Hello There,

I have a few questions in question number 1 of CPL performance exam 2 (untimed).

First, the pressure height that I am getting is 3000ft.

It says short dry grass in question and the lines in solved answer is going straight.

also, we always had a basic principle in mind to take off in wind so if we take off into the wind the slope percentage should be 2 percent up whereas, in the answer it is going for 2percent down slope and if we are ok to choose any of the runway than why the answers differ?

Thank you so much for reading my question

Nixor

Hi Nixor.

Correct pressure height is 3,000ft
There is no surface correction for short dry grass.

Using 2% down slope and 5 kts tailwind, I get a maximum take-off weight of 2,400 kg
Using 2% up slope and 5 kts of headwind I get a maximum take-off weight of 2,440 kg or there abouts.

What answer does the exam give?

Cheers Wayne

HI Wayne,

Thank you for your response, if you look at the solution, he is using 2500 pressure height and the answer is 2560KG. Please refer to the picture below:

Thanks,

Nixor

Hi Nixor,

I have just redone my Take-off chart and get an answer for 2% downslope and 5 kts of tailwind of 2,540 kg. Often, if you have a choice, T.O. performance is better with a down slope, even with a tailwind. I think it must be a typo where it says 2500ft Pressure Height as 3,000 ft is marked on the example.

Cheers W

Hi Nixor, WayneJ ... the answer justification in yellow does say [You can take-off down-hill downwind or up-hill into wind. Try both, not much in it really."
I see that WayneJ you agree taking off downhill, but why and how ??? to date, the unoquivocal rule has been take off into the wind.
On my 'extended' logic, there is ALSO a bit in it isn't there ??? ... as in a whole hundred kilos and possibly a different answer being 2,450kg/answer_"C"

We do tend to Take-Off into wind, but some times we may need to take-off with a small tailwind for noise abatement/ ATC requirements etc. If we also consider in the ERSA that most airports in OZ have a runway slope of less than 1%, the best take off performance will be with a headwind. In this case, the Echo question gives us the choice to use 2% downslope with a tailwind, which gives a greater Take-off weight. While it may only be a 100 kgs, if your flight is fuel critical, then you would take it the extra weight.

Re answer "C". This is an example of a tease - 2540 kgs or 2450 kgs

I'll leave working the example unless you have continuing problems with it. I will presume that Wayne's calculations are appropriate.

Some observations.

It says short dry grass in question and the lines in solved answer is going straight.

Anytime you see a carpet with a reference line, that indicates that the basic, underlying chart has been drawn for the carpet parameter coinciding with the reference line. In this case, the reference line is for SDG so you may presume that the basic chart is drawn for SDG conditions. It follows, then, that there is no need to worry about correcting SDG conditions to SDG conditions ? For the other surfaces, however, one does need to correct for the differences.

As an aside this carpet doesn't really have much to do with grass, rather it relates to several different rolling coefficients of friction which have been chosen to be more or less relevant to the specified surfaces. This is a fairly rubbery part of the takeoff chart philosophy. It is important to note that there is NO basis for any sort of interpolation with this carpet as you don't know the coefficient values and you would have no idea as to how to go about figuring how the actual coefficient might vary. This can be done. As I recall, I have a neat little paper tucked away in the tech library by an Italian OEM which describes such a process which they dreamed up for a small military jet trainer, years ago.

we always had a basic principle in mind to take off in wind

Yes, there has been a rule in place since forever which prescribes into wind operations; (I haven't looked it up in ages but I presume something like the original words still exist in the regs.).

However to do so, religiously, would seriously impede operations. So one appeals to another rule which says that one has to operate within the confines of the approved flight manual (generally contained in the POH for a light aircraft). If the POH gives you data for, say, 5 or 10 knots tailwinds, then you can so operate.

Distance

In post #3, the data entry for TODA is way off the mark. Please do have another looksee at that.

to date, the unoquivocal rule has been take off into the wind.

Not so, see the earlier comment in this post.