ATC giving Mach number vs speed in kts

[PT6ER]

Flying into San Francisco I was listening to the ATC channel since the movie was cr#p!

I noticed that during the cruise portion of the flight the ATC asked for speeds using mach numbers but as we approached San Franciso they changed to knots. I suspect height had something to do with it....can any body out there educate me as to why the different speed nomenclatures??

Thanks in advance.

[ifleeplanes]

As you climb up to about 26000 ft you will fly a constant airspeed in knots an IAS (indicated airspeed). As you climb keeping this IAS constant the Mach Number will be slowly increasing. There becomes a point where you cannot fly at this IAS in knots anymore since you will overspeed due to the mach number increasing beyond the aircraft capability. At this point you now climb at a constant Mach No and the airspeed in kts will decrease slowly as you continue to climb.

The reverse occurs in decent....hope this explains it a little.

[BOAC]

Have a look at Tech Log (http://www.pprune.org/forums/showthread.php?s=&threadid=167609&highlight=Mach+AND+number) and Tech Log (http://www.pprune.org/forums/showthread.php?s=&threadid=165531&highlight=Mach+AND+number) and This one................... (http://www.pprune.org/forums/showthread.php?s=&threadid=146658&highlight=Mach+AND+number)

[Byrna]

I read the postings from the TechLog but, not being a pilot and being unfamiliar with the theory, I'm having some trouble understanding something really basic here: WHY does IAS go down when MACH remains stable above the transition altitude? I know pressure and temperature are involved but I'm unclear as to how they effect MACH versus Indicated airspeed. In other words to be more precise - what is it about the CHARACTERISTICS of MACH versus IAS which causes this phenomenon?

Also, what exactly is CAS? (is it constant airspeed?)

Any help would be appreciated.

John

[ifleeplanes]

CAS is Calibrated Airspeed. Airspeed indicators have various errors and each has to be taken into account to give an accurate reading of airspeed. An airspeed indicator features a static vent to maintain a pressure equal to atmospheric pressure inside the instrument. Position and placement of the static vent along with angle of attack and velocity of the aircraft will determine the pressure inside the airspeed indicator, and thereby, the amount of calibration error of the airspeed indicator. Therefore, a calibration error is specific to an aircraft's design.

The speed of sound (Mach 1) gets slower as the temperature decreases. At sea level and 15 degrees C its approx 660kts. Far faster than a jet passenger aircraft can fly so it is irrelivant. As the temperature decreases the speed of sound gets slower and at some point it becomes more limiting to the aircraft than the airspeed in knots. Aircraft have many limiting sppeds two of them are the maximum in knots and another is the maximum mach number.

[Byrna]

Hello ifleeplanes,

So, if I understand this correctly, the structural stress is caused by the aircraft approaching the speed of sound, or more precisely, its own maximum MACH limit as it climbs higher and higher. Can IAS be considered a "theoretical" airspeed as it is, after all, indicated and represents the speed the aircraft "should" fly at; so keeping IAS constant with the speed of sound going down with temperature means the aircraft is approaching its speed limit faster at higher altitudes.

Hence, in using a constant MACH above the transition altitude, we are keeping the "real world" speed limit within the design specs of the aircraft. Am I right?


John

[Teddy Robinson]

Dunno if the theories have smothered the question.

ATC ask us to fly at specific Mach Numbers during the cruise and specific indicated airspeeds (knots) for decent and approach both indicies being used for seperation and sequencing in that order.

Mach is a stable indicator of cruise velocity, and knots a stable yet easily variable indication of velocity during descent/approach.

Well thats my take on it .. an ATC practictioner could prolly clear this one up far better than one who just flies at the mach/speed one is requested.

TR

[ifleeplanes]

Bryna- I am trying hard to keep it simple to understand so bear with me.The main 2 main limiting speeds we are dealing with here are the IAS (indicated airspeed-what we see on the airspeed indicator) in knots and the IAS as a Mach Number.

At low level where the speed of sound is around 660kts the passenger jet aircraft cant reach the limiting mach number (typicaly in the region of 0.84M) since the limiting IAS in knots will be in the region of 340Kts ie way below the 555 kts that would equate to the mach limit of 0.84M.

We will generaly climb at the best speed for the economy of the flight, typicaly 290kts (well below the limit of 340kts). As we climb the temperature decreases. Just as we dont want to fly AT the max speed in knots we dont want to fly AT the max mach number so we will climb and cruise at a lower mach number ie 0.79M. As the temperature decreases the speed of sound decreases until at some point (usualy about 26000ft) the speed we were climbing at (290kts) is the same as the Mach No we will climb at (0.79M) At this point if we continued to climb at 290kts the mach number we are at would continue to increase and we would eventualy bust the limiting mach number. We therefore stop climbing at a speed constant speed in knots and climb at a constant mach number. As we continue to climb at this mach number the speed in knots now starts to fall.


The problem with high airspeeds in knots is generaly due to problems with the structural strength of an airframe where the problem with high mach numbers is due to problems with the way airflow acts at the speed of sound, and problems with controlability. Mach theory gets VERY complicated and is beyond this topic so that statement is very simplified (before I get shot down by other Pruners ;) )

Also the speeds and Mach No I have used are aircraft dependant and are just examples for this case but are close to those used by a B737NG

[captain cumulonimbus]

to simplify things,IAS is unreliable at higher speeds/altitudes due to density of air/pressure and temps,but mach is a variable scale,dependant on ambiant temp and local spd of sound,allowing for uniformity.thats why we use it in the cruise rather than IAS