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Why the 250kt speed limit below 10,000ft?

Is this for improved “see and avoid” or is it related to the certification requirements for airliner windows to withstand a specific weight bird at a specified velocity?

Shoreham EGKA, United Kingdom

Yes, it is for see and avoid.

I think I heard/read something it is because of see-and-avoid with the idea that at a higher speed it is difficult to do so. However I am not sure where I heard/read it and if I am not mistaken.


I believe the 250kt speed limit is for ATC purposes in controlled airspace. It avoids large speed differences between traffic, making trajectories more predictable for radar vectoring. Only a guess.

There are actually two kinds of speed limits. One is that the Rules of the Air impose a 250 kt speed limit below FL100 on all traffic which are not provided with full separation from other traffic. That means that there is no speed limit in class A and B and in class C only VFR has a speed limit.

The other speed limit is for ATC purposes and published in the AIP. It is decided by the ANSP and can be lifted by ATC, while the first one can’t be.

Last Edited by Airborne_Again at 27 Jul 08:13
ESKC (Uppsala/Sundbro), Sweden

Depends where – see FAR 91.117 (a) : Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 mph).

Last Edited by Shorrick_Mk2 at 27 Jul 08:23

500kts closing rate (worse if you’re at 9k feet given we’re talking indicated air speed) gives you 15 seconds to react if you spot a plane at 2nm (which isn’t that easy). Good luck with that!


According to SERA, the speed limit is only “in force” for C, D, E, F and G. There is no speed limit for A and B. Maybe because it’s only in A and B that ATC has full separation of all flights?

The 250 knot speed limit below 10,000 feet is in large part applied because birds are found below 10,000 feet (I’ve seen Herons at 6500’). Larger aircraft are required to demonstrate bird strike resistance, windshields in particular, but not above Vc, which would be in the 250 knot range for many aircraft. So if you plow into a bird at 350 knots in your LearJet, you might not survive the impact. There were many first generation jet fighters succumbed to this.

Home runway, in central Ontario, Canada

@Pilot_DAR: so in airspace A and B there are no birds or separation is applied?

If it was an aircraft protection thing, it would be an operating limit of the aircraft, not the airspace. There are many cases where airspace A/B from the ground or below FL100 enables aircraft to exceed 250kt IAS. I have witnessed more than once in Germany ATC reminding an aircraft to reduce speed to 250kt or below. I assume that was the ATCO taking his knowledge about wind into consideration and not some automated system.

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