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I read that takeoff distance was the distance for which the airplane is on the runway, as well as the distance for which it ascends to 15 meters off of the ground.

Does it get measured as the hypotenuse of the "triangle" the airplane makes, or as the projection of its movement onto the runway, which is a purely horizontal distance?

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    $\begingroup$ Welcome to aviation.SE! Are you asking about a specific country or regulator? There's a regulatory definition in the US (for transport aircraft) but you mentioned meters, so that might not be relevant to you. $\endgroup$ – Pondlife Jul 29 at 0:32
  • $\begingroup$ I was trying to understand the general concept, so the exact standard doesn't matter too much, just how the measurement is taken. Thank you ! $\endgroup$ – dnfost Jul 29 at 15:16
  • $\begingroup$ I think for basically any airplane for which takeoff distances are specified by the manufacturer (i.e., ignoring souped-up bush planes), the difference between "projected" horizontal distance and actual flight path distance is going to be negligible. In mathematical terms, sqrt(a^2 + b^2) ~= a when a >> b. That said, it's the horizontal distance that matters as explained in the answers. $\endgroup$ – TypeIA Jul 29 at 15:19
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Takeoff distance will always be measured horizontally along the ground, because it is used to calculate and reference required runway length.

Also, I should point out that while 15 meters (50 feet) is common in some regions, other requirements exist in other regions and under certain circumstances. For example, in the U.S., according to CFR §25.113, a height of 35 feet (10.7 meters) is also used.

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Takeoff distance is always measured as horizontal distance.

Under 14 CFR Part 23, which apply to aircraft seating 19 passengers or less and a maximum takeoff weight of 19,000 lb or less, the pre-Amdt 64 specifications are:

  • For multi-engine jets of 6,000 lb or more, or commuter aircraft, 23.59 applies. Takeoff distance is measured as the horizontal distance between start of ground roll to where the aircraft attains 35 ft off the ground.

  • For others, 23.53 applies. Takeoff distance is measured as the horizontal distance between start of ground roll to where the aircraft attains 50 ft off the ground.

Note that a lot more nuance is involved since the greater of one-engine-inoperative distance and that with 115 percent of all-engine-operating distance goes into the actual performance number.

For Part 25 aircraft (transport category), there is a further distinction between dry and wet runway (25.113):

  • On a dry runway, it is measured to 35 ft.

  • On a wet runway, it is measured to 15 ft.

This information is harmonized with other regulatory agencies, such as EASA and TCCA.

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