it is of passing interest to know how the performance data is derived.
I would say that in every case it is of more than a "passing interest" to decide if you have enought runway to land on or not...........unless you laways use 10,000ft runways in a C172.
Unless you follow exactly the method specifed in the manual - you will never get anywhere near the book figures. No one is going to do the book procedure perfectly so to allow for day to day slight differences we factor the figures.
However, As I pointed out earlier, if you look at the book figures derived under FAR-23 (been around for a loong time so not just new aircraft) or CS-23 you will see that while landing, the difference between the published landing distance and the ground roll
will not be anything close to 1000ft (300m) - 150m being more typical.
So if you fly a 3 degree approach to the 50ft point as the test pilot did, you can not - as many schools teach - continue this stable 3 degree line to a few feet above the runway, round-out and land without exceeding the book figure by some 150m or more.
That is the point I was making and that is where CS-23 etc comes into relevance.
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ei-flyer,
I prefer to leave my additional mental capacity intact for flying the aeroplane when it's required as opposed to memorising a hundred and one useless regs.
Some people can fly a perfect aerobatic sequence while reciting the 13 times tables. Not everyone is limited to the same extent.
Depending on what you fly you will have learned parts of CS-23 / FAR-23 and perhaps BCARs and Annex 8 and so many other documents that it would make your head spin - but of course, you simply saw "Pilot Training Manual" on the cover. So if you fly you do know (or did at some stage) "a hundred and one useless regs". You just don't know where they come from or have not realised yet that they were not invented by ther author of the manual you used to learn to fly or your instructor!!
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I like the use of PAPI(3 deg) on short final as it aides in sink rate recognition among other things, but I dont use it much further out than approx. half mile when on final.
Why do you need PAPI to make a judgement. Isn't the whole idea of runway shape / aspect that you don't need PAPI?
PAPI are less and less useful as one approaches the threshold and therefore
if you are going to use them it should be from as far out as possible - when you break cloud - until it is confirmed that you are stabilised but not in the last few seconds of the approach and never inside the threshold.
PAPI are only required in specific cases - most GA fields don't have them and don't need them. Even at night I prefer to have them off so that the student can learn the correct runway aspect and then have them turned on during an approahc to confirm to the studen that they can fly an appropriate approach without PAPI - provided that they get the picture correct. (note the singular of picture is used).