonable accuracy but don't sweat over it. do lots of practice calcs and you will be fine.

Cheers

[IO540]

I recall the old engineers' slide rules were quality tools with accurate scales. Isn't there any equivalent for the whiz wheel? I see Jep make a metal one - is that likely to be better?

I have a little Jepp CR-5 which is spot on. My CRP-1 (which I gave to somebody, and having passed the relevant JAA exams now I hope I never need again in the rest of my life) was spot on too.

The old straight slide rules could be read to 3 sig figures, but one could also use them with long multiplication and especially long division where they could be used to instantly give the next digit. I used it at school in the 1960s; a super tool. But these old tools have their place now: in a museum :)

After all the wind direction and speed aloft is not accurate to that degree is it?

True (one can do the calc in one's head using a rule of thumb, to an accuracy good enough for any wind aloft forecast) but totally irrelevant :) This is an EXAM you have to pass.

If it's multiple choice I expect you will see the answers differing by more than 1knot/1 degree.

Possibly, but the answers tend to be rigged to pick up common errors in iterative use also and these can be very small.

I do recall the mark-up method, Deeday, but was never taught it in the PPL. I wonder if it shifts the issue elsewhere. Even the Jepp instructions (http://jeppesen.com/download/misc/crinstructions.pdf) for the CR-5 show an iterative step. Actually the CR-3/CR-5 (http://www.stefanv.com/aviation/flight_computers.html) is clever in the way it does it, without a sliding portion.

[soay]

I don't think it is possible to solve the wind triangle geometrically in just one step.
It is, by using the wind mark-up method, which is the one I used to apply (when I was still bothering with the whiz wheel) precisely because it's not iterative, unlike the -down method.
The same can be said of a whiz wheel with a wind arm, such as the CRP-1W (http://www.pooleys.com/prod_detail.cfm?product_id=566&PageNum_rs_product=1&product_category_id=0&product_sub_category_id=0). I found it made wind triangle calculations more intuitive, and I didn't need to cover it in pencil marks either!

[Gertrude the Wombat]

For my exam I think there was more leeway than that. After all the wind direction and speed aloft is not accurate to that degree is it? If it's multiple choice I expect you will see the answers differing by more than 1knot/1 degree.
Only just. My IMC exam had answers differing by four degrees, which meant you had to measure to two degree accuracy. After trying several times my measurement remained exactly between the two answers - so I guessed, and got the wrong one.

[24Carrot]

For Wind Correction Angle, The clock code and Max drift is almost good enough for one degree accuracy, though it fails for strong winds around 60 degrees off track.

If you use a scientific calculator and use the sin function instead of the clock code you can get one degree accuracy with this formula:

wca = 60 x wf x sin(wmt)

wca is Wind Correction Angle.
wf is the fraction: wind speed / true air speed
wmt is wind bearing minus track.

I checked it with a spreadsheet up to wf of 0.6, ie wind speed was 60% of airspeed.

Once wca is over 36 degrees, you have to use arcsin instead of multiplying by 60, ie use the actual formula which is:

wca = arcsin( wf x sin(wmt) )

arcsin on most calculators is shift-sin, looking a bit like sin-1.

The bog-standard Casio fx 85 in maths mode makes entering the formulae much easier than writing them out in text. And you are allowed them in CAA exams.

[FlyingStone]

The problem is that JAA/EASA most probably uses electronic calculators such as ASA CX-2 to calculate possible answers in the question bank. Therefor, they can afford to put possible answers for let's say, calculating unknown wind:

a) 050°/10 kts
b) 049°/11 kts
c) 051°/9 kts
c) 048°/10 kts

Anyone that has ever done a cross-country flight would know that this kind of accuracy isn't really required, since nobody can't fly (VFR) and maintaining heading accurate to a half degree or similar. But of course, JAA/EASA doesn't care about applying theoretical knowledge into practical flying...

But to make things even more fun (when is dealing with EASA anything else but extreme fun after all? :ugh:), the ASA CX-2 (or similar electronic computers) doesn't seem to be using accurate trigonometric functions. For example, if you calculate most questions in JAA ATPL with regular calculator (using Matlab's precision doesn't help either), you get the wrong results, since they are probably too precise - I assume that CX-2 rounds up result after each step of calculation - which is not the way how to calculate something to a certain degree of accuracy.

So to sum up: buy/borrow an electronic computer for the exams, but for actual flying, rules of thumb are usually more than sufficient for accurate navigation.

[IO540]

Most long distance flying is done using GPS anyway, or some other form of radio nav, so you get continuous lateral guidance. I have never once been interested in the exact E6B calculation, except as a curiosity, and even then one needs to know the heading pretty accurately (relative to the GPS track) and my compass/HSI have never been that accurate. A compass error of say 2 degrees really throws off the wind calculation.

[Genghis the Engineer]

Most long distance flying is done using GPS anyway, or some other form of radio nav, so you get continuous lateral guidance. I have never once been interested in the exact E6B calculation, except as a curiosity, and even then one needs to know the heading pretty accurately (relative to the GPS track) and my compass/HSI have never been that accurate. A compass error of say 2 degrees really throws off the wind calculation.

Entirely true, but not particularly relevant to somebody worrying about passing their PPL writtens, nor useful to him until after passing his PPL when he's actually allowed to use advanced devices like VOR or ADF for anything other than diversions (and GPS at-all!).

G

[Intercepted]

I did the PPL-Nav two years ago and the IMCR one week ago, using the AFE ARC1, and I didn't get any of the flight computer related questions wrong or ended up between two answers. Maybe the papers I did was different from others or my plastic AFE had an exeptionally good build quality (it's now dumped in the bottom of a drawer, never to be used again). :ok:

I think the key to success is to slow down and spend extra time on the questions where you need to involve the whizz wheel. I used the wind-down method (don't forget the iterations nedeed) and made the "dot" with a white-board marker. If you are not happy with the "dot" it's easy to wipe it of and re-apply. Another source of error are track measurement on the map. To reduce the errors I used the ASA-CP-R-ROTATING-AZIMUTH-PLOTTER (http://www.transair.co.uk/sp+Navigation-Plotters-ASA-CP-R-ROTATING-AZIMUTH-PLOTTER+5409). It's very good if you want to be accurate, but be careful to not use the statue miles scales printed on one of the sides.

[Gertrude the Wombat]

(don't forget the iterations nedeed)
In my experience the iterations are not actually needed for practical flying, because doing the iterations properly only makes a significant difference (ie one that would actually cause me to fly differently) if there is such a howling wind that I would never have taken off in the first place.

[Intercepted]

In my experience the iterations are not actually needed for practical flying, because doing the iterations properly only makes a significant difference (ie one that would actually cause me to fly differently) if there is such a howling wind that I would never have taken off in the first place.

Absolutely true for practical flying, but for the test you have to do it if you want to end up with the correct answer. I believe current ppl/imc tests are something from the past and they are, in my opinion, detrimental to real knowledge development. Why not learn how to program the Garmin 430 instead?

[IO540]

nor useful to him until after passing his PPL when he's actually allowed to use advanced devices like VOR or ADF for anything other than diversions (and GPS at-all!).Not quite true. GPS is permitted to be used within PPL training, except in the NAV exercises where dead reckoning is required, and similarly on the skills test. This one was done to death on Flyer after that CFIT.

The myth that GPS cannot be touched until one has the PPL in the bag is an enduring one, believed even by most instructors.

The reason I mention GPS in these threads is because a lot of pilots chuck in flying more or less right away. Of the 20 or so I trained with, all but 1 or 2 chucked it in within a few months. In many cases they give up because they have no confidence to go anywhere.

I believe it is worthwhile mentioning that there is a better method of navigation than the stupid pointless crap error-prone tools taught in the PPL; this gives the student something to look forward to. I continued my PPL because I already knew there were better ways.

Currently I am grinding through Met and Air Law (JAA IR). Never seen so much irrelevant bull***t. Would I be doing this if I thought it was actually needed to fly? No, because it will be forgotten the day after the exam and aviation would end tomorrow if all pilots had to know this stuff. It is just an ego trip for exam writers - just like the slide rule and the calculator ban in the PPL exams. I am doing it as an EASA insurance policy. I already have an IR and have no problem using it.

[24Carrot]

In my experience the iterations are not actually needed for practical flying, because doing the iterations properly only makes a significant difference (ie one that would actually cause me to fly differently) if there is such a howling wind that I would never have taken off in the first place.

True for another reason as well: howling wind forecasts are even less accurate than the normal kind!

IO540, I have just completed those exams and you have my sympathy. IR Nav was one of the most bizarre exams I have ever sat - a kind of speed test on measuring bearings and distances, and then a memory test on GPS and radar frequencies and the like. Zero utility for flying. If you really need to manually measure bearings and angles, you do it until you get it 100% right, not till you get 75% right within a time limit. Knowing those frequencies is also pointless: the pilot cannot tune them.

On the plus side I didn't need the CRP 5 I foolishly bought. There was one wind triangle question, but the answer was obvious once you figured out whether the head/tail wind was from the left or right. And given there were almost no wind qustions, I could have handled them with the calculator anyway.

Re GPS, I found my examiners very keen to move new PPL students onto using GPS. My fixed wing examiner even gave me a demo as we headed back to the airfield after the airwork. And there never seems to be any GPS debate in the helicopter community: everybody uses it.

[PAPI-74]

I haven't read every post, but I don't think that anyone has mentioned that you need to 'balance the drift' to get a more accurate answer.

The CAA will have answers that are balanced and unbalanced. It is too early to explain how to do it right now though, and it has been 6 yrs since I have seen my CRP-5.

[riverrock83]

I've just bought a second hand Pooleys CRP-1 off eBay and I'm surprised about the build quality.
The whole thing isn't symmetrical which is evidenced by if I put the slide in upside down, the centre mark is off by 1 square at the 40 mark. When in the correct way, the centre mark is just off the centre line on the slide.
A second thing is that the white plastic back (the wind triangle side) overhangs the slide rule side at both top left and bottom left (but doesn't overhand on the right side).
I'm sure I could post photos if anyone is interested, and I'm sure it will still be fairly accurate but is this normal? The seller said they had used it about a dozen times about 2 years ago and it has been sitting in a cupboard since... I haven't tried using it yet so I don't know its actual accuracy.
I'm just a bit disappointed - for the money I expected perfection (and I got it 1/2 price through eBay!)

[tmmorris]

I haven't read every post, but I don't think that anyone has mentioned that you need to 'balance the drift' to get a more accurate answer.

Isn't that the same as the iterative method? In which case, you're right, you haven't...

Tim

[IO540]

It is the same thing, yes.

But it is hardly relevant to normal flight, because the 2nd step merely takes out a tiny 2nd order effect, whose magnitude will be far smaller than the error in any wind aloft forecast.

In fact the entire wind side of the circular slide rule is pointless, because a simple rule of thumb will be as accurate as the wind aloft forecast allows anyway.

One of the other things you can do with the slide rule is the backwards calculation i.e. working out what the wind aloft is, given your GS, TAS, heading, track. But this is an illusion for the most part, because in typical enroute flight the wind aloft is of the order of 10-20% of your TAS and a 1 or 2 degree error in your heading (which is virtually certain to be present) makes a big difference to the calculation. To do meaningful wind along calcs (other than more or less pure head/tailwind cases which are trivial, assuming your ASI is accurate) needs a very well calibrated slaved compass system.

But then over 90% of the IR theory (right across the 7 exams) is not relevant to practical aviation...

[FlyingStone]

One of the other things you can do with the slide rule is the backwards calculation i.e. working out what the wind aloft is, given your GS, TAS, heading, track. But this is an illusion for the most part, because in typical enroute flight the wind aloft is of the order of 10-20% of your TAS and a 1 or 2 degree error in your heading (which is virtually certain to be present) makes a big difference to the calculation. To do meaningful wind along calcs (other than more or less pure head/tailwind cases which are trivial, assuming your ASI is accurate) needs a very well calibrated slaved compass system.

But you wouldn't use slide rule (except during training and examination) in the air, especially with its "accuracy" ,when any decent GPS (296/496, 430/530 etc.) can do this quite well. If you are interested in precise (as far as it can be), you supply the GPS with your indicated altitude, QNH (or QNE if you fly above TA), magnetic heading (well, you can always check at the compass and compensate compass heading for deviation and you get magnetic heading) and CAS. The latter is in normal cruise (e.g. 60-70% power, clean config) usually the same as IAS, a knot difference perhaps. But for practical use, personally I just use rule of thumb, it's usually accurate enough to estimate wind direction within 20-30 degrees and to 5 knots. Anything less than let's say 5 knot wind is irrelevant in practical flying, since WCA and difference between TAS and GS would be very small.

But I agree, theoretical JAA examination have very little with real-life flying and problems which you meet in aircraft.

[tmmorris]

Quite. Even my superannuated Lowrance Airmap 1000 (sad to see that go, but there are no database updates any more) can do that.

Tim

[IO540]

Indeed; any half decent GPS contains the E6B calculator, but you still have to enter a heading there - unless you have an IFR GPS connected to an air data computer in which case the heading field is filled in automatically, but the ADC still relies on accurate heading data from the slaved compass system, if you want to have any pretentions of the wind data being anywhere near right for low values of wind speed...

It is virtually irrelevant in flight because, in flight, all that matters is your ground speed. That determines your landing fuel on board at the currently programmed destination, and in any case the rough direction of the wind is obvious within a minute of getting airborne.

In 1300+ hours of flying I have never used any of these calculators.