ATC: Knots vs. Mach?

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?

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.

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?

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

"Really Me" wrote in message
m...



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

Any reason?

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?

"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 pressu-- knots indication is inaccurate

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?

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?

"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 pressu-- knots indication is inaccurate

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