Echo is the old DCA P-chart format (DCA was the original incarnation of CASA). Not 100% on who designed the format - a bit before my time - but, almost certainly, either Icko Tenenbaum or John Fincher who were the head men in Head Office performance back then. Superb engineers and top blokes, both of them. The beauty of the approach, for those of us playing engineers out in Industry, was that the format was standardised and they all worked to the same set of equations. DCA published a couple of tech memos for information - as I recall, those were authored by Ian Cohn and Ron Saunders who worked for Icko and John - I'd have to dig in the dustiest of dusty filing cabinets to locate my copies to double check on the memory cells. If neither Icko nor John developed the technique, I suggest that Ian and Ron would have done the deeds. I thought they were a pretty good attempt at a standardised chart structure and the approach was good so far as practical accuracy was concerned. More to the point, any developmental flight testing only required very simple work - read low cost for the applicant - and was easy work to do.

The grids aren’t exactly even
(a) 11mm for the PH,

These are just density height grids. The individual lines ("parameters") are pretty linear (straight) and the intervals between the lines are pretty linear as well (would fit reasonably on a straight line were we to read some points and cross plot them. Pragmatically, one would interpolate by Mk. 1 eyeball. If you wanted to do a bit better, the sloping rule trick would be just fine.

The distance grids are modestly non-linear (and not well drawn for the takeoff chart) but, for all practical work, treat them as linear for interpolation.

The surface grid provides for three accepted rolling coefficients of friction. Interpolation is inappropriate.

(b) slope % near radiates outwards

As to whether the lines meet somewhere, I have never even thought about that. I would need to revisit the first principles equations to make an assessment. In any case, not of any real relevance to anything in particular. If you were after better accuracy, it would be necessary to cross plot. However, for practical purposes, linear interpolation would be close enough so, again, the sloping rule trick is fine.

(c) wind speed are 1mm apart on the takeoff chart

First, be very aware that there is a sharp discontinuity in the interpolation at zero wind. This is due to there being a 50% conservative factor for headwind and a 150% pucker factor for tailwind. So never try to interpolate across the zero wind line. For headwinds, the interpolation functionally is linear, as is the tailwind so, again, the sloping rule trick works fine.

Here is an aid to accurate interpolation that I have included in the CPL Performance book.

Thank for this Bob, I saw this in your book beforehand. I've tried using this trick but the graph's aren't 10mm apart, so skewing the ruler doesn't fit the box. It's printed on an A4 page, so I'm not sure what I'm doing wrong

the graph's aren't 10mm apart, so skewing the ruler doesn't fit the box.

You're missing the point, I fear. The idea of the sloping rule is to aid the eye in interpolating even divisions - works on the principle of similar triangles. Doesn't matter what the actual dimensions might be. More to the point, you are using the rule as a convenient set of equally-spaced divisions - no more, no less.

If you want to use the trick, set up whatever number of divisions fits comfortably in the dimension available to suit the requirement.

Typically, if you can fit in 10, that's great for most needs. If you only can fit in 5, run with it. If 2, that's all you can manage. It still is better than a simple eyeball interpolation for the typical pilot. On the other hand, with a fair bit of practice, eyeballing and simple pencil marks can work really well. An experienced engineer or draftsman does this all the time to aid in drawing work.

Also, keep in mind that GA P-charts are not microscopically precise and have a bit of in-built fat in the numbers. I've made quite a few of the Echo style charts in years gone by - they are of reasonable and pragmatic accuracy but, by no means, precision data. Different matter with recent heavy aircraft where the numbers are processed far more accurately and, invariably, drawn using high accuracy computer kit.

Looking at Bob's post No. 12, the parallel lines example is linked up to the vertical scale. This doesn't appear to be a very easy way to pick a point or line between two of the graphed lines as far as I can see. It seems that we are normally trying to pick a position between two graph lines rather than between two scale positions. Am I missing something here ?

In an earlier post, there is a statement indicating that the nav computer can be used to get an exact wind component answer. How does this work ?

On your the wind side of your E6B, rotate the wheel to your magnetic wind direction, going up the headwind scale mark your wind speed, say 30kts. Then rotate to your runway heading, the mark you made will he out to one side, read down from your mark and this will show you crosswind, read across it will give you headwind.

That is a simple and straightforward graphical solution but not all that precise. I had the feeling that the comment made earlier referred to some other way of doing the calculation.

Some comments to the previous posts -

the parallel lines example is linked up to the vertical scale

That's just a consequence of how he told the tale in his text which followed from the post in the thread identified earlier. The important point is that you should only use the rule technique if the scaling is, or is reasonably close to, linear. The axes scales, on the chart used is linear so you can use the technique. Should you be playing, say, with logarithmic graph scales, it doesn't work.

It seems that we are normally trying to pick a position between two graph lines rather than between two scale positions

If you want to interpolate between graph lines, then you use the technique in a similar manner between two graph lines rather that the axis scale. Obviously, you will need to be pragmatic deciding how many intervals you can use.

a statement indicating that the nav computer can be used to get an exact wind component

Some background, first -

The Jeppesen CR style of navigation computer uses some simple trigonometry for the wind triangle solution and you can leverage off that however you might choose. The outer scales on the wind side of the computer are just the normal C/D slide rule scales such as you see on the calculator side of the computer. However, the inner scale on the wind side has been scaled at the appropriate location for the sine - if you go around clockwise (or cosine - if you go around anticlockwise) - of the angles engraved. In addition, the "10" position is engraved as "TAS". One additional thing you need to know is that the CR usually marks cosine (the black bit) to 45 degrees. You can keep going around past 45 degrees as sin (angle) = cos (90 - angle). So, at the 40 degree mark (beyond the black bit) you can read off sin (40) = cos (50) = 0.643. (One additional caveat, though: many of the CR machines are engraved in a pretty rough and ready manner so the scales may not be terribly near the precision of what you would find on a scientific slide rule - you get what you pay for, I guess). Notice that the sine scale continues around over the black bit. So sin (50) = cos (40) = 0.766.

You can check this easily by looking up some sine and cosine values on your PC or calculator and comparing those values to the scale value on the computer.

So, if you put the "TAS" mark on the inner scale against the wind speed on the outer scale, then if you read around clockwise to the angle that the wind is off from the runway, then the outer scale answer gives you windspeed x sin (wind angle). Similarly, if you read around anticlockwise, the outer scale gives you the value for windspeed x cos (wind angle).

Sure, it all sounds a bit complicated but, once you have done a few examples, it becomes pretty straightforward.

Now, if you look at the usual graphical wind component solution, as Bob posed at post #3,

(a) the headwind/tailwind component is just windspeed x cos (wind angle)

(b) the crosswind component is just windspeed x sin (wind angle)

Easy peasy - that's what we had set up on the navigation computer. Looking at the graphic Bob posted, the wind is 30 degrees off at 40 knots and we can read off a headwind of about 35 knots and a crosswind of about 20 knots.

If we set up the solution on the CR - set "TAS" to 40, then, if we read

(a) clockwise to 30 degrees, we read the crosswind as 20 knots (actual value is 20 knots), and

(b) anticlockwise to 30 degrees (on the black scale), we read the headwind as 34.7 knots (actual value is 34.64)

Which way to go ? If you practice the computer solution, you will find it very quick, very easy, and quite accurate for any reasonable pilot purpose.

On your the wind side of your E6B

We need to be a bit careful with terminology. The Dalton computer (with the slide) originally was styled E-6B by the US Army when it came into use. When Dalton's patent expired and every man and his dog started making copies, the usual descriptive term used for the civil Daltons became E6B or E6-B. Somewhere along the line ASA decided (I guess for marketing reasons ?) to style their copy of the Jeppesen CR computer "circular E6B". This is a source of much confusion and, on historical grounds, is pretty silly but, there you go.

I had the feeling that the comment made earlier referred to some other way of doing the calculation.

Indeed. See the earlier discussion.

Thanks for the clarification, John. Makes a lot more sense now knowing how it all works, much appreciated!