IAS vs. Mach Number

[BizPilot]

Question for you "Major" guys......(maybe an ATC question as well)

What dictates the use of either "IAS" or "Mach Number" when dealing with speed restrictions? In other words, why does ATC assign an "ISA" versus a "Mach Number" when decending inbound. What altitudes does the change-over occur at and why?

I was crusing along today and wondered why? Maybe the answer is in the AIM???

Thanks Dudes!

"BizPilot"
Cheyenne Capt'n

 

[Doberman]

They are identical at approx. 25500. Above this should be Mach, below should be IAS.

 

[jetblue320]

Although they are identical at about 25,500 (depending on temp. and density and a few other factors) it is generally accepted by ATC and us "major guys" that the transition from Mach to IAS occurs at about FL290. In other words, unless they request your speed in knots and you are at FL300 or above, chances are they want it in Mach.


Your mileage may vary, depending on whether or not the month has an "R" in it.

C yaaa

 

[FarginDooshbahg]

Yea, on the "R" months at my company, the change over occurs at FL270.

 

[C-150ETOPS]

99% of the time it comes with a descent clearance to an altitude <29-30K. We usually just "wag" it above that in Mach and do the requested at crossover.

Had a Fort Worth Center guy giving out assigned indicated speeds well into the mid 30's. 10 guys along with us just said "Raj', XXX Kts" , RJ guy gets the same thing and starts lecturing the controller about the IAS/MACH scientific legalese definition. ATC replied quickly something like pretty funny like "just fly it junior".

 

[8vATE]

What dictates the use of either "IAS" or "Mach Number" when dealing with speed restrictions?

I was crusing along today and wondered why? Maybe the answer is in the AIM???

Hey Cheyenne Capt'n..
It's a good question and to give you some more info on the why...
You may not be getting in to the speed/altitude regime where its a big factor unless you're tooling around in a Cheyenne 4 hotrod maybe?

Anyways... there is a "crossover" speed relating to IAS and MACH..

As you climb out next time.... maintain a set IAS.... your MACH will slowly climb.... say your manual says best economy climb is 280/.70.... you maintain the 280 until you reach the .70M... then you maintain the mach... if you just maintained 280 kts...your mach would continue to climb as you get higher while all the effects of mach takeover, mainly temperature and compressibility ...etc..

the same thing happens as you descend and are maintaining a set mach in the descent.. at some point you "crossover" to IAS.... maintain .70 mach until indicating 280.... your IAS will slowly increase in the descent to the 280 as the Mach effect diminishes...

Basically you gotta be up pretty high (as people have mentioned)...
Mid to high 20's .....

see ya around...

 

[321 busdriver]

If atc says " say airspeed, they want ias. If they want mach no., they will specifically ask for it. Above roughly 25,000 mach and ias diverge. It is much more accurate to give everyone the same ias for spacing purposes.

 

[G4G5]

You guys need to get a G5. It takes into account the pressures and temperatures and then calculates the exact Mach to IAS change over altitude. No math in public required.

 

[GoZags]

You guys need to get a G5. It takes into account the pressures and temperatures and then calculates the exact Mach to IAS change over altitude. No math inpublic required.

I'm sold. I'll pick one up Friday (after payday).

 

[rptrain]

when in doubt, look it up.

FAA Order 7110.65 - Air Traffic Control
http://nwp.natca.net/Administration/Documents/Airtrafficpublications/ATC/ATC0507.HTM

excerpt:
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.).

lots of other info, like:

NOTE-
1. On a standard day the Mach numbers equivalent to 250 knots CAS (subject to minor variations) are:
FL 240-0.6
FL 250-0.61
FL 260-0.62
FL 270-0.64
FL 280-0.65
FL 290-0.66.

 

[rptrain]

Reading this thread again, it becomes more absurd. Mach and airspeed diverge above FL250? Diverge from what? As if 250 knots equals mach 0.6 all the way down to sea level? Mach and IAS are never equal. Mach ranges (for most of us) from 0 to 0.85. When is that ever equal to IAS, other than sitting at the gate? Mach is a percent of the speed of sound. IAS is a much different measurement.

The only reason for using Mach at high altitudes is that IAS becomes so low that it is an inappropritate and unintuitive measurement of true airspeed.

 

[C-150ETOPS]

I don't know about others, but I've never heard a Mach# assigned less than .77

They must use that page of the ATC manual for doodling.

 

[cactuspilot]

The engineers at Boeing use 23,456'. The important thing to remember is when asked to "say speed" your answer should be "_ _ _ assigned".

It's a good thing Lincon didn't have an airplane.

 

[Stby One]

They are equal at 25500 feet??????? They are two totally different speed mesurements, one measures your dynamic pressure and one measures your TAS in relation to the speed of sound. As rptrain said before only at the gate are they the same (0).

The crossover altitude depends on temperature and IAS/MACH climb or descent schedule. If climbing at 250/.85 your crossover altitude will be probably be a good bit above 35000 feet. While as climbing at 340/.60 it would probably happen below 20000 feet.

 

[G4G5]

I don't know about others, but I've never heard a Mach# assigned less than .77

They must use that page of the ATC manual for doodling.

Then you have never flown a Hawaker or a Cessna Jet. For those flying speed bumps MMO is .77

 

[Flying Illini]

was given "slow to .82" the other day. That sucked.

 

[FarginDooshbahg]

I don't know about others, but I've never heard a Mach# assigned less than .77

I have been given .74 regularly when sequenced behind a 737. Not necessarily picking on 737s, it's just that there are a lot of them out there, and that always seems to be the number. Hey, many times when I was flying a 737, we were given the "maintain Mach .74 or less for sequencing" which later becomes "maintain 250 kts for sequencing." Or going into LAX it's "maintain 250 kts", 3 secs later "can you pick it up to 320" followed shortly thereafter by "slow to 250 kts" then about 5 seconds later this time "can you pick it up to 320"....and so on, and so on.

These days .74 or 320 would be a full-on speed rush.

 

[ex208driver]

I had iah center asked us for a gs readout then he proceeded to give that as a assignment to the next two guys behind us. It was a strange request but kept the spacing equal.

 

[C-150ETOPS]

I've done less than .77 plenty of times

FK100 was a speedbump too. I always loved our rocket scientists at the DFW masterbase flight planning a Janesvilles arrival into ORD at .65 Mach without any regard to the west coast DC10's behind house. We always blew it off and "rocketed" at .76/max IAS anyways.

Also did a Nat Track at .65 Mach in the Hawker. Really not a bad gig with suspect nav boxes. When you get passed 30 times by .86 Mach 747's directly above you, you know your on course Later on with even crappier 1st gen Litton's, we got out of a GNE just by a charming personality (or maybe some Shanwick holiday?) while flying a GII at the same relative speed as everyone else.

 

[SiuDude]

I had iah center asked us for a gs readout then he proceeded to give that as a assignment to the next two guys behind us. It was a strange request but kept the spacing equal.

I wonder why that's not done more often these days with GPS and all but it would require playing with the throttle a whole lot more, so, nevermind.