Mach to Airspeed Transition

[SELCAL checks]

The 230-240 HI/LOW sector boundary is probably the most common, but not the rule. e.g. Denver Center. Plenty of other exceptions as well.

As a rule of thumb, ATC assigns IAS FL280 and below and Mach FL290 and above.

 

[BillJBrake]

Stupidpilot, you obviously haven't flown to enough regions of the country, I could name NUMEROUS center sectors that go above/below FL240. There are plenty of areas where the separation from high/low sectors is FL280, and the ultra-highs start at FL330. But you knew that...right?

As avbug is saying, there are two questions at hand: when can Mach be assigned, and when do can pilots (or are they required) switch from Mach to Airspeed, vice versa. In the descent, FL270-280 seems pretty standard, but atc will often assign an airspeed at a higher altitude, knowing he's transitioning down and mach numbers may be assigned at FL240 and above, but are usually only assigned below FL280 if they plane is climbing higher (as I mentioned in post #8).

Randy, my only claim to you initially was that FL240 is a bit low to be used as a standard (which is true, most people seem to transition between FL270-280) and that while you are correct ATC "may" assign a mach above FL240 (which I never argued) I also am disagreeing that FL240 is a definitive transition between high and low altitude sectors, at there are tons of sectors out there throughout the NAS that have stratifications higher and lower than that.

Avbug is right on the money...I've never been assigned a mach number at FL240, even though the controller can technically use it....if he's spacing me at FL240 with someone at FL320, there will be a large discrepancy in the mach#'s. Anything below FL280 as a requested final, and you're gonna see a KIAS assigned, imho.

 

[pilotyip]

no right or wrong

I think the subject has been covered, but we're also hearing correctly about two different subjects.

The issue of FL240 is one subject, for ATC phraseology with respect to assigning mach or indicated. It refers primarily to a general guideline as to whether ATC will give an assignment based on mach or indicated airspeed. In general, ATC will assign a mach speed above 240, and indicated below, but this isn't always the case, and this isn't the value to which the question posed by the original poster refers.

The issue of maintaining 300 KIAS at the transition doesn't refer to FL240. It's referring to riding mach during the descent until indicated airspeed reaches a predetermined value, then maintaining that value. In this case, 300 KIAS.

This is a common descent profile, and one I typically use. .84 M1 until transition, then 300 KIAS in the descent until 10'000' or any other restrictions apply.

so basically everyone is right and it boils down to do what ATC instructs, BTW I am ont going ot change my ground school slide, but wil ldiscuss it thanks for all the inoput

 

[stupidpilot]

Stupidpilot, you obviously haven't flown to enough regions of the country, I could name NUMEROUS center sectors that go above/below FL240. There are plenty of areas where the separation from high/low sectors is FL280, and the ultra-highs start at FL330. But you knew that...right?

Sure, I've flown all around the world in excess of 100X plus I've been flying for over 20 years. I guess I know nothing.

 

[callowayhd]

Just to throw it out there at FL280 to FL290, 290KIAS-300KIAS ~ .73M-.75M

Most aircraft flying above FL290 climb and descend in the 290 knot range and descend using the same. Above FL280 or FL290 Mach is used for speeds during normal operations. If you climb at 290KIAS and transition to .74M, you will likely see a transition near the FL280-FL290 range.

As for FL240, in my opinion that has nothing to do with transition speeds. It is my understanding, as well as other previous posters, that maintain 300 KIAS in the transition means that when your current Mach number equals 300 KIAS, while descending, you maintain 300 kts. Since most aircraft cruise.75M-.80m This just happens arouns FL280-FL290. Again depending on ISA

 

[Singlecoil]

Where the transition occurs is solely a function of OAT.

 

[Griff]

Don't the high and low sectors have agreements with each other as to where the their respective sectors end/begin? Meaning that it varies.

I do appreciate the explanation on mach/indicated airspeed. Thanks All!

Griff