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Author Topic: Speed readings  (Read 9386 times)
bcrosby
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« on: October 05, 2005, 04:06:17 PM »

Why do some controllers ask for the speed in knots or in mach?

"Whats your speed" - "350 knots"

or

"Whats your mach number" "0.8"
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Jonathan_tcu
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« Reply #1 on: October 05, 2005, 04:45:52 PM »

Mach speeds I assume are greater than 500 knots.  I hear a lot of oceanic flights confirm their altitude and mach speeds for the crossing, when exiting Montreal's airspace, prior to handing off to Gander's airspace.  I think when I used to listen to the Killaloe sector in NOrth Bay on 121.22, I used to hear Toronto handing off flights to Montreal and mach numbers came up.
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FSS wannabe, just curious about stuff, that's all.
Jason
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« Reply #2 on: October 05, 2005, 04:50:56 PM »

Quote from: Jonathan_tcu
Mach speeds I assume are greater than 500 knots.  I hear a lot of oceanic flights confirm their altitude and mach speeds for the crossing, when exiting Montreal's airspace, prior to handing off to Gander's airspace.  I think when I used to listen to the Killaloe sector in NOrth Bay on 121.22, I used to hear Toronto handing off flights to Montreal and mach numbers came up.


Mach speed is not referred to being greater than 500 knots.  It's altitude specific.  In the states I believe the transition alt between KIAS and Mach is dependant on the aircraft; each aircraft has its own transition altitude.  Since true airspeed is higher at high altitudes, KIAS can be misleading at those high crusing altitudes.
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Jonathan_tcu
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« Reply #3 on: October 05, 2005, 04:58:45 PM »

I learned a lesson here.  Thanks for that.  I thought I would post this link I found about mac speeds:

http://www.lerc.nasa.gov/WWW/K-12/airplane/mach.html
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PHL Approach
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« Reply #4 on: October 05, 2005, 05:19:23 PM »

Last time I checked, Mach numbers are not converted by how fast your going but I believe at or above FL230. I heard that on a ZNY sector one night. Anyway, the main reason for asking them is to make sure the sequencing on a particular jet/victor airways isn't gonna be ruined by improper/various speeds. I'm going to explain how they make sure their spacing should remain within standards. Generally, you'll hear it go like this. Let's say there are 4 guys level at FL220 going southwest bound on J75 into DCA, BWI and IAD.

(Comair 5100 is climbing out of kennedy and is cleared direct COPES to join J75 and he will be in the lead of this group)


"New York Center, Comair 5100, 17,300 for Flight level 220"

"Comair 5100, New York Center, Roger. What's your speed in the climb?"

"We're holding 300, Comair 5100"

"Comair 5100, Maintain 300 knots or greater, your leading the pack"

"300 or better, Comair 5100"


(Meanwhile American 226 is 15 miles in-trail and gaining


"American 226, What's your speed?"

"310, American 226"

"American 226, Maintain 300 knots"

"300 on the nose, American 226"


(Another aircraft, Jetblue 449 is also 15 in trail of the American)


"Jetblue 449, What's your indicated?"

"We're at normal speed, given to us by the last sector, Jetblue 449"

"Jetblue 449, Maintain 300 knots for in-trail spacing into Dulles"

"300 knots, Jetblue 449"


(And the last aircraft in this group, at the back of the pack is United 990 who is about 12 in-trail of the Jetblue)


"United 990, What's your speed?"

"310 for United 990"

"United 990, Maintain 300 knots or less"

If you understand, the guy at the front should maintain a speed or greater. The guys in the middle maintain the speed alone, no more and no less. The guy all the way in the back can maintain that speed or slower. I have heard ZNY do this with packs up to 15 aircraft long. So this is a somewhat short example. Sorry for the essay.

Ed
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bcrosby
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« Reply #5 on: October 05, 2005, 05:34:28 PM »

Quote from: HPNPilot1200

Mach speed is not referred to being greater than 500 knots.  It's altitude specific.  In the states I believe the transition alt between KIAS and Mach is dependant on the aircraft; each aircraft has its own transition altitude.  Since true airspeed is higher at high altitudes, KIAS can be misleading at those high crusing altitudes.


Interesting.. is there a resource someplace that explains why this is so and how does the plane determine its mach speed?
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Jason
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« Reply #6 on: October 05, 2005, 05:37:08 PM »

Each aircraft has it's own transition altitude.  It's stated in the a/c's POH and saftey/performance charts/manuals.

Mach speed is not necessarily "calculated" by the pilot; It's displayed in the cockpit.
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davolijj
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« Reply #7 on: October 05, 2005, 09:59:34 PM »

And just to augment Ed and Jason's comments...

Mach speed references may only be used at or above FL240.
Controllers will frequently instruct an aircraft to "...maintain maximum forward speed," or "slowest practical speed."
The phraseology to maintain 300 knots or less would be "...do not exceed 300 knots."
There are recommended airspeeds used when slowing arrival aircraft for a sequence.  They are:

above 10,000 MSL - not less than 250
below 10,000 MSL (with more than 20 nm to the airport)
   210 for Jets
   200 for props
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JD
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« Reply #8 on: October 05, 2005, 10:25:09 PM »

Damn I was off by 1,000 feet or the controller was wrong with his info... Thanks for the clarification. Yea I have heard "Do not exceed 300 knots" before. But, I used the phraseology that I had grown up to hearing on ZNY, so Im used to it. Don't let Ms. Blakey find out!  evil I see your already picking up on your Enroute stuff for shipment to ZME  cheesy
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davys747
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« Reply #9 on: October 05, 2005, 11:39:09 PM »

For the 7110.65P...

http://www.faa.gov/atpubs/ATC/Chp5/atc0507.html#5-7-1

e. Express speed adjustments in terms of knots based on indicated airspeed (IAS) in 10-knot increments. At or above FL 240, speeds may be expressed in terms of Mach numbers in 0.01 increments for turbojet aircraft with Mach meters
(i.e., Mach 0.69, 0.70, 0.71, etc.).


As was previously stated, Indicated airspeed is calculated using by differences in pressures between the outside air and a pitot tube (located at the front of the aircraft). At higher altitudes, the air is not a dense, thus you get huge variations in the actual airspeed (called TAS or True airspeed) and the indicated airspeed (IAS). Im going to guess that Mach. Numbers (I assume everybody knows that a Mach number is a percentage of the speed of sound?) are easier to deal with than TAS at higher altitudes. Im not sure im not a pilot but im going to guess that it would be easier to separate on speed by saying M.81 or M.80 than it would be to say TAS of 464 and TAS or 459 or whatever.

Im just looking at a table of Mach 1 vs altitude (http://www.aerospaceweb.org/question/atmosphere/q0112.shtml). Its quite interesting. The speed of sound does some funny things. At about FL250, the value for M1 is 602kts.
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David Walsh
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« Reply #10 on: October 06, 2005, 08:30:49 AM »

It's probably obvious to everyone who follows aviation closely, but I didn't see it mentioned here for newer listeners.

Mach 1 is the speed of sound. It varies depending on altitude and atmospheric conditions. At sea level Mach 1 is "about" 720 mph.

If an aircraft reports his Mach speed as .83, then he's doing 83% of the required speed to break the sound barrier at his present altitude.
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binky
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« Reply #11 on: October 08, 2005, 09:20:29 AM »

The simple answer is the altitude that the aircraft is at.  In the low level regime (mid FL20's and below) controllers will reference indicated airspeed in knots because that is how the pilots reference their airspeed.  Into the higher FL's the controller and pilot will begin to reference Mach number as the primary measure of indicated airspeed.  Aircraft that are in the climb (like a previous example) may be given a speed to maintain (IAS) in knots but they must be given a mach number to transition to as they reach the higher FLs.  The 'transition' word you hear used by ATC has nothing to do with aircraft performance at all.  It is used to link the two measures of airspeed as an aircraft climbs or descends through the regime where speed measuring crosses over from IAS in knots to mach number.  For example, "AAL123 maintain FL330 speed 310, transition M.78" then "UAL123 maintain FL330 not above 300 kts and transition to M.77 or less".  The reverse happens for sequencing aircraft in descent.  ATC issues a mach number to maintain until transitioning to an indicated speed in knots.
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bcrosby
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« Reply #12 on: October 08, 2005, 09:26:28 AM »

I think it also has to do with keeping everyone on the same page.

500 kts at FL340 is not going to be the same speed as 500kts at FL450.

Mach speed takes into account temperature and air density. Therefore Mach 0.7 at FL340 is the same speed as Mach 0.7 at FL450
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JetScan1
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« Reply #13 on: October 08, 2005, 11:21:42 AM »

Quote
Therefore Mach 0.7 at FL340 is the same speed as Mach 0.7 at FL450


The only variable effecting the speed of sound/mach is temperature. If the temperature is different then the true airspeed for a given mach number will be different.

TAS = mach (644.3 X SQR (1 + (temp(C)/273)))

If the temperature at FL340 is -45C then mach 1 = 588 knots  (mach 0.70 = 412 knots)

If the temperature at FL450 is -60C then mach 1 = 569 knots  (mach 0.70 = 398 knots)

DJ
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bcrosby
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« Reply #14 on: October 08, 2005, 02:53:28 PM »

Quote from: JetScan1
The only variable effecting the speed of sound/mach is temperature.


Ahh yes... I stand corrected Tongue
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