sunwriter
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§ 91.117 Aircraft speed

Sun Jan 13, 2019 6:43 pm

I have noticed recently that some commercial aircraft on decent are substantially exceeding the following.
§ 91.117 Aircraft speed.
(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.). (For reference, I live about 40 miles from a large airport, and the path for some aircraft to this airport intersects with the city I live in.) This started when I began hearing jet sounds occasionally that were somewhat similar to being at an airshow where military jets were making high speed passes. After using some commercially available, and a custom real-time based software tool, I was able to calculate that these commercial aircraft were substantially above 250 KIAS (~300 KIAS) speed and substantially below 10,000 ft (~8000 ft) altitude. It seems to me to be similar to someone going past a 45mph sign on the road while still doing 70mph, albeit slowing down at the time. The point is that doing 70mph as you "pass" the sign is still illegal, even if you are slowing down. I believe it would be the same with commercial aircraft.

My questions to those that may know are these.
Is there some regulation I am not aware of that permits this?
Is this just an acceptable practice and the FAA doesn't enforce?
Is there a reason some commercial airlines would use this practice? Fuel savings or something other?

Just curious,

Thanks.
 
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atcsundevil
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Re: § 91.117 Aircraft speed

Sun Jan 13, 2019 7:34 pm

sunwriter wrote:
My questions to those that may know are these.
Is there some regulation I am not aware of that permits this?
Is this just an acceptable practice and the FAA doesn't enforce?
Is there a reason some commercial airlines would use this practice? Fuel savings or something other?

While CFR 91.117 restricts aircraft speeds to 250 kts below 10,000ft, there are numerous exceptions. It doesn't apply more than 12nm from the coastline, it's dependent on aircraft characteristics, and exceptions are made for military aircraft, for example.

In this case, it's probably based on the procedure the aircraft are flying. If they are operating on a descend via clearance, there may be a fix to cross at 10,000ft and 250kts (just as an example). However, if they are given a speed to maintain prior to the fix, or if they are instructed to resume published speeds at that fix, the requirement isn't to cross that fix at 250K; they must begin their speed reduction to 250K from that fix, while also complying with subsequent altitude windows. Airplanes have a tough time slowing down and descending for reasons which should be obvious, so this can result in aircraft operating in excess of 250kts below 10,000ft, provided they are slowing to 250kts or less.

Aircraft are often given a speed to maintain or told to delete speed restrictions prior to fixes to prevent compression. A line of aircraft spaced in-trail while transitioning from enroute to the terminal environment need to maintain speeds to preserve spacing. In some cases if they were to slow prior to the fix, it could result in compression, which makes it difficult to preserve in-trail spacing.

Tl;dr — sometimes it's for ATC operational advantage, sometimes it's because the aircraft asks to go fast for as long as possible (cough, Southwest) and the controller obliges. Either way, it's all legal.
 
ilovelamp
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Re: § 91.117 Aircraft speed

Sun Jan 13, 2019 8:38 pm

How did you calculate their indicated air speeds? You’d have to know winds and temps aloft. Extreme temps and pressure can have huge impacts on TAS. Add in big tailwinds it all can add up to high ground speeds.

Are you sure your calculations didn’t produce ground speeds?


ILL
 
GalaxyFlyer
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Re: § 91.117 Aircraft speed

Sun Jan 13, 2019 8:41 pm

The TRUE airspeed at 10,000’ is about 288 knots, so if you watching Flight Aware, there’s that plus any wind. Not sure how you did the calculations. There’s not a traffic cop with a radar gun checking either, but from my experience, we try to get below 250 at 10,000.

GF
 
sunwriter
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Re: § 91.117 Aircraft speed

Sun Jan 13, 2019 9:10 pm

ilovelamp wrote:
How did you calculate their indicated air speeds? You’d have to know winds and temps aloft. Extreme temps and pressure can have huge impacts on TAS. Add in big tailwinds it all can add up to high ground speeds.

Are you sure your calculations didn’t produce ground speeds?


ILL


Thanks for the reply. Winds aloft and temps were factored in to the final number.
 
sunwriter
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Re: § 91.117 Aircraft speed

Sun Jan 13, 2019 9:18 pm

Thank you for the explanation. It all makes sense. Not sure why I haven't heard this in the past, maybe the route is closer to my house now, who knows. It's loud enough that my neighbors have noticed it as well, which is why I decided to post the question. Thx

atcsundevil wrote:
sunwriter wrote:
My questions to those that may know are these.
Is there some regulation I am not aware of that permits this?
Is this just an acceptable practice and the FAA doesn't enforce?
Is there a reason some commercial airlines would use this practice? Fuel savings or something other?

While CFR 91.117 restricts aircraft speeds to 250 kts below 10,000ft, there are numerous exceptions. It doesn't apply more than 12nm from the coastline, it's dependent on aircraft characteristics, and exceptions are made for military aircraft, for example.

In this case, it's probably based on the procedure the aircraft are flying. If they are operating on a descend via clearance, there may be a fix to cross at 10,000ft and 250kts (just as an example). However, if they are given a speed to maintain prior to the fix, or if they are instructed to resume published speeds at that fix, the requirement isn't to cross that fix at 250K; they must begin their speed reduction to 250K from that fix, while also complying with subsequent altitude windows. Airplanes have a tough time slowing down and descending for reasons which should be obvious, so this can result in aircraft operating in excess of 250kts below 10,000ft, provided they are slowing to 250kts or less.

Aircraft are often given a speed to maintain or told to delete speed restrictions prior to fixes to prevent compression. A line of aircraft spaced in-trail while transitioning from enroute to the terminal environment need to maintain speeds to preserve spacing. In some cases if they were to slow prior to the fix, it could result in compression, which makes it difficult to preserve in-trail spacing.

Tl;dr — sometimes it's for ATC operational advantage, sometimes it's because the aircraft asks to go fast for as long as possible (cough, Southwest) and the controller obliges. Either way, it's all legal.
 
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atcsundevil
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Re: § 91.117 Aircraft speed

Sun Jan 13, 2019 9:36 pm

sunwriter wrote:
Thank you for the explanation. It all makes sense. Not sure why I haven't heard this in the past, maybe the route is closer to my house now, who knows. It's loud enough that my neighbors have noticed it as well, which is why I decided to post the question. Thx

It might answer your question if you could determine which arrival procedure routes over your house. These speed/altitude windows could help shed some light. It's hard for me to be more specific when I don't know where you're referring to.
 
747Whale
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 1:52 am

sunwriter wrote:
ilovelamp wrote:
How did you calculate their indicated air speeds? You’d have to know winds and temps aloft. Extreme temps and pressure can have huge impacts on TAS. Add in big tailwinds it all can add up to high ground speeds.

Are you sure your calculations didn’t produce ground speeds?


ILL


Thanks for the reply. Winds aloft and temps were factored in to the final number.


Unless performance required the higher speed, ATC doesn't have authority to waive the 250 knots speed limitation. That means that either your calculations are incorrect, or the crews were violating the regulation.

I'd question your calculations first.

ATC does monitor speed; high or low, ATC will see it and will notice it. Not long ago I had 300 knots approaching Chicago. It was late, light traffic. I was given 10,000' and typically that is a point at which I'd slow to 250. I did slow to 250 as I was being vectored to a downwind, and the controller returned to ask why I'd slowed. I was told to increase to 300. As soon as cleared below 10,000, I didn't need further clearance to slow, and was expected to slow to 10,000.

There are clean speeds on departure when I need to be faster than 250 knots; my minimum speed may be 280, for example. I'll include "request high speed climb," and ATC will approve it, though I neither need to ask for it, nor can ATC approve a deviation: it's automatically approved if performance dictates. It's good form to ask, though.

On arrival at landing weights, I'll be at slower speeds.

The sound of traffic passing overhead won't really tell you about airspeed of the aircraft.
 
FlyHossD
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 5:57 am

sunwriter wrote:
§ 91.117 Aircraft speed.
(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.)...

My questions to those that may know are these.
Is there some regulation I am not aware of that permits this?
Is this just an acceptable practice and the FAA doesn't enforce?
Is there a reason some commercial airlines would use this practice? Fuel savings or something other?


I suspect you're confusing ground speed for indicated airspeed. At typical temperatures at 10,000', 250 knots is 280 knots true airspeed. So the groundspeed at 10,000'/250KIAS would be 280 knots of groundspeed assuming NO wind.

Your post reminds me of a phone call from an ATC Tower many years ago - we had just started service there. They wanted to know why weren't complying with 91.117b - 200 knots in their Class D. They, too, didn't understand the differences between indicated airspeed, true airspeed and groundspeed.
My statements do not represent my former employer or my current employer and are my opinions only.
 
Woodreau
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 4:08 pm

When ATC or an arrival procedure imposes a speed restriction, it doesn't really matter what the winds or ground speed are. As every aircraft is complying with the speed restriction in indicated airspeed, all of the aircraft on the arrival will all be doing the same speed. anyone that is faster or slower would be immediately apparent to a controller, as the aircraft that is not compliant with other aircraft will not maintain it's position in the arrival stream. - it will either catch up to the aircraft in front or aircraft behind will catch up to it.

It's like being on an interstate highway - if everyone was doing the exact speed - say the speed limit - there would be no need to pass anyone, because you in theory shouldn't catch up to the person in front nor should anyone pass you. But because you have people who don't obey the speed limit- there's always people looking to pass. And because there are people who drive well below the posted speed limit (especially in the passing lane), you will need to pass them.

Once your place is set on the arrival, that's where you are and you rarely pass anyone to get to the airport any faster than anyone else. Everyone is doing the same speed. The controllers have built-in spacing to account for aircraft in front slowing to approach speed on the approach, or transitioning from the arrival into terminal airspace.

Since ATC in the US has no way of telling what our indicated airspeed is, it was always comply with altitude first, speed second, if you have to comply with both speed and altitude but find yourself unable to do both. The transponder extended squitter today reports everything to ATC, indicated airpseed, indicated heading, altitude set in the altitude selector, among some items (not a comprehensive list). ATC in Europe sees the extra information and does know what your indicated speed is.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
IAHFLYR
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 4:31 pm

atcsundevil wrote:
Airplanes have a tough time slowing down and descending for reasons which should be obvious, so this can result in aircraft operating in excess of 250kts below 10,000ft, provided they are slowing to 250kts or less.

Aircraft are often given a speed to maintain or told to delete speed restrictions prior to fixes to prevent compression. A line of aircraft spaced in-trail while transitioning from enroute to the terminal environment need to maintain speeds to preserve spacing. In some cases if they were to slow prior to the fix, it could result in compression, which makes it difficult to preserve in-trail spacing.

Tl;dr — sometimes it's for ATC operational advantage, sometimes it's because the aircraft asks to go fast for as long as possible (cough, Southwest) and the controller obliges. Either way, it's all legal.


This is an interesting interpretation above of 91.117!! Please explain as "Paragraph "a", clearly states the regulation.....and as you mentioned "paragraph d" covers those exceptions, "(d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed."

Having worked a busy approach control for more than 29 years I have to agree with those who mentioned that you slow to 250 KIAS before leaving 10,000' MSL on the descent if you'd been given a higher speed previously. Thousands of times I'l tell someone to maintain 320 KIAS at 10,000' MSL or above and when I descended them below 10,000" MSL expected them to slow first to 250 KIAS, then descend below 10,000' MSL and that is exactly what happened, even with Southwest! :)

747Whale wrote:
I was told to increase to 300. As soon as cleared below 10,000, I didn't need further clearance to slow, and was expected to slow to 10,000.

There are clean speeds on departure when I need to be faster than 250 knots; my minimum speed may be 280, for example. I'll include "request high speed climb," and ATC will approve it, though I neither need to ask for it, nor can ATC approve a deviation: it's automatically approved if performance dictates. It's good form to ask, though.


:checkmark: :checkmark:
Any views shared are strictly my own and do not a represent those of any former employer.
 
GalaxyFlyer
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 4:58 pm

People need to remember that this is another regulation written in blood after several mid-airs in the Sixties involving high speed jets and small planes.
 
arcticcruiser
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 5:10 pm

[list=][/list]
GalaxyFlyer wrote:
People need to remember that this is another regulation written in blood after several mid-airs in the Sixties involving high speed jets and small planes.


True. But based on politics and available technology at the time. Speeds higher than 250 below 10.000´ work in the rest of the World... and these were not all small planes.
 
GalaxyFlyer
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 5:33 pm

In the rest of the world GA traffic at low altitudes is a small fraction of US traffic and well segregated from major airports. Ther is nothing like a TEB or Linden NJ airport in most countries. Then there’s bird strike protection to think of
 
LH707330
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 6:18 pm

Here's a good article on the topic: http://code7700.com/airspace_class_b_woes.htm
 
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Starlionblue
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Re: § 91.117 Aircraft speed

Mon Jan 14, 2019 11:47 pm

GalaxyFlyer wrote:
People need to remember that this is another regulation written in blood after several mid-airs in the Sixties involving high speed jets and small planes.


The strict, no exceptions, speed regulation below 10000 is rather an American thing though. In other parts of the world, you'll see higher speeds than 250 below 10000 all the time, subject to ATC clearance.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
GalaxyFlyer
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Re: § 91.117 Aircraft speed

Tue Jan 15, 2019 12:47 am

True and experienced many times being approved for high speed; anyone for 310 knots on downwind for visual to Berlin?

GF
 
BoeingGuy
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Re: § 91.117 Aircraft speed

Tue Jan 15, 2019 8:10 pm

Starlionblue wrote:
GalaxyFlyer wrote:
People need to remember that this is another regulation written in blood after several mid-airs in the Sixties involving high speed jets and small planes.


The strict, no exceptions, speed regulation below 10000 is rather an American thing though. In other parts of the world, you'll see higher speeds than 250 below 10000 all the time, subject to ATC clearance.


The real accident that motivated this regulation was the 1960 mid-air over Staten Island between the UA DC-8 and the TW Constallation. One of the contributing factor was that the DC-8 was hauling a** at a low altitude.

There are two exceptions in the US that I know about. One is for performance reasons as others have mentioned, such as a very heavy 747.

The other exception is that you can request a waiver for flight test purposes. A certain airplane manufacturer I know of will request waivers when they need to exceed 250 knots below 10,000 feet. One example is flying at Vmo during a TCAS test - which uses a truck with a Transponder to simulate the other airplane - to confirm that the aural alert is loud enough at high airspeed. Another example is during some GPWS testing, such as descending at 5000 fpm over a lake to test the SINK RATE and PULL UP alerts. You request a waiver in case you happen to exceed 250 knots during the dive and recovery, so you aren't in violation.
 
GalaxyFlyer
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Re: § 91.117 Aircraft speed

Tue Jan 15, 2019 8:31 pm

Yes, I remember that mid-air vividly as a youngster reading the news. The -8 flew thru the assigned holding fix, colliding with the Connie, bodies and debris spread all over NYC. Truly horrific. I think it also brought up the rule to slow to holding speed 3 minutes prior to the fix and the requirement to notify ATC of equipment failures. The UA crew was trying to identify the fix by cross-tuning the one operative VOR.

GF
 
eidvm
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Re: § 91.117 Aircraft speed

Sun Jan 20, 2019 5:25 pm

Starlionblue wrote:
GalaxyFlyer wrote:
People need to remember that this is another regulation written in blood after several mid-airs in the Sixties involving high speed jets and small planes.


The strict, no exceptions, speed regulation below 10000 is rather an American thing though. In other parts of the world, you'll see higher speeds than 250 below 10000 all the time, subject to ATC clearance.


This is true, I was very surprised to hear about how rigorously this was enforced in America, in the rest of the world is seen/used as more of an advisory or suggested guidance than a hard fast rule.

In fact at many airports in Europe if you're assigned a speed of 280kts, 300kts, 320kts etc in the descent you're expected to maintain it until advised (typically 6-7,000')or to inform ATC that you're slowing down.

Slowing down to 250 as you pass 10,000' in Europe without informing ATC if you've been assigned a speed has been known to result in delaying vectors to move you to the end of the queue as you're considered to have held others up as well as a slap on the wrist.

Quite the difference to learn of the American enforcement of 250kts as an actual limit.
 
barney captain
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Re: § 91.117 Aircraft speed

Sun Jan 20, 2019 6:21 pm

Some of the anecdotal explanations notwithstanding, as mentioned it’s simply a matter of Indicated Airspeed vs True Airspeed (+wind). Those aircraft, likely without exception are at 250 KIAS.
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