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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 Oct 22 '16 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. $\endgroup$ – Jay Carr Oct 22 '16 at 12:41
  • $\begingroup$ I'm curious to know why you asked the question. $\endgroup$ – user6035379 Oct 23 '16 at 11:40
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The tricky part is that a de facto rule is unlike a de jure rule, it's hard to pinpoint its origin as it is not based in law. The more I dig, the more it seems it is indeed a de facto rule that found its way to Annex 11.

It would take a team of aviation historians hunting through countless meeting points to get to the bottom of it.

However, I gathered some clues, mostly in reverse chronological order:


For the 250 knots during climb I found:

ICAO's REVIEW OF NOISE ABATEMENT PROCEDURE RESEARCH & DEVELOPMENT AND IMPLEMENTATION RESULTS

Target climb speeds are given as V2 plus, no attempt was made to specify the actual target speed as it is accepted that the segment is flown to the manufacturer’s specified safety speed.

It just so happens 250 is a numerical rounding that matches most aircraft, except for heavies loaded up for long-haul flights.

Numerous parties were involved:

... various parties, including universities, regulatory agencies, manufacturers, air carriers and airports.

So everyone came together, and agreed to 250 knots. But that's recent and has to do with noise.

ICAO Journal TOWARDS A PERFORMANCE-BASED NAVIGATION SOLUTION

Pilots should also avoid airspeeds of more than 250 knots below 10,000 feet above ground level (AGL), especially at times of the year when birds are migrating. Aircraft speed is more critical than bird size (body mass) in causing collision damage.

Someone should tell the geese to stop flying at FL400. (Related post.)

Activities of the European Office from 1954 to 1958 (link)

They discussed the advent of the jet age, and how the prop aircraft speeds of 250 knots are being exceeded leading to research into the consequences.

At that time, namely what affected the ATC and navigation systems because jet planes were relatively crossing sectors much faster.

It's not much but this section hints at the origin of 250 knots:

... significant increase in cruising speed of the jet-aircraft over the propeller-driven airline [...] around 400 knots = 750 km/h versus 250 knots = 450 km/h ...

REPORT OF THE EIGHTEENTH MEETING OF THE APANPIRG ATM/AIS/SAR SUB-GROUP

Appendix 4 to Annex 11 speed limitations of 250 kt below 10 000 ft AMSL are required for certain airspace classes.

Annex 11 history:

In October 1945, The Rules of the Air and Air Traffic Control (RAC) Division at its first session made recommendations for Standards, Practices and Procedures for Air Traffic Control. These were reviewed by the then Air Navigation Committee and approved by the Council on 25 February 1946. They were published as Recommendations for Standards, Practices and Procedures—Air Traffic Control in the second part of Doc 2010, published in February 1946.

Chapter 2

2.6.3 The requirements for flights within each class of airspace shall be as shown in the table in Appendix 4.

Appendix 4 lists the now de facto limit.

It doesn't say why, so it's most likely it's what everyone agreed on at that convention and consequent meetings.

8 December 1959 is likely the date when an amendment was added to Annex 11 that detailed those figures in Appendix 4 we now know.

That's 8 years before the FAA. Check this post for the FAA's side of the story if interested.

Without access to meeting points, that's the most I could find.


Short version of the above:

At numerous conventions and meetings beginning in the 50's, many parties met again and again to discuss the jet-to-prop traffic separation, the noise impact of jet planes, and the dangers of bird strikes. They concluded V2 plus speeds should be used, but since civilian jet aircraft have the same performance more or less, the 250/10,000 rule found its way to the requirements of airspaces.

It's not hard evidence, but given what I found, it's not far fetched to assume the above is correct. Over to you.

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    $\begingroup$ That's some impressive digging you've been doing, thank you! $\endgroup$ – J. Hougaard Oct 23 '16 at 5:43
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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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