Flight Planning interpolation troubles with CAP697

[sshd]

Hi,

I am going through Flight Planning at the moment using CAP697 but I am finding impossible to understand how interpolation is done for lines which aren't parallel like the pressure altitude ones on the following image:

Say page 17 of the CAP pdf you want to find the line for 3000ft pressure altitude or 11500ft, how would that be done? Given that precision is key to resolve the questions, I am struggling to find a way to correctly draw it.
Thanks!

[Arena_33]

Its a bit crap but you have to estimate and 'eye ball' it with 2.1 and a few other graphs. It's unlikely you'll get an SEP or even an MEP graph in your exam anyway, but if you do you've just got to do the best you can with the graph in question.

It gets even worse when looking a fuel planning graphs for MRJT like graph 4.3.1 etc when interpolating there but again you've just got to do your best to eye ball it and estimate as accurately as possible

[paco]

Accuracy for questions is down to one square

[TayBee]

If you want the 'perfect' interpolation, I was taught to (and did in my actual exam successfully, as I got all those questions right) use my calculator and effectively calculate a proportion:

Take for example 3000 feet OAT 0C, there are 4.8 squares between the 2,000 and 4,000 lines. 3000 is half way (1000/2000) between 2 and 4, so 1/2 * 4.8 = 2.4 squares, so you can draw a dot there and travel across.
The 11500 example, is 1500/2000 of the way between 10 and 12,000 which simplifies to 3/4. So at 0C OAT there are about 5.5 squares between the 10 and 12 lines, so 3/4 * 5.5 = 4.1 squares from the 10 line.

Obviously your pencil is thicker than 0.1 of a square so there is an element of guesswork, but it gives more than enough precision.
If it is multi-choice, you should be able to eyeball its successfully, as the difference between the answers is usually fairly hefty.
For type-ins, the calculator method is probably the safest bet. Trying to draw full lines across as the example lines do is generally not worth it, dashes at each point creates less markings and if you need to erase and try again, it reduces the risk that you get sucked into following the shadow of your previous line.

[Lekker1]

If you want the 'perfect' interpolation, I was taught to (and did in my actual exam successfully, as I got all those questions right) use my calculator and effectively calculate a proportion:

Take for example 3000 feet OAT 0C, there are 4.8 squares between the 2,000 and 4,000 lines. 3000 is half way (1000/2000) between 2 and 4, so 1/2 * 4.8 = 2.4 squares, so you can draw a dot there and travel across.
The 11500 example, is 1500/2000 of the way between 10 and 12,000 which simplifies to 3/4. So at 0C OAT there are about 5.5 squares between the 10 and 12 lines, so 3/4 * 5.5 = 4.1 squares from the 10 line.

Obviously your pencil is thicker than 0.1 of a square so there is an element of guesswork, but it gives more than enough precision.
If it is multi-choice, you should be able to eyeball its successfully, as the difference between the answers is usually fairly hefty.
For type-ins, the calculator method is probably the safest bet. Trying to draw full lines across as the example lines do is generally not worth it, dashes at each point creates less markings and if you need to erase and try again, it reduces the risk that you get sucked into following the shadow of your previous line.

Did you follow the same method interpolating between the trend lines for MRJT graphs? Specifically at the fuel required part?
also i’m confused wether to use the solid or dashed lines? For example solid indicates 10 and dashed 37, for a pressure altitude of 33 000 will i need to interpolate between the dashed or solids or both?