ATPL Perf Example Query - error in CAP?

[StatorVane]

In CAP 698 there is an example for calculating the Field Length Limited TOW (Section 2 - SEP 1, Page 4).

In the worked example they derive a figure of 3530 lb which for the life of me I cannot replicate. In fact, with absolute accuracy, I obtain 3430 lb!! I wonder if the mistake is their's rather than mine?

Here is their written example:

IMG_0001 | Flickr - Photo Sharing! (http://www.flickr.com/photos/79095664@N05/7320343414/in/photostream/lightbox/)

And here is my effort:

IMG_NEW | Flickr - Photo Sharing! (http://www.flickr.com/photos/79095664@N05/7320301454/in/photostream)

Tackling this another way I did this with their figure of 3530 lb as the mass and worked out a TOD (to 50') as being 2700 lb.

I'd be delighted to be proved wrong here so I can move on with the the graphs but either I'm doing something wrong or their example is flawed.

All/any input appreciated, including that which proves me to be an idiot!! :{

[pudoc]

I used my 698 and I got 3500lbs which is close enough.

In the picture of your graph, it's too small for me to accurately see each square so I can't comment on how accurate you are. But the example doesn't seem flawed.

[FlyingStone]

At the first glance it looks you've calculated it correctly. Have you checked what's the nearest answer in the question bank? With this chart reading you usually don't get the exact result as the answers provide, but most of the time the closest answer is the correct one.

Besides, am I the only one who doesn't see point in using ASDA for SEP performance calculations? Or is this just some UK way of complicating things?

[StatorVane]

Hi pudoc.

I appreciate what you're saying. I have scrutinized it to death now and cannot improve on my accuracy. More worryingly, if I work this in the "forward" sense using their weight of 3530 lb I derive a take of distance (no factors applied) of 2700'.

Here's a talk through on what I'm doing just in case I've cocked up! On the right of the graph the boxes are in increments of 100 so I enter at one block above 2500. Paralleling down to the ref line and left across to a 10kt headwind as accurately as I can, then up a short slope to that ref line - and I'm happy with the accuracy having used these graphs umpteen times before.

Next its a straight line across the mass grid which I have drawn absolutely straight. Now the fun begins; using their already plotted data (PA 5653' OAT 15C) I drift down to intersect my line at precisely 3430! Very confusing and frustrating isn't it?

Afternoon FlyingStone. Not sure what you mean, if I'm honest, about the question bank since this is a worked as example as provided in the CAP itself. I agree about natural variations but its quite a bit on this one plus I'm thinking that since they got 3530 and I got 3430 there is some silliness going on.

[pudoc]

I don't see the point in doing any distance performance calculations for a SEP unless flying out of somewhere with a tiny runway. My 152 would never struggle getting off our 4000ft runway.

--

Yes it's very annoying. Don't worry about it. In the exams they give you a good gap so you'd probably pick the right one. Hand it into your school instructors and see what they think.

[StatorVane]

pudoc, I quite agree but since it is for the exam who knows what I might get asked? Or are you suggesting that this stuff won't come up in the exam?

[pudoc]

No I'm not saying that. What I mean is you might be given the question in the example and the answers could be:

3500
3900
3100
2900

And with your answer, you'd go with 3500. The examiner doesn't expect you to be 100% accurate so he gives lee way in the answers.

[StatorVane]

Thanks pudoc. I see what you're saying now. So it doesn't appear to be an error on my part.

[pudoc]

Correct. And remember, you can always appeal a question in the exam if you're sure you've done everything right and there's no suitable answer. You must appeal it during the exam though.

There are, unbelievably, questions out there which have no right answers.

[keith williams]

With this type of question it is all a matter of accuracy.

Looking at the right hand edge of your graph, you do not appear to have made an accurate job of parallelling the spaces between the sloping lines.

I zoomed in to your graph until the gap between the lines at the right hand edge was 31 mm, I then measured the gap between the left hand (lower) end of these lines and it was 16 mm.

At the right hand edge I also measured the gap between your red line and the next sloping line up. This gap was 5 mm.

Parallelling accurately between these lines would get the following

16 mm / 31 mm = X mm / 5 mm

Rearranging this gives X mm = ( 16 x 5 ) mm / 31 mm = 2.58 mm

This means that in the zoomed in condition described above, the left hand end of your red line should be 2.58 mm below the next sloping line up. Measuring it on my computer screen gave me something more like 4 mm.

So you sloping red line is sloping down too much.

Also at the left end of the wind area you appear to have dropped down slightly from your upward lsoping line before drawing your hohizontal line to the left.

These two error would put you too low when moving into the centre of the graph.

These are small errors but the slope of the curevd line sin the centre of the graph is very shallow. The difference between 3430 and 3530 is only a vertical distance of 1 small square on the graph.

As other posters have said, there should be a reasonable distance between each of the options in the questions. But you should also do the best that you can to be accurate in using the graphs.

[StatorVane]

Thank you Keith for this. I am impressed with your explanation. Is this the technique to employ in the exam? I was concerned that the reports were of guys running out of time and whilst this wasn't a rush job for me I didn't know to measure in this way.

[keith williams]

Is this the technique to employ in the exam?


If you mean "should you paste a graph onto a hosting website, then zoom in until you could measure it accurately?" then the answer is quite obviously, no you should not.

But as you had already posted the graph and asked why your answer did not match that in the CAP698, the most onvious way of addressing your question was to check your solution for accuracy. This check revealed two areas where your solution was inaccurate. The purpose of my explanation was to enable you to see where these inaccuracies lay.

Over the past 12 years there have been many cases in which exam candidates have argued that the options in some of the graphical questions were too close. In some cases the appeals have been accepted and in some cases they have been rejected.

The standard advice from the examiners throughout this period has been that you must be able to achieve an overall accuracy of within 1 small square. Unless you are prepared to look critically at your work you will not achieve this level of accuracy.

[StatorVane]

Thanks for your advice Keith. I most certainly am prepared to be critical of my efforts Keith, after all that is why I dropped this question and my graph on you guys.
Thanks also pudoc and FlyingStone too. Hope you all have a great weekend!

[StatorVane]

Hello again. I'm having issues with getting the right answers with these graphs. This may sound arrogant but I honestly don't think I can improve on my accuracy any more than this without resorting to magnifications, not to mention time, that won't be available in the exam. So here we go with another question:

For this question refer to CAP 698 SEP 1 Figure 2.4. With regard to the landing graph what is the minimum headwind component, given:

Actual landing distance = 1300feet
Rwy Elev = MSL
Weather = ISA
Mass = 3200 lb.

a) No Wind
b) 5 kt
c) 15 kt
d) 10 kt.

If you've got a second just run this through and let me know what you get? I did think about the correction factor to be applied to a headwind (use only 50%) but I am led to believe that this chart incorporates that parameter.

Cheers:ok:

[Dick Whittingham]

Why don't you go directly to your instructor at your school?

[StatorVane]

Since it is a Bank Holiday and since I am sat at my computer doing this I thought I might get some help from here as I did yesterday. It is a polite request. Have I committed some kind of forum faux pas? :O Apologies if so - feel free to take this thread down if that's the case. Incidentally, I have indeed contacted the school who no doubt are busy doing other things at this time.

[keith williams]

Doing it freehand I got about 9 knots.

Starting at the right hand edge 1300 ft is 2 small squares above the bold sloping line that is between the two arrow heads.

The sloping lines are almost parallel to each other, so going down to the bottom left end we should end up 2 small squares above the bold sloping line.

Draw a horizontal line to the left across the wind area.

At the left end of the graph the MSL and ISA lines cross 18.5 small squares up from the bottom of the graph.

Draw a horizontal line to the right from this point to the weight REF LINE. This should meet the REF LINE about 2 small squares below the bold sloping line that is below the two arrow heads.

Move to the right parallel to the bold sloping line until you are directly above the 3200 lb mark. The bold sloping line only goes down about half of a small square over this distance so you should now be 18 small squares above the bottom of the graph, directly above 3200 lbs.

Now draw a horizontal line to the right to meet the wind REF LINE. You should hit yhis line 18 small squares up from the bottom of the graph.

Move down to the left parallel to the wind lines until you cross the line that you drew from the right. Drop down to the bottom of the graph to read off approximately 9 knots.

The closest option to this, which is at least 9 knots is 10 knots.

This is the correct answer.

[StatorVane]

Cheers Keith. I just think its what you said earlier about accuracy. I have started to count the increments between the drift lines at either end as you do. This works better than what I was doing which was trying to eyeball a solution between the two.

Thanks again. I think I can buzz off now and leave you all in peace!

[Lightning Mate]

I constructed the CAPs on behalf of the CAA.

The examples are accurate to within 0.1 mm.

There are no errors!!!!!!