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Take-off performance charts.

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Take-off performance charts.

Old 21st Feb 2015, 00:15
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Take-off performance charts.

Hello everybody,

I have my Flight Planning and Performance PPL exam on Sunday

When I look at the take/off graph I feel like I am hitting a brick wall.



Where you have Pressure altitude, above temp, what if your pressure altitude is like 652 feet?

Do you draw a line directly up to sea level and move across to the reference line, or do you go below it?

For some reason my mind gets blocked on this performance chart, everything else I have learned so far has been pretty much easy.

Help please!!!!

Thank you, in advance of course....
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Old 21st Feb 2015, 02:44
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Hey Banana, You might need to provide a bit more info as there are different performance charts for different planes i.e Cessna etc. I know they are confusing, having just done my PPL: you're never sure where to start and which direction to go. It all depends on what information you have and what you need to know as to which direction you go round the block.
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Old 21st Feb 2015, 03:25
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Welcome to PPRuNe, Banana.

Without seeing the chart you are using, it's dificult to help.

If it's the one i'm thinking of, you would start at the temp. at the bottom, draw the line up to the pressure altitude, then across to the reference line.

If you could scan and post the chart that would help.


MJ
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Old 21st Feb 2015, 03:44
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Welcome Bananaflier,

First of all, like ALL math to need to do while you fly, guess at the answer first, then work it out to get to a ore precise answer which is in line with your guess.

As said, without seeing the chart, it's hard to offer a good answer, but you can help yourself by guessing the answer first. You have a "standard atmosphere" performance value, which you should be easily able to find for your circumstances. Will the difference in your atmospheric conditions improve or decrease your performance? By much? Or just a little? Now, you've bracketed the answer you're looking for - you won't get it wrong, just less right - continue looking at the chart, and/or, doing the math.

Generally, other than for extreme temps, a Palt of 600 feet will make little difference to performance, but a tiny decrease, so now you, know what you're looking for, a slightly longer TO distance.

If you're still stuck, post the image of the chart (600 x 800 max pixels please), and you're conditions, and we'll try to help you out...
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Old 21st Feb 2015, 06:07
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I took this example here for the take/off distance.

http://flight-school.appspot.com/med...figures/41.jpg

Say if everything was the same, temp 15 degrees, weight is 2950lb and H/W component 9.0knots.

BUT, pressure alt was (is I just done the calculation for my aerodrome)

Palt is 672ft, do I draw a line from the 15 degree mark and where would I stop my vertical line with regards to the pressure altitude?

Its just this really guys, I am just wondering at what point do I stop so I, can carry it across to my reference line.

Meant to go in to see instructor today, have really bad tooth abscess so don't really want to make the journey.

Thank you guys for the open arms welcome.

Last edited by Bananaflyer737; 21st Feb 2015 at 08:32.
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Old 21st Feb 2015, 08:28
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Start from temperature, go vertically up until you get to the pressure altitude which will be just over a quarter of the distance between the surface and 2000 ft lines ( you will need a very thin pencil!) then horizontally across to the reference line.
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Old 21st Feb 2015, 08:40
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Cheers dobbin1,

Making a bit more sense now.
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Old 21st Feb 2015, 08:55
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Start with pencil on 15 degree mark at the bottom
Line goes vertically to a point about one third the way between SL and 2000 lines
Go horizontally across the chart from that point to 2950 lbs which is just the reference line in this case
Continue across to the head wind speed (9kts in the example) and here kink your line to parallel the drawn line below until you reach the correct speed.

Now the tricky bit which isn't shown well on the diagram.

Track across horizontally to the far right and read ground roll distance, 1375 in the example,
Then go back to where your line crosses the zero reference line and follow the diagonal line to read the 50 ft clearance distance, 2300 ft in the example.



Hope that helps
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Old 21st Feb 2015, 09:53
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Johnm,

Do you still have to add the safety factor of 1.1 for 1000ft increase in altitude and 10 degree in temp??

Or does the graph take this into account?

Thank you, you're a star mate!!
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Old 21st Feb 2015, 11:19
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You'll need to check the pilot operating handbook, but generally the performance charts will take those factors into account, that's why temperature and altitude are considered first. However where density altitude is significant the charts will make assumptions about engine management which also need to be considered.

Hope it goes well
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Old 21st Feb 2015, 11:23
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Thank you very much Johnm.

I have my Nav exam tomorrow too, I'm pretty confident about that one.

Thanks again mate.
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Old 21st Feb 2015, 12:05
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You'll need to check the pilot operating handbook
Well worth repeating that. I have seen several variations on performance charts in POH. First time I encounter a new type, I read through the POH.

The Performance section typically contains a detailed example on how to apply the performance charts. But they might use certain terms (like Pressure Altitude or Density Altitude) that may be defined elsewhere in the document. If not, then the E6B will allow you to work out PA from DA and vice versa.

The performance chart will also specify the pilot technique and aircraft configuration, if that's relevant. For instance, the PA28 POH has two charts. One is for a regular take-off with zero flaps, the other is for a short-field take-off, with two stages of flap.

And, as said, the performance charts may or may not take into account a safety factor. Similarly, they may or may not take into account things like wet runways, grass fields, and upslope/downslope. This should be mentioned in the POH. If the charts don't take this into account, I apply the standard CAA safety factors for those.

The CAA exams, by the way, should provide you with similar information. They don't give you a chart in isolation but they will give you some additional information. Like whether things are based on DA or PA, and whether the safety factors are included or not. That information may be on the chart, or in the question, but the question can be answered with the information given in or with the question. (You do need to be able to work with PA and DA though.)
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Old 21st Feb 2015, 12:20
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Thank you Backpacker,

I am looking through some of my flight schools POH's on the PA-28, I am looking all through the charts and they make a lot of sense now.

I do know my PA and DA in terms of being able to do the calculations necessary, so I have no problems with that.

I am training on the DA/40, as soon as I check out I will transition onto the PA-28.

I recently fell in love with it and its cheaper too.

Thanks Backpacker
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Old 21st Feb 2015, 16:42
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Now that you have passed the exam you can operate in the real world. When using the charts just go to the next higher solid line for each value rather than trying to extrapolate. Rounding up will provide you with an extra safety margin

Remember the charts were done by a factory engineering test pilot flying a new airplane. When you get into your club's 35 year old, well shagged PA 28, "your mileage may vary".
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Old 21st Feb 2015, 17:27
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Additional Safety Factors are only applied if a Performance Chart does not take them in to account. Most allow for the variables of Temp & Press (Density Altitude) and whether there is a Headwind or Tailwind.
Adjustments for Runway slope is not always given so a Safety Factor would need to be applied in this case - as previously stated check the graphs in the POH for the aircraft you will be flying - and for the exam study the one given to see exactly what variable is, and what is not, included.

As an extra safety margin, for CAT an additional 25% is required to be added to the calculated TODR and 43% to the calculated LDR.
However, because these are mandatory, most operating manuals for aircraft which are mainly used for CAT will have Performance Charts that already apply these (they will state on them whether they do)

For GA (including the PPL written exam) you will be expected to apply the CAA recommended Safety Factors of an additional 33% to the calculated TODR and 43% to the calculated LDR.

The reasons for this are as stated by BPF above - along with taking in to account the 'possibility' that the aircraft Mass, the Temp, the Press & any Headwind are not necessarily exactly the values used in any calculations.

So, in your exam, if you calculate a value for TODR which seems to be within 1-2 metres of a possible answer; but there is also a possible answer that seems to be exactly 1/3 longer than the distance you calculated - Then the longer distance is the correct answer.

NB; Make sure you can easily convert between feet and metres as sometimes the Performance Charts will be in feet, but the declared distances/possible answers will be in metres.
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Old 22nd Feb 2015, 17:13
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Thanks guys,

Had my exams today, didn't go too bad.

Cheers.
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Old 23rd Feb 2015, 18:03
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Aced my exams guys!!

Thanks for the help.
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Old 23rd Feb 2015, 21:02
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Aced my exams guys!!

Thanks for the help.
It's nice to hear a happy outcome, thanks for the update! A bit more learning, and you'll be returning the helpful advice to others before long.....
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Old 23rd Feb 2015, 21:08
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Thank you Pilot Dar
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Old 24th Feb 2015, 08:11
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During my ATPL Flight Planning and Performance exam, we were given one really useful tip - "Sharp Pencil".

Some of those graphs are flippin' ridiculous to read, let alone trying to interpolate 1/4 of a square on, then draw an accurate line across.
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