Jeppesen CR-3 Instruction Booklet

[japanam]

Just sent my questions to Jeppesen for clarification, but thought I would try posting it here as someone else may be able to answer my questions sooner. So here it goes!;)

(Below is the e-mail I sent to Jeppesen with my question)

I am writing this e-mail in regards to the "Jeppesen CR-3" instruction booklet as downloadable at (http://www.jeppesen.com/download/mis...structions.pdf). As I was reading through the booklet, I came across the section finding True Air Speed using the CR-3 on page 14 through 16. Please forgive me if I'm the one missing something here, but I am having a real difficult time understanding the context and was hoping I could get someone at Jeppesen to clarify. This may become confusing, therefore I suggest you have a copy of the booklet in question in hand prior to reading any further.

The booklet does an excellent job explaining "temperature rise" and "recovery coefficient" on pages 14 and 15. However, towards the bottom of page 15 it starts to get confusing. The booklet states,

The cursor on the CR is marked with a straight hairline and a curved line to the right of it (see FIG. 11), with recovery coefficients plotted for CT values of .8 and 1.0.

The recovery coefficient of CT = .8 is the straight line. On the CR-2 and CR-3 there are two lines plotted for the CT value of 1.0. The solid line is for the standard stratosphere temperature of -55 degrees Celsius (35,000'), and a dashed line is for the standard sea level temperature of +15 degrees Celsius. (etc. etc.)

Then on page 16 at the top it states, "In all problems in this book, it is assumed that the recovery coefficient is the more common 1.0, unless otherwise stated."

Following the aforementioned statement, the booklet explains a sample True Air Speed problem. The explanation provided is as follows:

1. Place CAS (400 kts) opposite PA (15,000').
2. Move cursor until hairline crosses intersection of long spiral with 30 degree wiggly line.
3. Read TAS under hairline on scale below.

My confusion with these instructions are:

1. It is my understanding that the "straight hairline" and "curved line" are two different lines, and that the curved line is not labeled as a "hairline". If my understanding is correct, why is it that in Fig. 12 on page 16, instruction box 2 has an arrow pointing at the curved line when the instructions explicitly states to "Move cursor until hairline crosses ......"?

In extension of the question above, the same instruction box uses terms such as "long spiral" and "30 degree wiggly line". Which line is the long spiral line? And which one is the wiggly line?

I need to move the cursor until which part of which line intersects with what?

Again, if this example is using the more common recovery coefficient of 1.0, then although contradictory to my initial understanding, I would suppose the instructions are referring to the curved line. It was my understanding that the straight line was referred to as the "hairline" and the curved one was just a "line".

Thanks:ok:

[Pelikanpete]

I'm not sure what the problem is here - both CT 0.8 and CT 1.0 lines are hairlines. Your aircraft will have one probe or the other fitted (or less likely something in between that needs interpolating). Know which type is fitted and then only use which ever hairline is appropriate - ignore the other completely. The dotted line exists for extreme accuracy and can generally be ignored.

If you're using this computer for the ATPL exams use the CT 1.0 hairline not the CT 0.8.

Diverging from the question a bit but - one of the enduring myths is that the UK ATPL exams are 'designed' for the Pooleys CRP5 and that other computers will come up with a different answer (that is not present in the multiple choices) - this is not true. Many years ago when it was UK CAA exams, maybe, but since it has been a Europe wide JAR exam the tolerances of the questions are such that any computer can be used. Besides if used correctly all computers come up with an answer within 2 or 3 kts of the others. If they don't you are doing something wrong.

The usual exam question mistake is when trying to calculate an IAS from a TAS on the Jepp CR. The CR will come up with the exact correct answer but it requires the user to manually incorporate the ram rise temp into the calculation, using a sort of trial and error until it fits. This is easy (with practice and understanding) and takes seconds but seems to confuse people who use the wrong temp and get the wrong answer.

Having used a number of slide rules, the Jepp CR2 and CR3 are by far the best (fastest, most versatile and most capable and also cheap and indestructible). The Pooley's CRP5 is expensive, mathematically clumsy and generally a couple of generations behind the Jepps in terms of 1950's sliderule technology. It's only used because it always has been, so most text books use it for explanations.

The downside of the Jepp CR is that because it can do so much more, there is more to learn when you first get one - but it is worth it and some functions are the same on all circular slide rules.

It's worth investing in the Jeppesen CR manual/workbook BW2 (about £12) and taking the time to work through it. Without it, you are on your own as many ground school instructors only use the CRP5. The Workbook is better written and easier to understand than every ATPL textbook I have ever read and contains a lot more than the free downloadable CR manual. Lots of really useful methods that will make you love the thing rather than hate it. You will also benefit enormously in understanding the subjects and save many valuable minutes in the exams compared to the CRP5.

[Flatface]

http://www.jeppesen.com/download/misc/crinstructions.pdf

[Tinstaafl]

A Jepp CR was quite ok for the UK national exams. I and a bunch of others used CRs for our British ATPLs in 1999 ie prior to JAR. We found that they were faster to solve some problems and more accurate overall. Our spread of answers were nearly always a tighter cluster around the correct answer compared to CRPs. Considering mine is a CR5 (the smallest at 3 3/4" and therefore the least accurate) I still did better than many CRPs, I have to wonder at the death grip CRPs have on the market in the UK.

I've been using mine since 1987 for Oz IR, Oz ATPL, FAA ATP & UK ATPL and god knows how many flights across the outback & North Sea between Shetland & Scotland. Damned if I'd want to go back to a slide type - and my slide E6B is a Kane, one of the better ones that used to be on the market (no good for high speed stuff though).

[paco]

I can confirm that the CR 3 is probably the best - I've been using mine since the 70s and used it for my own CAA exams way back when. It even gives you sine/cosine values around the front face. An instrument well worth getting to know well, especially for the wind triangle calculations. So much quicker.

For information, this is from a student with a maths background who recently took the Nav exam:

"For the Nav I found that a number of questions did not offer the actual solution. I was using a Jep CR-3 computer which gives slightly different answers than theirs. I then did a spot check on a number of their problems solving them by trig. The trig is really only GCSE level (sin/cos/tan, and then the law of sines). What dumbfounded me was NONE of their answers were correct. I am at a complete loss of what to do with this. All the wind problems were Euclidian so really quite simple. Also, I don't use the 60-1 small angle approx since I had a calculator (and the CR3 does trig nicely) and they are very quick. Even with these simple ones, they did not have the exact answers, with the correct answers almost smack in between two others.

For my next attempt I am at a loss for what to do. If the "correct" answer is not correct, then what kind of arbitrary, "just happen to be the answer"are they looking for? Do they understand that in this simple math there is exactly one answer? Do they understand the difference between accuracy and precision? These too have tight definitions, but that seems to be lost on them. They seem to be happy with precision being that they all get around the same wrong answer, but either don't understand the simple math or don't care how to get accuracy. Unless they are going to specify the exact model of flight computer, to include the date of manufacturing since tolerances change, how can they do this? Perhaps I should send them ISO 5725."

Not good, is it?

[Tinstaafl]

Perhaps those questions were based on using 1:60 to solve? In which case a correctly written question has to specify the method to be used when there is more than one way to solve - especially if different methods give different results. That sort of question is really testing that a particular technique is known, not just whether an answer can be calculated.

[paco]

Perhaps, but there are too many cases where an answer might be, say 33 degrees for 1 in 60 or 35 for trig. Both answers would be correct given the accuracy of the average whizzwheel.