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In certain airspace classes, a speed limit of 250 knots IAS is imposed for aircraft flying below FL100/10.000FT, according to the airspace classes defined by ICAO. How did this limit find its way into the ICAO SARPS?

The answer to this question gives a good reason why such speed limits were introduced in the USA by the FAA. I am interested in finding out why the same speed limits were de facto rolled out world wide by being included in the ICAO SARPS. Did the ICAO include the 250 KIAS below FL100 restriction for the same reason the FAA did, or are there other (additional) reasons?

For example, was this done to give pilots a better chance of spotting other traffic? To reduce the severity of bird strikes? To provide consistency with FAA rules? To allow more efficient air traffic management? To reduce noise? Or for other reasons entirely?

I am looking for actual sources on this matter, not guesses and opinions.

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    $\begingroup$ That seems to provide two main advantages in a busy area at a moment pilots have a heavy workload: All aircraft have limited speed differences in the horizontal plane (easier ATC separation), and vertical rate is reduced (more time for ATC and crews to react). This may be related to the higher number of slower VFR flights. Just a guess. $\endgroup$
    – mins
    Commented Oct 22, 2016 at 9:59
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    $\begingroup$ Part of it could simply be that most of the slower planes out there (GA, single engine, prop planes) can't fly much higher than 10,000 (without supplemental oxygen at least). So above 10k jets don't need to worry about planes that fly almost 500knots slower, but below 10k, there's a chance there are much slower aircraft you may be flying with. $\endgroup$
    – Jae Carr
    Commented Oct 22, 2016 at 12:41
  • $\begingroup$ I'm curious to know why you asked the question. $\endgroup$ Commented Oct 23, 2016 at 11:40

2 Answers 2

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VFOP (1985)

The speed limits are in Appendix 4 to ICAO SARPs Annex 11, which is all about the A–G airspace classes, and this is the clue I've been missing all along, because the ICAO airspace classification compared to the Jet Age is relatively new.

The speed limit in USA became effective in 1967 (as you linked), but for ICAO it was two decades later:

In 1985, the ICAO Visual Flight Rules Operations Panel (VFOP) proposed a new system of airspace classifications aimed at achieving a common standard of rules and service [...] The VFOP introduced a new[note a] concept of a speed limit of 250 kts for certain flights below FL100 (10 000 ft) in some classes of airspace, the philosophy being that where separation was not wholly provided by ATC, and visual flight in low visibilities was permitted, higher speeds would greatly reduce the ability of the pilot to see and avoid other traffic.

— Walker, W. J. V. "UK Airspace Planning–The New ICAO Airspace Classification System." The Journal of Navigation 46.3 (1993): 336-342. pp. 336–337.

So, finally, this confirms it was "done to give pilots a better chance of spotting other traffic" as you proposed in the question, and also establishes the when.


note a:

Assuming "new" is vague:

New in the UK since this is a University of Cambridge publication?
New for being limited to "certain flights"?[note b]
New because it's for "some classes of airspace" and not all?[note b]
Wholly new for ICAO?

It doesn't matter; 1985's VFOP is the reason for what the SARPs say as asked.

note b:

The "some" and "certain" are because this ICAO speed limit does not apply in Classes A and B, and to IFR in Class C. (Page attached.)

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The reasons for this restriction are the same- to give better chance of spotting traffic, reducing noise, preventing bird-strikes from compromising the structural integrity of the aircraft, consistency, etc. A Canadian govt. consultative document has some details (I'm unable to find the original though):

The 250 KIAS maximum airspeed below 10,000 feet MSL was a rule of flight that was introduced a number of years ago to address the bird strike hazard of modern high-speed aircraft.

Air Traffic Control adopted this maximum airspeed below 10,000 feet MSL as an air traffic management tool ...

It is also noted that ICAO rules are identical to the FARs

The primary reason appears to be safety, from the view of bird strikes and also in spotting the aircraft. Another aspect is the use of Airborne Collision Avoidance System (ACAS). From the ICAO Airborne Collision Avoidance System (ACAS) Manual:

The surveillance range of ACAS has been designed to ensure that sufficient warning time can be provided for collision avoidance even in environments with heavy electromagnetic activity. This assumes that each aircraft does not have an excessive True Air Speed (TAS). Below FL 100 ACAS warning times will only be guaranteed when both aircraft have TAS less than 250 kts.

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