Mach/IAS transition question

[khedrei]

Looking to get a few more opinions on this to "resolve" a disagreement.

Instruction: "Descend FL230, Maintain M 0.70, transition 250 knots"

Do you:

a) maintain 0.70 until the barber poll moves up high enough that you can fly 250kts then maintain 250kts from there onward or,

b) maintain 0.70 until the aircraft avionics make the change from mach to IAS and then reduce speed to 250kts.

If using option b), it is my understanding that the aircraft avionics are programmed to transition based on the aircrafts maximum speed. IE, if the aircraft max speed/mach is 340/0.87, then the avionics flip over when 340kts IAS and Mach 0.87 are equal. Maybe somewhere around 31000 ft. I'm simply guessing. Regardless, my aircraft normally transitions around 27000 ft.

If I were to use option a) then I would simply override the avionics automatic switch and make the switch myself early, somewhere around 30000 ft.

If I were to use option b) in the above scenario (in my aircraft) then maintaining 0.70 would have me going up to about 275kts IAS and then slowing down AFTER the avionics made the switch.

I have been told by my superior that option a) is technically correct, but that most people will use option b) and simply wait until the aircraft avionics does it and that is considered acceptable. ATC doesn't really know when we transition and each aircraft transitions at a different altitude so they plan for some discrepancy when they are sequencing aircraft in descent.

Curious on what the airline guys and corporate guys do. I can't find anything about this in the AIM.

[DHC-1 Jockey]

In your example, I would descend at 0.70, and watch the IAS increase. Once the IAS increased to 250kts, I would then continue descent at 250 kts.

I can't think of a time where I've been told to transition at 250 kts, but knowingly accelerated past 250 as I started descent, only to need to reduce my descent rate to allow myself to slow back down to 250.

Option A is much smoother because it allowed me to maintain a more constant rate of descent instead of option B where you need to descend, level off to bleed off speed and then start descent again

[‘Bob’]

The reason for a transition speed assignment from ATC is generally for spacing. It should be read as “maintain 0.7 or 250, whichever is less”.

Some avionics are smart at figuring it out. Some aren’t and just have a set transition altitude based on normal transition speeds at ISA.

But if you see either 0.75 or 275 on the PFD, you’re going too fast.

[cdnavater]

Looking to get a few more opinions on this to "resolve" a disagreement.

Instruction: "Descend FL230, Maintain M 0.70, transition 250 knots"

Do you:

a) maintain 0.70 until the barber poll moves up high enough that you can fly 250kts then maintain 250kts from there onward or,

b) maintain 0.70 until the aircraft avionics make the change from mach to IAS and then reduce speed to 250kts.

If using option b), it is my understanding that the aircraft avionics are programmed to transition based on the aircrafts maximum speed. IE, if the aircraft max speed/mach is 340/0.87, then the avionics flip over when 340kts IAS and Mach 0.87 are equal. Maybe somewhere around 31000 ft. I'm simply guessing. Regardless, my aircraft normally transitions around 27000 ft.

If I were to use option a) then I would simply override the avionics automatic switch and make the switch myself early, somewhere around 30000 ft.

If I were to use option b) in the above scenario (in my aircraft) then maintaining 0.70 would have me going up to about 275kts IAS and then slowing down AFTER the avionics made the switch.

I have been told by my superior that option a) is technically correct, but that most people will use option b) and simply wait until the aircraft avionics does it and that is considered acceptable. ATC doesn't really know when we transition and each aircraft transitions at a different altitude so they plan for some discrepancy when they are sequencing aircraft in descent.

Curious on what the airline guys and corporate guys do. I can't find anything about this in the AIM.

Based on your question and two possible options, I’m going to suggest your training was not adequate!
The “transition” is when Mach.70 equals 250 IAS, you would descend in Mach and transition to IAS at the assigned speed of 250.
On the RJ the system transitions from IAS to Mach or Mach to IAS at 31600’, the speed bug does not move, it simply now displays the Mach or IAS depending on climbing or descending. Incidentally if you were to climb at 253 KIAS, at 31600 it would transition to Mach 70. On descent at Mach .70 would transition to 253 KIAS.
So the altitude is only relevant to when the IAS is equal to the assigned Mach transition which is not aircraft specific, Mach .70 is relative to TAS which is IAS adjusted for altitude and temperature which would not change on the aircraft.
ATC does know when you transition because ADS B has airspeed readouts on the controllers screen!

[digits_]

Based on your question and two possible options, I’m going to suggest your training was not adequate!
The “transition” is when Mach.70 equals 250 IAS, you would descend in Mach and transition to IAS at the assigned speed of 250.
On the RJ the system transitions from IAS to Mach or Mach to IAS at 31600’, the speed bug does not move, it simply now displays the Mach or IAS depending on climbing or descending. Incidentally if you were to climb at 253 KIAS, at 31600 it would transition to Mach 70. On descent at Mach .70 would transition to 253 KIAS.
So the altitude is only relevant to when the IAS is equal to the assigned Mach transition which is not aircraft specific, Mach .70 is relative to TAS which is IAS adjusted for altitude and temperature which would not change on the aircraft.
ATC does know when you transition because ADS B has airspeed readouts on the controllers screen!

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

[khedrei]

Looking to get a few more opinions on this to "resolve" a disagreement.

Instruction: "Descend FL230, Maintain M 0.70, transition 250 knots"

Do you:

a) maintain 0.70 until the barber poll moves up high enough that you can fly 250kts then maintain 250kts from there onward or,

b) maintain 0.70 until the aircraft avionics make the change from mach to IAS and then reduce speed to 250kts.

If using option b), it is my understanding that the aircraft avionics are programmed to transition based on the aircrafts maximum speed. IE, if the aircraft max speed/mach is 340/0.87, then the avionics flip over when 340kts IAS and Mach 0.87 are equal. Maybe somewhere around 31000 ft. I'm simply guessing. Regardless, my aircraft normally transitions around 27000 ft.

If I were to use option a) then I would simply override the avionics automatic switch and make the switch myself early, somewhere around 30000 ft.

If I were to use option b) in the above scenario (in my aircraft) then maintaining 0.70 would have me going up to about 275kts IAS and then slowing down AFTER the avionics made the switch.

I have been told by my superior that option a) is technically correct, but that most people will use option b) and simply wait until the aircraft avionics does it and that is considered acceptable. ATC doesn't really know when we transition and each aircraft transitions at a different altitude so they plan for some discrepancy when they are sequencing aircraft in descent.

Curious on what the airline guys and corporate guys do. I can't find anything about this in the AIM.

Based on your question and two possible options, I’m going to suggest your training was not adequate!
The “transition” is when Mach.70 equals 250 IAS, you would descend in Mach and transition to IAS at the assigned speed of 250.
On the RJ the system transitions from IAS to Mach or Mach to IAS at 31600’, the speed bug does not move, it simply now displays the Mach or IAS depending on climbing or descending. Incidentally if you were to climb at 253 KIAS, at 31600 it would transition to Mach 70. On descent at Mach .70 would transition to 253 KIAS.
So the altitude is only relevant to when the IAS is equal to the assigned Mach transition which is not aircraft specific, Mach .70 is relative to TAS which is IAS adjusted for altitude and temperature which would not change on the aircraft.
ATC does know when you transition because ADS B has airspeed readouts on the controllers screen!

Thanks for the dig big guy. I wouldn't expect anything less from you.

Is your long winded answer saying that option a) is correct? The way you explain it, seems to suggest that. You descend at .70, then when .70 = 250, you fly 250. I said nothing about a speed bug moving... you did.

I would argue that my training is adequate, because thats what I did, and I believe that option a) is correct. I asked because someone who I am working with very much believes I am wrong. This person told me that I should fly .70 until the aircraft avionics displays IAS at the top, then slow down to 250 from about 275-280 once it does. I believe that is wrong. That being said, I was told that a lot of other people do that. In scenario a) the transition altitude is well above the normal 27000 ft that the avionics makes the change in this aircraft.

I also understand that the spot that .70 = 250 is not aircraft specific, but the point at which the avionics switches mach/IAS is.

I will thank you for the info that ATC gets the info on their screen from ADSB. I didn't know that. I have seen the targets, and I don't remember them having that much information on them. I suppose it can be customized to each controllers preferences to display certain info and leave other info out.

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

I wouldn't say it's luck, or not. I gave this example because it was my exact situation. It would make no difference for 0.70 to 230 kts. The transition would happen even earlier in descent. Maybe 33000 feet. Or higher perhaps.

Not sure why this has generated so much discussion. I thought the answers were pretty simple. From what I can tell, one is right and the other is wrong.

[digits_]

Not sure why this has generated so much discussion. I thought the answers were pretty simple. From what I can tell, one is right and the other is wrong.

Oh, that's easy, technically option A is the only correct one. However, depending on how the avionics on your airplane are set up, it might be a bit cumbersome to do exactly that. 'dumber' airplanes where most stuff is happening manually might be easier in that regards vs the smarter ones where the transition happens automatically, but perhaps not exactly where it should. For example at a fixed altitude or a fixed speed.

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

I wouldn't say it's luck, or not. I gave this example because it was my exact situation. It would make no difference for 0.70 to 230 kts. The transition would happen even earlier in descent. Maybe 33000 feet. Or higher perhaps.

Yes, but it was my understanding that cdnavater's example mentioned that the RJ always transitions at 31600 ft. in which case the transition will only be correct for M 0.7 / 253 kts, not for other airspeed combos at M 0.7.

But I might have misunderstood.

[khedrei]

We may be getting a big confused here.

The meat of my question worded more simply is this:

In descent, should you start flying your assigned IAS when the assigned mach = IAS or should you continue flying the assigned mach and wait for the aircraft avionics to switch to IAS and then make an IAS adjustment? In my specific example, in my aircraft, this would have me needing to slow down from about 275 in order to reach 250.

The first response in the thread seemed to understand what I was asking and gave an answer to the question.

I mainly wanted to know what the practice is with the more experienced guys.

[Tbayer2021]

Another thing to keep in mind is the planned descent on the FMS. If you don't change the speed to the new ATC requested speed and simply wait until you're at the required IAS and speed intervene, you'll quickly find yourself off path. Not a big deal most times but it can increase workload if your planned descent on the FMS was 310 and center is asking you so transition 250 and you don't bother changing it.

It has been happening often lately approaching YYZ from the east where they ask for 250 on the transition and each successive controller slows us down to 200 well before the first speed restriction on the STAR.

[khedrei]

Another thing to keep in mind is the planned descent on the FMS. If you don't change the speed to the new ATC requested speed and simply wait until you're at the required IAS and speed intervene, you'll quickly find yourself off path. Not a big deal most times but it can increase workload if your planned descent on the FMS was 310 and center is asking you so transition 250 and you don't bother changing it.

It has been happening often lately approaching YYZ from the east where they ask for 250 on the transition and each successive controller slows us down to 200 well before the first speed restriction on the STAR.

I understand what you're saying, but this is getting off track.

Also, I don't have auto throttle, so it doesn't matter what's in the FMS. Its a rule/theory/procedure question only.

[Tbayer2021]

Another thing to keep in mind is the planned descent on the FMS. If you don't change the speed to the new ATC requested speed and simply wait until you're at the required IAS and speed intervene, you'll quickly find yourself off path. Not a big deal most times but it can increase workload if your planned descent on the FMS was 310 and center is asking you so transition 250 and you don't bother changing it.

It has been happening often lately approaching YYZ from the east where they ask for 250 on the transition and each successive controller slows us down to 200 well before the first speed restriction on the STAR.

I understand what you're saying, but this is getting off track.

Also, I don't have auto throttle, so it doesn't matter what's in the FMS. Its a rule/theory/procedure question only.

You don't need auto thrust for this to apply. The FMS calculates a descent based on speed. Plenty of planes without A/T have VNAV.

But anyways, yes. When the IAS increases to whatever ATC wants me to transition to, then that becomes the speed I use.

[khedrei]

Another thing to keep in mind is the planned descent on the FMS. If you don't change the speed to the new ATC requested speed and simply wait until you're at the required IAS and speed intervene, you'll quickly find yourself off path. Not a big deal most times but it can increase workload if your planned descent on the FMS was 310 and center is asking you so transition 250 and you don't bother changing it.

It has been happening often lately approaching YYZ from the east where they ask for 250 on the transition and each successive controller slows us down to 200 well before the first speed restriction on the STAR.

I understand what you're saying, but this is getting off track.

Also, I don't have auto throttle, so it doesn't matter what's in the FMS. Its a rule/theory/procedure question only.

You don't need auto thrust for this to apply. The FMS calculates a descent based on speed. Plenty of planes without A/T have VNAV.

But anyways, yes. When the IAS increases to whatever ATC wants me to transition to, then that becomes the speed I use.

Thank you.

I see what you mean. I was of the impression the VNAV calculates a 3 degree descent path (at least it does in mine). The faster the forward speed, the more VS needed, and vice versa. The speed won't matter unless it would require a crazy VS in order to meet the 3 degrees. Perhaps im wrong about the programming in the FMS but I thought the speed profile was there for the auto throttle and fuel calcs, not the descent path. None the less, I appreciate the answer.

[digits_]

The speed won't matter unless it would require a crazy VS in order to meet the 3 degrees. Perhaps im wrong about the programming in the FMS but I thought the speed profile was there for the auto throttle and fuel calcs, not the descent path.

It matters if you try to fly as economically as possible. Generally the flightplan/FMS settings are set up for the optimal situation. If ATC wants a certain speed, it might be more economical to start your descend earlier/later with a more 'normal' VS than it would be to start at the precalculated point but with an unusual VS. Then again, very often there's little you can do, but if the speed restrictions are anticipated to last a while, it might be worth it to recalculate.

[pelmet]

Old jets, maintain cleared Mach on descent and watch the cleared airspeed for descent move to match the Mach and then follow the airspeed(or maybe it was the Mach indication that moved.

New jets, just put the IAS/Mach combo in the FMC and let it do the rest by following command bars(at least some new jets - maybe some others are different).

[Tbayer2021]

I understand what you're saying, but this is getting off track.

Also, I don't have auto throttle, so it doesn't matter what's in the FMS. Its a rule/theory/procedure question only.

You don't need auto thrust for this to apply. The FMS calculates a descent based on speed. Plenty of planes without A/T have VNAV.

But anyways, yes. When the IAS increases to whatever ATC wants me to transition to, then that becomes the speed I use.

Thank you.

I see what you mean. I was of the impression the VNAV calculates a 3 degree descent path (at least it does in mine). The faster the forward speed, the more VS needed, and vice versa. The speed won't matter unless it would require a crazy VS in order to meet the 3 degrees. Perhaps im wrong about the programming in the FMS but I thought the speed profile was there for the auto throttle and fuel calcs, not the descent path. None the less, I appreciate the answer.

Speed certainly matters and how much it matters depends on when they give it to you. If you're descending at say, 300 and ATC asks you to reduce to 250, you'll probably end up above path if the boards aren't enough to slow you down while still keeping you on path. Some times we even get warnings that the required descent is too steep for the given programmed speed and restrictions on the FMS.

[Tbayer2021]

Old jets, maintain cleared Mach on descent and watch the cleared airspeed for descent move to match the Mach and then follow the airspeed(or maybe it was the Mach indication that moved.

This is exactly what glass/FMS/A/T equipped jets do. At least the Boeings and Airbus I've flown do this. They maintain desired Mach during descent and will transition to IAS when the IAS increases to match the current Mach. This is literally the definition of "transition". That's why the altitude the transition happens will vary depending on the specific speed. It will also vary slightly depending on temperature since Mach is a function of temperature. But that won't change it much.

[cdnavater]

Based on your question and two possible options, I’m going to suggest your training was not adequate!
The “transition” is when Mach.70 equals 250 IAS, you would descend in Mach and transition to IAS at the assigned speed of 250.
On the RJ the system transitions from IAS to Mach or Mach to IAS at 31600’, the speed bug does not move, it simply now displays the Mach or IAS depending on climbing or descending. Incidentally if you were to climb at 253 KIAS, at 31600 it would transition to Mach 70. On descent at Mach .70 would transition to 253 KIAS.
So the altitude is only relevant to when the IAS is equal to the assigned Mach transition which is not aircraft specific, Mach .70 is relative to TAS which is IAS adjusted for altitude and temperature which would not change on the aircraft.
ATC does know when you transition because ADS B has airspeed readouts on the controllers screen!

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

I guess I could have been more clear, in the RJ to switch the speed bug between Mach and IAS you simply press the center of the speed bug selector, so if your IAS transition is higher or lower than the automatic switch over, press the center of the bug selector until your Mach equals the assigned transition.

[cdnavater]

Looking to get a few more opinions on this to "resolve" a disagreement.

Instruction: "Descend FL230, Maintain M 0.70, transition 250 knots"

Do you:

a) maintain 0.70 until the barber poll moves up high enough that you can fly 250kts then maintain 250kts from there onward or,

b) maintain 0.70 until the aircraft avionics make the change from mach to IAS and then reduce speed to 250kts.

If using option b), it is my understanding that the aircraft avionics are programmed to transition based on the aircrafts maximum speed. IE, if the aircraft max speed/mach is 340/0.87, then the avionics flip over when 340kts IAS and Mach 0.87 are equal. Maybe somewhere around 31000 ft. I'm simply guessing. Regardless, my aircraft normally transitions around 27000 ft.

If I were to use option a) then I would simply override the avionics automatic switch and make the switch myself early, somewhere around 30000 ft.

If I were to use option b) in the above scenario (in my aircraft) then maintaining 0.70 would have me going up to about 275kts IAS and then slowing down AFTER the avionics made the switch.

I have been told by my superior that option a) is technically correct, but that most people will use option b) and simply wait until the aircraft avionics does it and that is considered acceptable. ATC doesn't really know when we transition and each aircraft transitions at a different altitude so they plan for some discrepancy when they are sequencing aircraft in descent.

Curious on what the airline guys and corporate guys do. I can't find anything about this in the AIM.

Based on your question and two possible options, I’m going to suggest your training was not adequate!
The “transition” is when Mach.70 equals 250 IAS, you would descend in Mach and transition to IAS at the assigned speed of 250.
On the RJ the system transitions from IAS to Mach or Mach to IAS at 31600’, the speed bug does not move, it simply now displays the Mach or IAS depending on climbing or descending. Incidentally if you were to climb at 253 KIAS, at 31600 it would transition to Mach 70. On descent at Mach .70 would transition to 253 KIAS.
So the altitude is only relevant to when the IAS is equal to the assigned Mach transition which is not aircraft specific, Mach .70 is relative to TAS which is IAS adjusted for altitude and temperature which would not change on the aircraft.
ATC does know when you transition because ADS B has airspeed readouts on the controllers screen!

Thanks for the dig big guy. I wouldn't expect anything less from you.

Is your long winded answer saying that option a) is correct? The way you explain it, seems to suggest that. You descend at .70, then when .70 = 250, you fly 250. I said nothing about a speed bug moving... you did.

I would argue that my training is adequate, because thats what I did, and I believe that option a) is correct. I asked because someone who I am working with very much believes I am wrong. This person told me that I should fly .70 until the aircraft avionics displays IAS at the top, then slow down to 250 from about 275-280 once it does. I believe that is wrong. That being said, I was told that a lot of other people do that. In scenario a) the transition altitude is well above the normal 27000 ft that the avionics makes the change in this aircraft.

I also understand that the spot that .70 = 250 is not aircraft specific, but the point at which the avionics switches mach/IAS is.

I will thank you for the info that ATC gets the info on their screen from ADSB. I didn't know that. I have seen the targets, and I don't remember them having that much information on them. I suppose it can be customized to each controllers preferences to display certain info and leave other info out.

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

I wouldn't say it's luck, or not. I gave this example because it was my exact situation. It would make no difference for 0.70 to 230 kts. The transition would happen even earlier in descent. Maybe 33000 feet. Or higher perhaps.

Not sure why this has generated so much discussion. I thought the answers were pretty simple. From what I can tell, one is right and the other is wrong.

I guess it’s how you worded option a), you are correct fly Mach .70 until that equals 250 then fly 250.
When flying a specific speed do you have a speed bug on your airspeed indicator to target that speed?
Can your AP fly a speed descent?

[Eric Janson]

Looking to get a few more opinions on this to "resolve" a disagreement.

Instruction: "Descend FL230, Maintain M 0.70, transition 250 knots"

Do you:

a) maintain 0.70 until the barber poll moves up high enough that you can fly 250kts then maintain 250kts from there onward or,

You maintain 0.70 until the IAS pointer hits 250 then fly 250 knots.

Very simple - I do it regularly at busy airports.

[khedrei]

Based on your question and two possible options, I’m going to suggest your training was not adequate!
The “transition” is when Mach.70 equals 250 IAS, you would descend in Mach and transition to IAS at the assigned speed of 250.
On the RJ the system transitions from IAS to Mach or Mach to IAS at 31600’, the speed bug does not move, it simply now displays the Mach or IAS depending on climbing or descending. Incidentally if you were to climb at 253 KIAS, at 31600 it would transition to Mach 70. On descent at Mach .70 would transition to 253 KIAS.
So the altitude is only relevant to when the IAS is equal to the assigned Mach transition which is not aircraft specific, Mach .70 is relative to TAS which is IAS adjusted for altitude and temperature which would not change on the aircraft.
ATC does know when you transition because ADS B has airspeed readouts on the controllers screen!

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

I guess I could have been more clear, in the RJ to switch the speed bug between Mach and IAS you simply press the center of the speed bug selector, so if your IAS transition is higher or lower than the automatic switch over, press the center of the bug selector until your Mach equals the assigned transition.

That's exactly what I did. The assigned transition happened earlier than the transition that the avionics does. So when .70 matched 250 I pushed the button. I was told this was incorrect I should wait for the automatic switch which would have us going 275 then slowing down.

The AP will fly a bugged speed on a random decsent, but NOT on a VNAV descent. Without auto throttle, it prioritizes the path so I control the speed manually with the throttles. I can bug it, but its just a reference.

[Tbayer2021]

Is that not simply luck of the draw that the OP's example is M 0.70 / 250 kts?

What should he do if the instruction was 'descend M .70 transition 230 kts'? Then the automatic scenario above wouldn't work anymore.

I guess I could have been more clear, in the RJ to switch the speed bug between Mach and IAS you simply press the center of the speed bug selector, so if your IAS transition is higher or lower than the automatic switch over, press the center of the bug selector until your Mach equals the assigned transition.

That's exactly what I did. The assigned transition happened earlier than the transition that the avionics does. So when .70 matched 250 I pushed the button. I was told this was incorrect I should wait for the automatic switch which would have us going 275 then slowing down.

The AP will fly a bugged speed on a random decsent, but NOT on a VNAV descent. Without auto throttle, it prioritizes the path so I control the speed manually with the throttles. I can bug it, but its just a reference.

I’m going to suggest your captain's training was not adequate!

[Eric Janson]

I know we have ATC people on these Forums.

Perhaps one of them would like to clarify (even though the correct procedure has been listed multiple times).

[kevenv]

ATC here. As stated a few times, I expect you to maintain whatever mach is assigned in descent until it matches the assigned IAS then maintain that.

[Eric Janson]

Thanks kevenv.