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I came across this the other day

It says that, for most GA aircraft, that if you have not reached 70% of your takeoff speed by the time you have reached 50% of the length of the runway, you should abort the takeoff.

One thing I wonder is how do you judge where you are on the runway. If there is a taxiway coming off at the 50% point, that’s useful, but if there are multiple ones, you have to read the letter codes on the signs while trying to keep the plane going straight, which is not a good idea.

I have learnt that as a bush pilots’rule.

Taught to me as resultant of two considerations:

1 Typical acceleration curves suggest that below 2/3 t/o speed at 1/2 TODA makes it unlikely to reach it in the rest (on an even flat surface having started at the end ;-) )

2 when aboring there, the rest of the runway hives you enough distance to decelerate safely.

From my freshman physics book, for constant acceleration, V**2 = V0 +2AS where V is speed, V0 is initial speed, A is a constant acceleration, and S is the distance.

Assuming initial speed V0 is zero and takeoff is via a constant acceleration, the equation reduces to V**2 = 2AS.

Assume that V1 is the required liftoff speed at distance S and that V2 is the speed at distance S/2.

Substitute V1 and V2 into the general equation and divide the results. V1**2 = 2AS and V2**2 = 2AS/2, so dividing the second equation into the first and cancelling like terms yields V1 **2 / V2**2 = 2. Solving for V2 yeilds V2 = V1 / SQRT (2) = 0.707 V1.

That is where the 70% rule comes from.

I wish all runways would have half way markings on them.

Steve6443 wrote:

if you’ve not touched down by this half way board, go aroundWhile for takeoff rolls the 50/70-rule makes sense due to the distance being covered during acceleration (assuming you have started your takeoff roll at the beginning of the runway), this obviously does not apply to landing an arbitrary airplane on a runway of arbitrary length.

But even on takeoff the rule has to be applied with care, as it assumes nominal acceleration even after the “decision point”. If you have used up 3000 ft of runway to get a 172 to 70% of takeoff speed in a hot-and-high-scenario, chances are that you will never reach flying speed or (worse) that you will be able to liftoff in ground effect only to find out that the airplane does not climb out of it.

tschnell wrote:

While for takeoff rolls the 50/70-rule makes sense due to the distance being covered during acceleration (assuming you have started your takeoff roll at the beginning of the runway), this obviously does not apply to landing an arbitrary airplane on a runway of arbitrary length.

But even on takeoff the rule has to be applied with care, as it assumes nominal acceleration even after the “decision point”. If you have used up 3000 ft of runway to get a 172 to 70% of takeoff speed in a hot-and-high-scenario, chances are that you will never reach flying speed or (worse) that you will be able to liftoff in ground effect only to find out that the airplane does not climb out of it.

Sorry, should have indicated, my homebase is 600m, the runways I land on are usually around that length…..

(Yorkshire Accent on) 3000 foot runway? Luxury. Where I came from, we considered ourselves fortunate to have a 20 inch long grass runway.

(second Yorkshire Accent) Grass Runway? You were lucky. We had to take off, in a bramble patch, going uphill, whilst fighting off the farmer for using his field…..