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Page 3.22 Example T/O Weight

  • brentonrule
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brentonrule created the topic: Page 3.22 Example T/O Weight

Hi Team. Can you please clarify for me.

Example for ECHO at Leongatha says rwy 04/22 is closed due wet soggy soft surface.

I used LONG WET GRASS in my calcs as runway 18/36 is also gravel/grass (northern end where the t/o run will commence).

The example in the example/answer uses 'unrated gravel' as the entry argument.

Why so?

Footnote on another couple of observations/questions:

1. Page 3.20 talks about TOD i flight tests (bottom of the page) having an allowance of +22% then says ref CAO 20.7.4 para 10.1 - I cannot see any mention in this section of +22%

2. Page 3.22 talks about a downslope of 6% being acceptable - where can I find this in the a/c data or CAO's ?

Thanks, Brenton
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Richard replied the topic: Re: Page 3.22 Example T/O Weight

Hi Brenton,

Here's a take on some of your questions:

ECHO Example on pg 3.22:
The Echo charts only have short dry, short wet/long dry or long wet lines. They don't have lines for gravel (or for any other surfaces for that matter) so you take a best guess. Gravel is simply an unsealed runway. So, since it has grass at one end (which is not explicitly declared as being wet or long) you can simply use the value for short dry.

Check out point 4.1 on page 16 of the CPL-PERF textbook supplement. It explicitly says to use short dry grass for sealed or gravel runways.


Page 3.20:
CAO 20.7.4 states that test pilot figures need to be adjusted by the following factors:

(a) 1.15 for aeroplanes with maximum take-off weights of 2 000 kg or less;
(b) 1.25 for aeroplanes with maximum take-off weights of 3 500 kg or greater; or
(c) for aeroplanes with maximum take-off weights between 2 000 kg and 3 500 kg, a factor derived by linear interpolation between 1.15 and 1.25 according to the maximum take-off weight of the aeroplane.

Since the ECHO has a MTOW take-off weight of 2950 we must take a linear interpolation between the factors 1.15 for 2000kg and 1.25 for 3500kg. This results in a value of about 1.22. If you multiply a number by 1.22 it is the same as adding 22%.

By the way, you don't have to do this calculation. The factor has already been worked out for the ECHO and applied to the values you will read off the charts. This factor of 1.22 is also stated explicitly in the information box at the bottom of the take-off and landing charts.


6% Downslope:

Take a look at the slope box on the Take-off chart. The last slope % listed is 6%. That means you can generate data for slopes of up to 6%. Any slope steeper than that has to be calculated as if it was a 6% slope. Remember, you can interpolate the data but you can't extrapolate the data (i.e. create new lines such as one for a 7% downslope).

One important thing to notice is the type of line that appears in the boxes. Check out the 2% upslope line and also the 5 KT tailwind line in the take-off chart for the ECHO. Notice how they are dashed and not solid?

A dashed line on one of these charts is actually a limit imposed on this aircraft. They are effectively saying you may not attempt a take-off with more than a 2% up slope in the runway and you may also not take-off if you have more than a 5kt tailwind.

Compare that to the line for the 6% downslope. That's a solid line. The manufacturer is therefore saying, "You may well use a runway with a steeper downslope (if you consider it safe) but we're not going to give you any performance figures for it."

Cheers,

Rich
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  • captainellzy
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captainellzy replied the topic: Re: Page 3.22 Example T/O Weight

Gday Brenton,

I see your thinking in regards to if 04/22 is wet then 18/36 would be too, however it took me a while to get there! I had a few too many last night.. So here's my take on it.

The grassed end of 18 wouldn't have nearly as much effect on the takeoff performance as if the whole runway was grass and so Bob uses the equivalent short dry grass.

As for the 22%, this is referring to the Echo MK4 (a fictional aircraft) chart information box. If you were to take a chart from the C152 I believe it is factored by 1.15. Richards explanation of the CAO reference is much better than my attempt.

6% downslope is acceptable because this is allowed for in the chart. Just the same as if the temperature reached 46C, you wouldn't be able to take off because you physically cannot get an accurate take off distance (it's off the charts so to speak :))

You may interpolate charts but may not extrapolate them. If you notice however in the supplement on pg 19 it says you can use more than 20kt wind even though this isn't on the charts because more headwind can only be a good thing and will reduce your TODR.

Do you have the exam booked in yet?

Cheers,

PS - Nice one Rich, did you have something to do with my 'session expiring'! :ohmy:
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Richard replied the topic: Re: Page 3.22 Example T/O Weight

Hehehehe, beat you to it Ellzy :P
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  • brentonrule
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brentonrule replied the topic: Wet or dry

Thanks Ellzy and Richard, this sort of feedback is very useful. Perhaps I could have been a bit more studious in my reading to extrapolate the 1.22 for the 22% buffer. I just could not see that when I first read the CAO.

On the wet/dry surface, I would still have sued wet grass becasue my assumption is that if one runway is 'sodden' then both would be. At leas tthat would be my logic as both these runways are made of the same stuff (well roughly anyway).

And the 6% downslope - YEP - it's on the charts - why didn't I see that? Forrest for the trees hey?
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Richard replied the topic: Re: Page 3.22 Example T/O Weight

PS - Nice one Rich, did you have something to do with my 'session expiring'! :ohmy:

Ellzy, I'm shocked you would even suspect such a down and dirty trick :P
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  • captainellzy
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captainellzy replied the topic: Re: Page 3.22 Example T/O Weight

All in good fun, all in good fun :D
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  • brentonrule
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brentonrule replied the topic: Re: Page 3.22 Example T/O Weight

G'day Ellzy. No eaxma booked in just yet - just started the study. Did the Aero exam last week and got 83% so quite happy with that one. I am having all sorts of trouble getting my head around some of these performance processes but no doubt it will fall into place the harder I work at it. Cheers.
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  • captainellzy
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captainellzy replied the topic: Re: Page 3.22 Example T/O Weight

Can't recommend the practice cyber exams enough for this one. It is tough. But as you say the harder you work the more you'll understand it. Good luck!
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