Mach Numbers or Knots

[MightyGem]

Listening in to ATC as SLF on a United flight in the USA recently, ATC were requesting speeds as Mach numbers when at cruising levels, but as soon as the descent started they then used knots.

Why the difference?

[Superpilot]

Take 30 minutes to learn how airspeeds are related. You need to have an appreciation of Indicated, True and Mach airspeeds and how one changes with respect to the other as you climb and descend. I will try to state the most basic reason (in lay terms) but this can make up for a detailed discussion.

Flying at a constant Mach speed during the descent will push your Indicated airspeed into the "overspeed" range and you will probably rip your wings off within 5 minutes of commencing the decent. Decending at a constant Indicated airspeed (the speed ATC gives), results in the aircraft's True and Mach airspeeds being gradually reduced during the descent therefore ensuring the aircraft flies within the speed boundaries it was designed to.

[hollingworthp]

Chicken Tikka Masala - works for me every time ;)

[Nimer767]

Superpilot (http://www.pprune.org/members/32074-superpilot) I Agree !

[BOAC]

MightyGem - I guess your plod chopper doesn't quite reach mach numbers, so to help you further (without the Indian take-away), jet a/c in general fly an IAS in climb and descent until/from about FL280 (I don't suppose you know where that is :)), hence the 'change' in the R/T

[Superpilot]

To be honest I'm surprised that this topic does not feature at all for any CPL/ATPL(H) theory :eek:

And before anyone reminds me how low choppers actually fly, I will remind you how much surplus "knowledge" there is in ATPL(A) theory :ugh:

[MightyGem]

Flying at a constant Mach speed during the descent will push your Indicated airspeed into the "overspeed" range

Thanks, SP, I am familiar with that concept, but the question was more aimed at why the reference to mach numbers at cruising levels. Surely two aircraft at widely separated FLs, flying at the same mach number will/could have differing IAS/TASs, so why not using IAS anyway?

[One Outsider]

Once you are above crossover altitude you climb/descend at a constant Mach number so using IAS as a speed reference would be pretty useless. Just as having cruising traffic use IAS and traffic changing levels use Mach would be. Changing back and forth between the two every time you change level is an unnecessary additional complication.

[TopBunk]

In cruise, we may have 190 knots indicated, but be doing Mach .84.

Sh1t dude, you must be seriously high for that!

I have M0.85 at FL430 as 240 kts in a B747 on camera!

[MightyGem]

In cruise, we may have 190 knots indicated, but be doing Mach .84. At another altitude, we may be showing 220 indicated, and still flying .84 Mach.
So given that those could be two different aircraft, both reporting .84 to ATC at different altitudes, would ATC be aware of the airspeed differential between the two, especially if they are on the same track/route?

[MightyGem]

OK, thanks.

[One9iner]

I P
C C
E D
T

-ectm+
+ectm- :d

[Northbeach]

MightyGem,

Did you get an answer?

I will try and make this simple without totally assaulting the topic. Most primary airspeed indicating instruments sample air pressure and convert the data into speed (how fast I am flying through the air-not over the ground). We use knots and miles per hour simply because of past practice and evolution of travel and they are the scales we are familiar with.


Ok simple enough, what happens when the jet flies up into the higher atmosphere where the air is much less dense? At some point simply measuring air pressure down the throat of an air sampling tube no longer yields as much useful information because the air is much less dense. (Remember this is a simplified explanation, so I am skipping the concept of static pressure).

The airspeed instrument is designed to display both knots and a Mach number, how fast I am flying in relation to the speed of sound under the current conditions.

The place where the sample ceases to display useful information called the cross over altitude, and it varies between airplanes types.

From the surface to this cross over altitude the instrument displays useful airspeed information in knots, and the pilots usually fly in knots. Above the cross over altitude the pilots pay more attention to the Mach number display as it reads tenths, hundredths and thousandths. The instrument will still function correctly but it is calibrated to display speed information differently at different altitudes.

At the higher altitudes ATC assigns mach numbers, as they are easier to fly because our display shows our Mach speeds in 3 digits. At altitude the airspeed needle may move a fraction of a distance, but it would be impossible for me to actually fly the extremely small variations. But I can easily read the 3 digits and make the necessary changes.

[Auto-throttles make all of this so much easier.]

Down lower in the atmosphere (below the cross over altitude) at the altitudes where the air is much denser the scale on my airspeed indicator is larger allowing me to see and fly airspeed in knots much easier. Mach number varies with temperature, so assigning two aircraft the same Mach number at different altitudes will likely not result in speeds being matched. So down lower in the atmosphere it is easier to assign airplanes airspeed in knots.

Hopefully you will find the above useful.

Respectfully,

Northbeach

[MightyGem]

Thanks, Northbeach, just the job.

[shitzig]

Hi,

I was thinking it could have to do with Vmo/Mmo,

consider an acft with
Vmo 350Kts and Mmo 0.84 that cruises at 0.82
climb speed: 310kts until crossover, 0.78 mach afterwards



During climb you should observe that the speed limit for the aircraft is first achieved in knots, rather than mach. to go near 0.84 at low altitude would totaly burst the 350 Knots as low as 20000 feet.

when you reach the crossover is when the chosen speeds encounter themselves and now you can fly mach because at a steady mach your knots will decrease
from that point you should observe your mach limitation, wich is now more easy to burst than the 350 knot

then you cruise at mach 0.82 and notice the airspeed has fallen to eg. 260 kts

so i believe this has to do with finding a way to keep airplane inside its limits.

just my 2 cents