Hey everyone, wanting a quick explanation on p charts before attempting the performance exam. On page 40 in the textbook, it says: "if more than a 20kt headwind is blowing the takeoff may proceed but only 20 knots can be used to establish mtow" So if I'm given a 25 knot headwind, do I just use 20 knots on the chart when calculating mtow or todr? When considering tailwind is it correct that we can only use a maximum of 5 knots? Question 11 in online exam 4 asks for mtow and gives ambient wind as 140 degrees at 5 knots and factors an uphill takeoff into wind or downhill takeoff downwind, with the lesser figure being the correct answer. In similar questions I've done, they provide a 10 knot headwind acting down the runway but only a takeoff into headwind is factored. I know that the echo has a tailwind limit of 5 knots, but does that apply to the other charts aswell? I've been told that these questions make or break the exam and I don't want to be second guessing myself.

"if more than a 20kt headwind is blowing the takeoff may proceed but only 20 knots can be used to establish mtow" So if I'm given a 25 knot headwind, do I just use 20 knots on the chart when calculating mtow or todr?

That's it in a nutshell. Different aircraft will have different data limits. 40 kts is a not uncommon maximum headwind and 10, sometimes 15/20 knots for tailwind.

When considering tailwind is it correct that we can only use a maximum of 5 knots?

That's it in a nutshell. You can't operate if you don't have the data.

Suggest you scan a few answer solutions to the thread so we can review and comment on some specifics.

thanks for the reply John. This is a question given to me by my instructor. I have highlighted the lines just so they are easier to see. What is the minimum takeoff distance required under the given conditions? runway: 08/26, tow: 975kg, slope: 1.5% down to NE, long wet grass, pressure height: 3000ft, temperature 15 degrees and wind 260/15. I haven't allowed for a takeoff downwind and downhill as tailwind would exceed 5 knots. When factoring slope, I know the echo and linear charts allow for slopes of up to 6%. If a question specified the use of the same chart as the question above with 6% up slope, is it correct that I have to use the 4% up line as we cannot extrapolate?

runway: 08/26 - for all intents and purposes, the wind is straight along the strip.

I haven't allowed for a takeoff downwind and downhill as tailwind would exceed 5 knots. - good, your chart doesn't give the data so you can't figure the numbers, so you can't takeoff downwind.

If a question specified the use of the same chart as the question above with 6% up slope, is it correct that I have to use the 4% up line as we cannot extrapolate? - first, you wouldn't even think about taking off into that sort of up slope. Maybe a maximum of 1.5 to 2 percent. Those sort of steep slopes dictate one-way strip operation only. Have a look at the one way strip photos in this thread www.pprune.org/pacific-general-aviation-...ghlight=runway+slope Post 23 is the one usually trotted out for steep slopes and associated fun and games.

As to your specific question, again, if you don't have data, you can't go off on a non-conservative use of the chart. So, if the strip slope exceeds the chart maximum up slope, you don't go.

Now, with this stuff, you can use a conservative truncation for the chart. So,

(a) if the lowest chart pressure height is sea level but you have a pressure height of -500 ft, then you do the sums for sea level but get the benefit of the lower actual.

(b) if the head wind is 30 knots but the chart only goes to 20 kt, you do the sums for 20 knots but are grateful for the extra headwind to get you up and away quicker than the chart might suggest.

(c) if the maximum chart strip length is 1000 m but the strip is 1500 m, you do the sums for 1000 m and enjoy the good view as you pass over the upwind threshold.

(d) if the maximum chart down slope is 2 percent but you have 3 percent, likewise (for takeoff considerations. If you have ASD charts, then they might limit you a bit).
etc.

For the specific chart example, the following thoughts are relevant -

(a) be a bit more careful in your interpolations

(b) ensure you use a reasonably sharp pencil for the exam and make sure you run parallel or proportional to the lines as is required. Much better to use a Douglas protractor for running parallel to the grid lines.

(c) in the rolling coefficient of friction grid (LWG, etc) you only run up to the reference line and then go horizontal - you have continued up to the distance line.

Thanks John, feeling a lot better with p charts now

Not sure what you meant about the tailwind. If the tailwind component was 10 kt, you don’t use 5kt and go ahead with the take off. You can’t take off at all if the tail wind component is above max. You would have to use the opposite end - or stay home and go to the pub!!