Mach Number Change Over

[ALF1 15]

Gotcha. I am at a loss explaining the difference between the -700s as I dont even own it.

 

Are you sure that the different changeover altitude is not due to different atmospheric conditions ?

Gotcha. I am at a loss explaining the difference between the -700s as I dont even own it.

 

Are you sure that the different changeover altitude is not due to different atmospheric conditions ?

[cbrdriver 0]

I'm gonna keep an eye on weather conditions, to see. I've noticed that the Mach number it goes to is rarely the same, so that never even crossed my mind.

Will let you know.

[ALF1 15]

Check also the setting in the FMC to make sure it is the same for the different airplanes. And the weight and balance. And engine power rating being used. Differences there could affect the changeover altitude even if the aircraft are both -700s

[cbrdriver 0]

I see where this is going.

 

I'll run a controlled test. Save a weather for consistency, then, same ZFW, same CG, same fuel load, etc.

 

I'll do that and reply.

[ALF1 15]

I have been thinking some more on this. Are you using a weather engine ? Reason I ask is because sometimes the temperature outside is not what it should be and this affects the IAS/Mach number conversion. Do you leave your TOC temperature in the FMC at default or plug the forecast temperature ?

[cbrdriver 0]

I leave it. And yeah, was recently gifted OPUSFSX.

[ALF1 15]

I just did a climb from KSEA to PAJN and plugged the weather (winds and temperature at TOC) as per ASN in the FMC. I did your climb using LVL CHG at 287/.76 target speed in the  climb page. Sure enough, the 737-800WL switched to Mach number at 26,200 ft where airspeed was 287 but Mach number was only slightly above .70. My guess is it should have waited for the airspeed to match the mach number at whatever altitude that happened given the temperature, and then climb at mach number while airspeed decreased further.

 

I don't know if the airplane behaves differently in VNAV than in LVL CHG. The ECON suggested climb by the FMC with my weight was 305/.78, and I am thinking that if in VNAV mode the changeover would have been at M.78 regardless of altitude.

 

I know controllers use speed restrictions as mach numbers above a certain altitude for traffic separation and KIAS further down but don't know exactly what altitude that is. Maybe the airplane switches to Mach number at a certain altitude when in LVL CHG I don't know. But I can certainly see your point and the reason that you ask.

 

And regarding my previous suggestions, come to think of it maybe weight and balance has nothing to do with changeover altitude, it should not depend on it. Scratch that.

 

I will do some more testing tomorrow and come back, its late right now.

[scandinavian13 5,192]

 

 

I know controllers use speed restrictions as mach numbers above a certain altitude for traffic separation and KIAS further down but don't know exactly what altitude that is.

 

FL240 (turbojets only)

 

7110.65 5-7-1 f

[ALF1 15]

Thanks Kyle. Do you think this has something to do with the c/o altitude in lvl chg ? In VNAV the plane seems to c/o when airspeed matches Mach number, not so on lvl chg.

[Spin737 102]

 

 

I don't know if the airplane behaves differently in VNAV than in LVL CHG.

 

Yes. This is why VNAV is handy. LVL CHG simply changes from CAS to whatever equivalent Mach that is at approx FL260.

 

The CLB page (eg. 287/.76) is for VNAV and will have no effect on LVL CHG.

This discussion is like when my wife orders food:

"I'll have the #2 Buger Combo, but instead of a burger, I'd like a salad. And instead of fries, I'd like the veggies."

 

"So, you want a number #5 Salad Combo?"

 

"NO! I want a #2 but make it like a #5."

 

"Okaaaaaaaay."

 

 

You guys want to use VNAV, but you've heard that some people don't like it, so you won't use it.

[scandinavian13 5,192]

 

 

This discussion is like when my wife orders food:

 

I worked at the Cheesecake Factory for a summer and people would do exactly that. Drove me nuts. Rang in the correct order (menu item versus "custom creation") each time and none of them realized it to my knowledge.

[JoeDiamond 67]

 

 

I know a pilot of an NG. He told me that he rarely uses VNAV.

 

That may be his personal technique, and there is nothing wrong with that, but based on my experience that places him in the minority.  99% of the pilots I have flown with use VNAV to climb and cruise and about 80% to descend.

 

I'd be hesitant to base your procedures on the advice of what a single pilot told you.

[scandinavian13 5,192]

 

 

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.

 

Southwest pilot, perhaps? Regardless, as Joe mentioned, such operations are a rarity. VNAV is going to give you the most optimal solution with the least amount of work.

 

 

 

but the HUGS make for a more precise landing, right?
But many choose not to use the HUGS.

 

You're missing the fact that the HGS is an option, and just like cars, options cost money. In the case of aircraft, they cost quite a lot of money. Pretty sure the collimator glass alone is a good $10K-15K.

 

VNAV, on the other hand, is not.

[ALF1 15]

 

 

Yes. This is why VNAV is handy. LVL CHG simply changes from CAS to whatever equivalent Mach that is at approx FL260.

The CLB page (eg. 287/.76) is for VNAV and will have no effect on LVL CHG.

 

 

 

Matt, this solves the whole mistery. Truth be told I never noticed this changeover thing because I never climb to cruise in LVL CHG. Coming from a real pro that the airplane uses altitude for changeover well that explains the whole thing.

[cbrdriver 0]

Lol, yes WN.

Matt, is there a reason for this? At this point, i guess, just a curiosity.

 

I downloaded real time weather, then saved it. My -900 with PMDG colors did not change over. My -700 with SW colors did, but my -800 with SW colors did not.

 

Maybe just a -700 thing?

 

Anyway. I really wasn't basing my technique (completely) on a single pilot. I didn't fully understand some concepts of VNAV so I asked, he told me that he always used LVL CHNG for climb and VS for descent. Just made me feel better about continuing to do that. (Though the beating I've taken here has me looking at VNAV again)

 

Not for nothing, though, but I would use the speeds on the climb page as targets. Reference, I guess. I knew they had no interaction with LVL CHNG.