Real Answers from Real Pilots

WHY DO PLANES Have to go less thsn 250 knots under 10,000 ft?

I never knew. It is due to noise or other fctors?

Daniel,

It’s due to the fact the below 10,000’ there’s LOTS and LOTS of stuff to hit. GA aircraft, birds, terrain, buildings, towers and these really neat places called airports. Funny thing about airports is that planes from all over seem to congregate at the them. Ever look at a spoked wheel? The further away from the hub the more space you have but as you get closer things get REALLY tight hence the need to SLOW DOWN. Make sense?

Adam

I see about ga planes but birds don’t go up to 10,000 ft.

Big birds fly WAYYYY above 10000 feet boss

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I mean birds as animals not planes bro.

Wanna bet? https://www.livescience.com/55455-how-high-can-birds-fly.html

Adam

1 Like

Yes that’s what I mean bro, Animals not planes

Of course this rule does not apply in Europe. While the 250 below 10 rule is on the books, controllers routinely issue “free speed” clearances that allow a pilot to accelerate to any speed they wish. I usually keep it at 250 or less, but many pilots accelerate right on up to 320, which is about as fast as a jet airliner can go at that altitude.

Chris

Yes, they do.

Just to throw in some perspective from a former air traffic controller… it’s all about the aircraft congestion below 10,000’, especially VFR aircraft. A few reasons (but not all):

  1. Most VFR flights are operating at or below 10,000, and “see and avoid” is much more difficult if one of the aircraft is moving over 250 kts. Both in terms of making visual contact and having time for evasive action.
  2. VFR flights rarely stick to victor airways and other corridors. Going all over the place makes them less predictable, and therefore more of a potential hazard. Ties into the first point.
  3. Many VFR flights choose to not communicate with air traffic control, so there is less opportunity to provide traffic alerts. Back to point one again.
  4. Fast moving aircraft are hard to fit into the traffic picture when ATC is sequencing or providing separation in congested areas. While most aircraft capable of those speeds are going to be on some sort of arrival or transition route anyways - and will be restricted to speeds listed on the route - the 250kt restriction helps keep everybody on the same page, so to speak.

There are a lot of other reasons, and that’s a pretty simplified explanation, but the TLDR is that a fast moving aircraft in a lower, congested area is like the guy weaving through highway traffic in rush hour. It’s potentially dangerous, especially in busier areas.

TX,

Ok I’ve got a question. When I’m below 10,000’ and I’m doing 250 why do you guys tell me best forward speed? Are you trying to get me in trouble? :wink:

Adam

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TX,

Great points, thank you for adding that.

Chris

@Adam - Haha. Nah. It’s almost always because somebody is coming up behind you and they don’t want them to get too close.

Just to dig deeper - this might be more than you really wanted/cared to know - let me contrast what’s in the CFRs, which the pilots are reading, and what’s in the 7110.65, which is what the controllers are reading. First, the 7110.65:

A pilot operating at or above 10,000 feet MSL on an assigned speed adjustment greater than 250 knots is
expected to comply with 14 CFR Section 91.117(a) when cleared below 10,000 feet MSL, within domestic airspace, without notifying ATC. … The phrases “maintain maximum forward speed” and “maintain slowest practical speed” are primarily intended for use when sequencing a group of aircraft. As the sequencing plan develops, it may be necessary to determine the specific speed and/or make specific speed assignments.

And then, 14 CFR 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 m.p.h.).

The information furnished isn’t really the same, so there’s definitely room for confusion. Very few controllers read the CFRs or AIM, too. The 7110.65 is a lot to memorize as it is.

As far as pilots go, you probably (edit: might) have an out by considering an ATC instruction as implicit “authorization by the Administrator”… but unless you turn on the afterburners and go 400 kts, it’s unlikely ATC would complain if it bumps up to ~265. Then again, should “maximum” be interpreted as 250? It’s not in there. Thanks, FAA.

Of course, if there’s any question or you want to verify, go ahead and speak up. We were expected to know the approximate speeds that an aircraft is capable of, so if it’ll put you at 300 or something, they’re probably unaware.

Last bit… some controllers don’t use the phrase at all, partially for the reason you just said. I personally preferred “maintain # knots” and would save best forward for aircraft I knew could never break 250, like a Caravan.

TX,

Thank you for that and no, it’s not more than I wanted, I love this stuff. On occasion (when I’m feeling froggy) I will query “so are you saying I can exceed 250?”. It’s usually followed by a long pause and then a “NO” :slight_smile:

Adam

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TX,

Have you ever heard of this old one? I heard a Houston TRACON controller issue “free speed” to a Speedbird a few months ago, so perhaps this is still in effect.

FDC 2/1957 - TX… EFFECTIVE IMMEDIATELY UNTIL FURTHER ADVISED. PURSUANT TO A SPECIAL DELEGATION OF AUTHORITY TO GRANT WAIVERS TO CODE OF FEDERAL REGULATIONS (CFR), PART 91, THE FAA SOUTHWEST REGION AIR TRAFFIC DIVISION MANAGER HAS WAIVED CFR 91.117A, (250 KNOT SPEED LIMIT) FOR DEPARTING AIRCRAFT IN THE HOUSTON, TEXAS APPROACH CONTROL AIRSPACE FOR THE PURPOSE OF TESTING THE EFFECT OF INCREASED DEPARTURE SPEEDS ON THE AIR TRAFFIC CONTROL ENVIRONMENT. NOTICE, AIRCRAFT DEPARTING FROM AIRPORTS IN HOUSTON APPROACH CONTROL AIRSPACE MAY BE AUTHORIZED TO EXCEED THE 250 KNOT SPEED RESTRICTION CONTAINED WITHIN CFR 91.117A, AT THE DISCRETION OF AIR TRAFFIC ONTROL (ATC). HOUSTON ATC WILL BE PERMITTED TO ASSIGN/ AUTHORIZE SPEED IN EXCESS OF 250 KNOTS TO DEPARTING AIRCRAFT USING PHRASEOLOGY “NO SPEED LIMIT” OR “INCREASE SPEED TO (NUMBER) KNOTS.” THIS TEST IS FOR DEPARTURE TRAFFIC ONLY AND MAY BE TERMINATED AT ANY TIME BY ATC. QUESTION SHOULD BE DIRECTED TO HOUSTON APPROACH CONTROL, PLANS AND PROCEDURES DEPARTMENT, AT 281-230-8400. WIE UNTIL UFN

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I remember that one!

Adam

@Adam - Good! Keep them honest. I usually appreciated those pilots.

@Chris - No, that’s really interesting. Know if it is current? I have a couple friends at Houston TRACON, I should ask them about it. I wonder if a/c are following the rest of the SID profile and just disregarding speed restrictions, or are given vectors when traffic is clear enough, or… lots of questions on that one.

Do you remember if Speedbird read back how fast he’d be going? I’m don’t remember climbout profiles well enough anymore to know how much of a benefit this would offer.

TX,

I do not know if it is current or not and I do not remember what speed Speedbird went. I would be interested to hear what your Houston friends have to say about this.

Chris