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I hope this is not as trivial as it sounds. Is there a wind gusts threshold based on which, at a given airport, incoming flights are diverted and outgoing ones are delayed?

I assume that aircraft are designed to handle a specific maximum wind gust, so if there is any such threshold, I expect it to be less than the design threshold. Is this correct?

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You have two questions here,

I hope this is not as trivial as it sounds. Is there a wind gusts threshold based on which, at a given airport, incoming flights are diverted and outgoing ones are delayed?

As far as I know there is no specific threshold for which they will divert. You may want to read up on wind shear here as an airport may advise sever or unsafe wind shear and urge pilots to divert. There will be a point (often at single runway airports) where a landing in a particular type of aircraft becomes unsafe or even impossible as a result of wind but this will vary by plane.

Also, I assume that aircraft are designed according to a specific max wind gust, so if there is any wind gusts threshold, I expect it to be less than the design threshold. Is this correct?

Yes, aircraft are designed with whats called a maximum demonstrated crosswind component. The key here is that this is a demonstrated limit not necessarily an airframe limitation. The number is published in the POH for the aircraft made after 1975 and provides a general guideline of the maximum cross wind the plane can be landed safely in however its more than possible to land it in higher wind situations.

Also it should be noted that some of this is up to the pilot in command. Just because a towered airport clears you to land does not mean it is safe for you to do so likewise just because a pilot diverts does not mean they were instructed to do so by the tower they may self elect based on their personal abilities.

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  • $\begingroup$ Probably worth adding that the aircraft operator will have a windspeed limit in their operations manual, which may depend on the pilot's experience as well as the type of aircraft. $\endgroup$
    – Dan Hulme
    Nov 29, 2015 at 13:18
  • $\begingroup$ @DanHulme You mean different limits are tabulated for different experiences of pilots? A fleet would have a wide variety of pilot experiences. How does an Ops Manual mandate all this? $\endgroup$
    – curious_cat
    Nov 29, 2015 at 15:28
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    $\begingroup$ @curious_cat The Ops Manuals at the 747 carriers I flew for in the 1990s provided for different experience levels by specifying that for certain circumstances, the captain had to be flying the airplane. Further, insofar as required weather minmums, Captains with less than 100 hours as captain had higher minimums. However, no differentiation was specified for crosswinds. FWIW, in 747 simulators, we would occasionally crank the xwind up to 40-45 knots, and land without problems. I never made an actual landing in that much of a xwind. $\endgroup$
    – Terry
    Nov 29, 2015 at 19:43
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The maxima of wind gusts which need to be considered in the structural design of aircraft to achieve certification are up- or downdrafts.

FAR part 25.341 says:

The following reference gust velocities apply:

(i) At airplane speeds between V$_B$ and V$_C$: Positive and negative gusts with reference gust velocities of 56.0 ft/sec EAS must be considered at sea level. The reference gust velocity may be reduced linearly from 56.0 ft/sec EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec EAS at 50000 feet.

Since wind rarely flows into or out of the ground, vertical gust speeds are less of a concern during takeoff and landing. Here, lateral speeds (crosswind) and changes in horizontal speed (gradients due to microbursts) constitute the most important limits.

For crosswind limits, FAR part 25.237 states:

(a) For land planes and amphibians, the following applies:

(1) A 90-degree cross component of wind velocity, demonstrated to be safe for takeoff and landing, must be established for dry runways and must be at least 20 knots or 0.2 V$_{SR0}$, whichever is greater, except that it need not exceed 25 knots.

In the end, it is up to the pilot to decide whether to attempt to land in adverse conditions, to go around, or to fly to an alternate destination. This answer gives some criteria which can be used by pilots to decide if they better go around. Airports are closed rarely and only when severe weather approaches, such as thunderstorms and tornados, but then it is not due to a specific wind speed, but to the expected maximum speeds, which then will far exceed certification limits.

Vertical gusts become more stressing with speed, because at the low lift coefficient of high speed flight a vertical gust will produce a relatively higher increase in lift coefficient than at low speed. In extreme cases, the wing will stall due to the gust, which will limit the additional structural loads.

Horizontal speed gradients, which are caused by downdrafts hitting the ground and spreading out, are most harmful at low speed. First the aircraft will encounter a headwind, which slows it down, and then rapidly cross into a tailwind which will cause airspeed to drop, in extreme cases below stall speed. This can even crash a large airliner.

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