ATC: Knots vs. Mach?

[AES/newspost]

Listening to audio channel 9 on United, at higher altitudes and during
cruise I hear ATC say things like "What's your mach?" and "Maintain
mach 77" and at lower altitudes and during approaches "slow to 180" or
"maintain 250 for spacing".

I appreciate that these are occurring in quite different ranges of
operation and the units involved are very different in scale, but I'm
still curious about the potential confusion, or at least possibility for
same, in using two different sets of units for the same quantity
(especially when you recall incidents where confusion over units has led
to loss of satellites, or airliners running out of gas).

Do different levels of ATC consistently use only one of these units and
not the other? Do cockpit instruments read in both units, or are there
two separate "speedometers" (or a switch that has to be thrown to read
in one or the other)? What's the smallest _civilian_ aircraft that will
have a mach meter? Do prop or turboprop airliners have a mach indicator?

[Really Me]

AES/newspost wrote:

> Listening to audio channel 9 on United, at higher altitudes and during
> cruise I hear ATC say things like "What's your mach?" and "Maintain
> mach 77" and at lower altitudes and during approaches "slow to 180" or
> "maintain 250 for spacing".
>
> I appreciate that these are occurring in quite different ranges of
> operation and the units involved are very different in scale, but I'm
> still curious about the potential confusion, or at least possibility for
> same, in using two different sets of units for the same quantity

I don't think a UA pilot is going to confuse 250 knots with 2.5 Mach or
180 Mach any more then he will confuse mach 77 (.77) with 77 knots.

[BTIZ]

IIRC.. from controlling traffic more than 25 years go..

Speed control for spacing, must use Mach numbers above FL290.. not Indicated
airspeed in knots.

Below FL290, or was it FL250, use of IAS in knots is standard..

Ref: 7110.65 (which I do not have handy right now)

BT

"AES/newspost" > wrote in message
...
> Listening to audio channel 9 on United, at higher altitudes and during
> cruise I hear ATC say things like "What's your mach?" and "Maintain
> mach 77" and at lower altitudes and during approaches "slow to 180" or
> "maintain 250 for spacing".
>
> I appreciate that these are occurring in quite different ranges of
> operation and the units involved are very different in scale, but I'm
> still curious about the potential confusion, or at least possibility for
> same, in using two different sets of units for the same quantity
> (especially when you recall incidents where confusion over units has led
> to loss of satellites, or airliners running out of gas).
>
> Do different levels of ATC consistently use only one of these units and
> not the other? Do cockpit instruments read in both units, or are there
> two separate "speedometers" (or a switch that has to be thrown to read
> in one or the other)? What's the smallest _civilian_ aircraft that will
> have a mach meter? Do prop or turboprop airliners have a mach indicator?

[Really Me]

BTIZ wrote:
> IIRC.. from controlling traffic more than 25 years go..
>
> Speed control for spacing, must use Mach numbers above FL290.. not Indicated
> airspeed in knots.
>
> Below FL290, or was it FL250, use of IAS in knots is standard..
>
> Ref: 7110.65 (which I do not have handy right now)
>

At or above FL240, express speed (IAS) in Mach
Below FL 240, express speed in knots

The document I read indicates the unit should still be given, like
"Maintain two eight zero knots" or "Increase speed to Mach point 72"
http://www.faa.gov/atpubs/ATC/Chp5/atc0507.html#5-7-2

[Mark Hewitt]

"Really Me" > wrote in message
m...
>
>

> At or above FL240, express speed (IAS) in Mach
> Below FL 240, express speed in knots

Any reason?

[Nathan Young]

On Wed, 07 Jul 2004 19:29:06 -0700, AES/newspost
> wrote:

>Listening to audio channel 9 on United, at higher altitudes and during
>cruise I hear ATC say things like "What's your mach?" and "Maintain
>mach 77" and at lower altitudes and during approaches "slow to 180" or
>"maintain 250 for spacing".

For the jet crew out there.... What's the transition point between
mach and airspeed callouts? FL180?

[Icebound]

"Mark Hewitt" > wrote in message
...
>
> "Really Me" > wrote in message
> m...
> >
> >
>
> > At or above FL240, express speed (IAS) in Mach
> > Below FL 240, express speed in knots
>
> Any reason?
>
>

high altitude, low air pressure:-->> knots indication is inaccurate

[kage]

For us it is variable. Mostly with temperature. For instance, we normally
climb at 270 knots until M 0.75, and continue referencing Mach.

Also, if you select a target speed with the FMS it is IAS until 24,350 and
then the numbers automatically turn to Mach by the computer.

I've never really though too much about it. If ATC wants your speed you give
them IAS. If they want your Mach, they ask for Mach. If a chart is
referenced IAS, we use IAS. Some charts are referenced Mach, then of course,
Mach is used.

Karl

"Nathan Young" > wrote in message
...
> On Wed, 07 Jul 2004 19:29:06 -0700, AES/newspost
> > wrote:
>
> >Listening to audio channel 9 on United, at higher altitudes and during
> >cruise I hear ATC say things like "What's your mach?" and "Maintain
> >mach 77" and at lower altitudes and during approaches "slow to 180" or
> >"maintain 250 for spacing".
>
> For the jet crew out there.... What's the transition point between
> mach and airspeed callouts? FL180?

[Charles Newman]

In Flight Simulator, I use knots below 14,000 and Mach above 14,000.
Below 14,000
true airspeed and indicated speed are pretty close, but above 14000, they
start to drift
apart quite rapidly.


"BTIZ" > wrote in message
news:yC4Hc.15044$z81.6157@fed1read01...
> IIRC.. from controlling traffic more than 25 years go..
>
> Speed control for spacing, must use Mach numbers above FL290.. not
Indicated
> airspeed in knots.
>
> Below FL290, or was it FL250, use of IAS in knots is standard..
>
> Ref: 7110.65 (which I do not have handy right now)
>
> BT
>
> "AES/newspost" > wrote in message
> ...
> > Listening to audio channel 9 on United, at higher altitudes and during
> > cruise I hear ATC say things like "What's your mach?" and "Maintain
> > mach 77" and at lower altitudes and during approaches "slow to 180" or
> > "maintain 250 for spacing".
> >
> > I appreciate that these are occurring in quite different ranges of
> > operation and the units involved are very different in scale, but I'm
> > still curious about the potential confusion, or at least possibility for
> > same, in using two different sets of units for the same quantity
> > (especially when you recall incidents where confusion over units has led
> > to loss of satellites, or airliners running out of gas).
> >
> > Do different levels of ATC consistently use only one of these units and
> > not the other? Do cockpit instruments read in both units, or are there
> > two separate "speedometers" (or a switch that has to be thrown to read
> > in one or the other)? What's the smallest _civilian_ aircraft that will
> > have a mach meter? Do prop or turboprop airliners have a mach
indicator?
>
>

[Charles Newman]

"Icebound" > wrote in message
able.rogers.com...
>
> "Mark Hewitt" > wrote in
message
> ...
> >
> > "Really Me" > wrote in message
> > m...
> > >
> > >
> >
> > > At or above FL240, express speed (IAS) in Mach
> > > Below FL 240, express speed in knots
> >
> > Any reason?
> >
> >
>
> high altitude, low air pressure:-->> knots indication is inaccurate

I find, in Flight Simulator, that accuracy starts dropping off at
14,000 feet. MSL.

>
>

[Capt.Doug]

>"AES/newspost" wrote in message > I appreciate that these are occurring in
>quite different ranges of
> operation and the units involved are very different in scale, but I'm
> still curious about the potential confusion, or at least possibility for
> same, in using two different sets of units for the same quantity

Controllers will preface the number with the appropiate scale. Additionally,
Mach will be a 2 digit number whereas Airspeed will be a 3 digit number.

> Do different levels of ATC consistently use only one of these units and
> not the other?

No

> Do cockpit instruments read in both units, or are there
> two separate "speedometers" (or a switch that has to be thrown to read
> in one or the other)?

Both units are on the same instrument simultaneously.

> What's the smallest _civilian_ aircraft that will
> have a mach meter?

The 6 passenger CitationJet comes to mind.

>Do prop or turboprop airliners have a mach indicator?

Most do not. Props are not efficient at airspeeds of Mach concerns.

D.

[Capt.Doug]

>"Nathan Young" wrote in message > For the jet crew out there.... What's
the >transition point between
> mach and airspeed callouts? FL180?

As we climb, the outside air temperature drops. As the temperature drops,
the speed of sound drops. We climb at indicated airspeed (not true airspeed)
until mach climb airspeed matches indicated airspeed, at which point we
transition over from indicated airspeed to mach airspeed. For the jet I fly,
the transition occurs around FL240 (24,000') depending on outside air
temperature.

D.

[Darrell]

Some flight guidance and control systems automatically switch from IAS to
Mach at a given Flight Level. Example would be the MD-80 DFGC. If the
IAS/Mach switch is pushed below FL 270 it shows and controls current IAS.
If the same switch is pushed above FL 270 is shows and controls current
Mach.

--

B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"Mark Hewitt" > wrote in message
...
>
> "Really Me" > wrote in message
> m...
> >
> >
>
> > At or above FL240, express speed (IAS) in Mach
> > Below FL 240, express speed in knots
>
> Any reason?
>
>

[Leopold Hamulczyk]

Yes, but remember that Filght Simulator is not a good benchmark. Flight Sim
is "dumb", it doesn't know anything. It uses lookup tables, not aerodynamic
modelling, for aircraft performance.

"Charles Newman" > wrote in
message ...
>
> In Flight Simulator, I use knots below 14,000 and Mach above 14,000.
> Below 14,000
> true airspeed and indicated speed are pretty close, but above 14000, they
> start to drift
> apart quite rapidly.
>
>
> "BTIZ" > wrote in message
> news:yC4Hc.15044$z81.6157@fed1read01...
> > IIRC.. from controlling traffic more than 25 years go..
> >
> > Speed control for spacing, must use Mach numbers above FL290.. not
> Indicated
> > airspeed in knots.
> >
> > Below FL290, or was it FL250, use of IAS in knots is standard..
> >
> > Ref: 7110.65 (which I do not have handy right now)
> >
> > BT
> >
> > "AES/newspost" > wrote in message
> > ...
> > > Listening to audio channel 9 on United, at higher altitudes and during
> > > cruise I hear ATC say things like "What's your mach?" and "Maintain
> > > mach 77" and at lower altitudes and during approaches "slow to 180" or
> > > "maintain 250 for spacing".
> > >
> > > I appreciate that these are occurring in quite different ranges of
> > > operation and the units involved are very different in scale, but I'm
> > > still curious about the potential confusion, or at least possibility
for
> > > same, in using two different sets of units for the same quantity
> > > (especially when you recall incidents where confusion over units has
led
> > > to loss of satellites, or airliners running out of gas).
> > >
> > > Do different levels of ATC consistently use only one of these units
and
> > > not the other? Do cockpit instruments read in both units, or are
there
> > > two separate "speedometers" (or a switch that has to be thrown to read
> > > in one or the other)? What's the smallest _civilian_ aircraft that
will
> > > have a mach meter? Do prop or turboprop airliners have a mach
> indicator?
> >
> >
>
>

[Bushy]

> > > > in one or the other)? What's the smallest _civilian_ aircraft that
> will
> > > > have a mach meter? Do prop or turboprop airliners have a mach
> > indicator?

Must get a new ASI for my "Hovey Delta Bird", maybe I can get one with both
scales?
;<)
Peter