Originally posted by 3WE
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Shortly after my post, it hit me...the problem is with the 20 knots head start, I can hit 80 in 20 seconds with a possibly real acceleration issue and continue my takeoff or worse my not_takeoff.
But then you go on asking how much variation can there be in the "20 sec to 80 kts" rule. How far can we be from about 80 kts after about 20 seconds.
That is not the useful question here.
Let's assume that the rule is pretty robust and that, no matter what, you will always be at about 80 knots after about 20 seconds.
The real question is how robust the take off performance is, given that that condition is met.
That is, how much variation can there be in the distance run to achieve V1 (compared to the theoretical one taken into account when the take off calculation was done). And the answer there is, a lot.
So, what is the value of checking that you achieved about 80 kts in 20 seconds if that means not much for whether your take-off performance is doing good or not?
Remember, we are trying to see if your take-off performance is the expected one, so you need two things: 1- A variable that is strongly correlated to the take-off performance and 2- A reference value to compare that variable against.
"Time to achieve 80 knots" could be a good variable (if the technique to initiate the take-off is solid consistent (like rolling vs standing, technique to advance the thrust levers and stabilize the engines... etc), which is quite questionable to begin with. But "about 20 seconds" cannot be a universal benchmark. If your take-off performance calculation is based on a headwind of 25 kts and you actually have your 25 kts, you better achieve 80 kts in much less than 20 seconds.
I would say that every take-off would need it uniquely calculated "time to achieve 80 kts" as much as every take-off needs its uniquely calculated V1, Vr, V2, trim setting, flap setting, and thrust setting. (and that, again, assuming a consistent technique at the beginning of the take-off roll, which is not the case).
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