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With most transport-category aircraft, even with a 5 km runway, gross weight close to ZFW, using maximum flaps for takeoff, performing an unpressurized, full-power takeoff, the maximum legal tailwind is limited to 10 knots (or 15 knots, depending on the manufacturer) -- why is this? Clearly in a case like this, the aircraft could take off just fine with a tailwind above the 10 or 15 knot limit, when it takes off with a tailwind at the limit under much less ideal circumstances.

It seems unreasonable that there should be a fixed aircraft limitation for the tailwind component, rather than a limit that varies with runway length, aircraft configuration, temperature, etc.. Yet hard tailwind limitations exist. Why?

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    $\begingroup$ What limitation are you referring to? Please be specific. $\endgroup$
    – GdD
    Feb 9 at 13:05
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    $\begingroup$ Well, you might as well go ahead and specify aircraft or general range of aircraft you are talking about. There are many aircraft in which opting to not use any packs or pressurization would not gain you anything, because they do not exist. I never had much luck with downwind takeoffs in a foot-launched hang glider. Paragliders would probably have an even harder time. Even when flying for commercial reasons (just noticed the tag). $\endgroup$ Feb 9 at 13:07
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    $\begingroup$ Transport airplanes normally have a certified tailwind limitation, usually 10 or 15kt, and it's simply to provide flexibility in situations where the runway distance is sufficient for a safe departure with a modest tailwind component, so you won't have to abandon a takeoff and go to the other end just because the wind shifts from headwind or crosswind to slight tailwind. There is also a runway slope factor in the performance data. $\endgroup$
    – John K
    Feb 9 at 14:04
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    $\begingroup$ It's not just the takeoff length you need to consider. Such a tailwind will impact the climb performance so you might not clear the terrain. $\endgroup$
    – Ben
    Feb 9 at 20:46
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    $\begingroup$ "using maximum flaps for takeoff" - Actually, the optimum flap setting for takeoff is generally well below maximum, since the later stages of flap extension add a little lift but a lot of drag, meaning that the aircraft actually needs more runway to take off at maximum flap than at midflap; at maximum flap, it lifts off at a slightly-lower speed, but takes considerably longer to accelerate to that speed. All transport-category aircraft can successfully take off with full flaps (this is a byproduct of the certification requirement that they be capable of successfully aborting (1/2) $\endgroup$
    – Vikki
    Feb 9 at 23:55

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Airline operations only happen where there is valid performance data. If Airbus or Boeing or whatever other manufacturer only provides takeoff performance data up to a 10 knot tailwind, then at 11 knots, you're a test pilot. Granted, we can probably extrapolate from the 8 knot to 9 knot to 10 knot data and have a pretty good guess what things will look like at 11 knots, but we aren't allowed to operate that way. Same reason that operations stop when the temperature gets too high... you go beyond the charted data, even by 1 knot or degree, you're done.

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    $\begingroup$ I totally agree with your answer. AFM limitations are absolute limits beyond which operations cannot legally be conducted. Doesn't mean the aircraft could not operate beyond the limits (within reason). On occasion an operator will pay the manufacturer to change a limitation and then provide that operator with a revised and approved AFM (assuming available engineering/testing data indicates that the change is appropriate). Also U.S. Op Specs generally limit a T/W to 10 kts. And ATC under most circumstances will change rwys at T/W speed of 5 kts or more. $\endgroup$
    – 757toga
    Feb 10 at 18:30

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