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cbrdriver

Mach Number Change Over

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I generally do not use VNAV for neither climb nor decent. I will use LVL CHNG and VS.

I've noticed that on some of my airplanes, the IAS will automatically change over to what ever Mach Number it happens to be at around FL263 and some do not.

 

Is there an option or switch in the config to control this behavior?


John Anderson

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Is there an option or switch in the config to control this behavior?

 

Nope, this is based more on physics than equipment. The switchover should be dependent on atmospheric conditions...., you didn't mention what "some of my airplanes" were and don't assume PMDG (this is a PMDG forum). Generally speaking then, there will be a point in the column of air where the set speed in kts is equal to the set speed in Mach number and that is where the changeover occurs in a constant speed climb.  Mach is dependent on the density of the air, which in turn is dependent on pressure, temperature and humidty.


Dan Downs KCRP

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But, I don't set a Mach number. They (yes PMDG) just change over. My target Mach number on the Climb page may be 287/.76, but at FL263, it will change to whatever 287 indicated happens to be. The change isn't as dynamic as I know the speed of sound is, it happens at FL263 whether that's .68 Mach number or .72 Mach number.

The point is, it's targeting a flight level, not a Mach number.

Did that make sense?


John Anderson

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But, I don't set a Mach number. They (yes PMDG) just change over. My target Mach number on the Climb page may be 287/.76, but at FL263, it will change to whatever 287 indicated happens to be. The change isn't as dynamic as I know the speed of sound is, it happens at FL263 whether that's .68 Mach number or .72 Mach number.

The point is, it's targeting a flight level, not a Mach number.

Did that make sense?

This is also dependent on the type aircraft and systems. For example, in the DC10, our mach change over was 29,000ft. There was no auto change, you just climbed at 330 until reaching .82 and selected mach hold(around 29,000). On the way down, you descend at .82 until reaching 320 and you selected IAS hold. In the Gulfstream 5/550, it changes automatically at 28,500. Passing 28500, mach becomes primary and low bank engages if in heading select. Just depends on the aircraft and system. To be honest, no one actually pays attention to the exact change over point when it happens. The more important thing is that it actually changes over. On the way up, you would run out of power in the climb trying to maintain indicated if it failed to swap. This would be very noticeable. On the way down, you run the risk of over speeding if it fails to change. I've seen guys get distracted while descending in manual mach hold and would over speed If i hadn't mentioned it. Its a big gotcha on descent.

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So when at FL263, it changes over to Mach number .68, then I should stay at .68 until I reach cruise altitude, then accelerate to .78?

 

That seems inefficient.

 

I would rather hold 287 indicated until reaching Mach number .76. I understand that it does shallow the climb, but doesn't that make for a cleaner, more efficient climb?

 

 

There was no auto change, you just climbed at 330 until reaching .82 and selected mach hold(around 29,000).

 

At that altitude, in many cases, my target Mach number would be overspeed.

And at FL263, in many cases, my target Mach number is overspeed.


John Anderson

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If you want to manage the climb via KIAS in level change, hit the C/O button in the MCP to revert back to KIAS from Mach. Level change will use N1 Climb power and will adjust pitch to your selected MCP speed.

 

Make sure you dont go above Mmo after 32,000 or so.


Alberto Ferracuti

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That's exactly how I've been handling it, Alberto, but why does my 700 with Southwest colors automatically change over, but not my 700 with PMDG colors. My 800 with Delta doesn't, but my 800 with Alaska does.


John Anderson

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So when at FL263, it changes over to Mach number .68, then I should stay at .68 until I reach cruise altitude, then accelerate to .78?

 

That seems inefficient.

 

I would rather hold 287 indicated until reaching Mach number .76. I understand that it does shallow the climb, but doesn't that make for a cleaner, more efficient climb?

 

 

 

At that altitude, in many cases, my target Mach number would be overspeed.

And at FL263, in many cases, my target Mach number is overspeed.

I gave an example, you could also check the manual for the climb speed schedule and perf it or follow it manually as I did in my DC10 days.

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Ah you got me there. My Alaska Spirit of Seattle just changed at 25,800. They should all change over at some point below 28000, the 737 NGX is a slow bird compared to the others. I guess that the climb at KIAS just becomes inefficient at that point for the airplane and hence the climb via mach number. The point is to get up as fast as possible to be efficient.


Alberto Ferracuti

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I would rather hold 287 indicated until reaching Mach number .76.

That's what VNAV does. You program the speeds you want. Why use LVL CHG and V/S if VNAV does what you want?


Matt Cee

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Gentlemen, thank you for your replies, but with due respect, I'm asking why it would be different from one 700 to another 700, and where is the option to control that behavior?

I do know that different airlines have different options and equipment, but I don't see any option to change this.


John Anderson

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John you are chasing information that once obtained will have little value.

 

As Matt suggested, why not fly her as intended?  I just left KLAX in a DAL B738 FMS ECON CLB 311/.789 and right on the money the indicated changed from 311 to .79 at 28200 on the altimeter.


Dan Downs KCRP

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I know a pilot of an NG. He told me that he rarely uses VNAV. What are the exact situations, I don't know, but it makes me ask, why have a LVL CHNG or VS speed modes if they weren't intended for use.

 

Maybe someone else can have a new perspective with the answer, maybe not, but this is how I (and my friend) fly.

 

Didn't think this was worth a ticket, just was hoping for an answer.

 

I do understand how the climb works, I do understand how the Mach number, I do know that it behooves me to go to Mach number once that number lines up with current IAS.

 

I just want to know why there is a difference and how to make my fleet more consistent.

Ah you got me there. My Alaska Spirit of Seattle just changed at 25,800. They should all change over at some point below 28000, the 737 NGX is a slow bird compared to the others. I guess that the climb at KIAS just becomes inefficient at that point for the airplane and hence the climb via mach number. The point is to get up as fast as possible to be efficient.

I'm sry, Alberto. I didn't see your response.

That does make since (a point that I'd not thought about)


John Anderson

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Hmm ok appreciate the comment but I think all points put forth by the other guys are valid and.....all say about the same thing in different words.

 

VNAV automates what the G550 flyer did manually in the DC10, which is what you are trying to do now.


Alberto Ferracuti

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You're right. Everyone is saying pretty much the same thing, but the basic question is. . . why does one -700 act this way, while a different -700 acts a different way.

 

I understand that VNAV automates the process, and for some, that's exactly what they want. I'm sure there are carriers out there that have VNAV use in their SOPs, but the HUGS make for a more precise landing, right?

But many choose not to use the HUGS.

The question was more about inconsistencies that I see, and how to fix or change them, than how to fly the airplane.


John Anderson

Windows 10, FSX:SE

I5 4690k

GA-Z97M-DS3H

EVGA GTX 950, 2GB

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