Transition Airspeed

[Jump Pilot]

At what altitude do you transition from mach to knots?

I hear airliners checking in on the arrival at FL330 with PD to FL240. Sometimes the controller will tell them to transition to 300 knots when able in the let down.

Thanks. Just trying to build up that knowledge base.

 

[flywithruss]

Varies by airplane and descent profile. Typically somewhere in the 20s if I'm not too much mistaken. Again, it would depend on your profile. Some airliners use a .78/290/250 descent profile, for example. However, we all know controllers are using speed to help sequence airplanes ... I hear "slow to 250 now please" and "300 knots or greater" all the time on the radio, so most people aren't flying their "standard" profile that often, unless perhaps going in to East Nowhere with no other traffic!

Good question!

R

 

[Boeingman]

At what altitude do you transition from mach to knots?

I hear airliners checking in on the arrival at FL330 with PD to FL240. Sometimes the controller will tell them to transition to 300 knots when able in the let down.

Thanks. Just trying to build up that knowledge base.

No "official" altittude so to speak. On the MCP panel I think it changes from mach to IAS (vice vs) around 27,300 on the 57 and 67. We just plan on using the indicated as soon as it is below (or mach) the barberpole for reporting and planning purposes.

The speed profile varies with the cost index entered on the FMC or what the dispatcher plans the flight at. Of course ATC instructions override any FMC desired descent or climb profiles.

 

[AAsRedHeadedbro]

The speed will vary with the aircraft, ATC permitting, of course. Personally, in a descent, I will hold cruise Mach (.77 to .78) until the aircraft reaches 315 to 320 knots indicated (320 indicated is the airplane's Vmo). I will then hold that speed until cleared to descend below 10,000'.

In my aircraft, I believe the Vmo will start decreasing in order to hold the .78 Mmo limit somewhere around 23,500 to 24,000 feet. At FL370, M.78 equates to about 250 knots indicated airspeed.

So, to answer your question, the altitude at which one will transition from mach to airspeed will vary with the particular aircraft, how it is being flown, and ATC directives. Sorry we can't give you a more concrete answer than that.

Cheers!

 

[Brother Francis]

It's the point during descent at which the requested IAS can be maintained without exceeding Mmo.

As you descend, the indicated airspeed for a certain Mach number will increase. For example, in the EMB 145, Mmo of M.78 is equal to Vmo of 320 KIAS at FL260; at FL370, M.78 occurs at 250 KIAS, with minor variation for non-standard temperature.

Therefore, if I were descending from FL370 at .78 and was asked to transition to 320 when able, it would occur at FL260. If I were asked to transition to, say, 290, it would occur at some higher altitude. If I were asked to transition to 250, I would hold 250 from the start.

 

[avbug]

I usually transition right about FL290. The climb is typically something like 250 KIAS to .74 Mach, and this generally seems to take place about FL290.

On descent, it's generally just below Mmo until reaching vne, and then a descent near or just below this to any airspeed restrictions given, depending on the flight, conditions, traffic, etc. Any previously given restrictions before initiating the descent, of course, apply. Exactly where this takes place depends on the specifics for the descent; it varies according to need, temperature, etc.

 

[banned username 2]

We usually climb at 280/0.78M or 300/0.80M the transition usually occurs around 31,000 ft or so...

On the way down we usually do .84M/340 (or 370 if we are in a hurry)