Common misunderstandings B737

[ImbracableCrunk]

While cruising, in turbulence, pointing to the Turb N1% on the CRZ page and then monkeying with SPD INTV until the N1 matches that Turb N1, and then saying, "This is how you get to the Turbulence N1."

[framer]

Entering the ground-level ISA deviation in the Descent Forecast page

hmmmmm, it's early and I'm half asleep but I think I do that....teach me something shaftsburn, what is wrong with that?

[framer]

Don't worry Shaft, I've had a read and guess what, I learnt something
Ta

Enter the average ISA deviation for descent in °C (+/–XX°C) or °F (+/–XX°F)
Enter the destination QNH altimeter setting (IN. HG. or MB). Do not enter a QFE

altimeter setting.

[misd-agin]

Optimum and max altitude should increase during climb. Typically 4500-5000 lbs fuel burn(737-800) to TOC so OPT/MAX altitudes should increase by approx. 700'(before non-standard temperature issues).

[framer]

Optimum and max altitude should increase during climb. Typically 4500-5000 lbs fuel burn(737-800) to TOC so OPT/MAX altitudes should increase by approx. 700'(before non-standard temperature issues).

But they don't.
The optimum alt on the cruise page stays the same throughout the climb until you are 1000ft from cruise alt. Thats not from a book, just from observation.

[misd-agin]

framer - 737-800 data - every 1000 lbs of weight decrease increases OPT/MAX altitude by 150'(most weights)

TOC fuel burn is 4500-5000 = OPT/MAX altitude increase of ~700'

I agree that it doesn't always increase as expected. It's been my experience from 17 yrs of observation that it doesn't always work as I explained, particularily with warm surface temperatures. In those cases altitude starts increasing with fuel burn about the time you reach cruise altitude which is what you've observed.

altitude increase per 1000 lb fuel burn (mid range weights) -

777 40'
763 70'
762 80'
757 100'
737 150'

[framer]

I agree that it doesn't always increase as expected.

I'l have to keep a close eye on it because I'm pretty sure that it doesn't budge until you are levelling out and even then only 100ft.
In the cruise you are reducing weight by about 2200kg an hour and the optimum increases 100ft every 6 1/2 - 7 1/2 minutes.
In the climb, if it was using the same computational logic, it would increase 100ft every 2-3 minutes and over a 20 minute climb you would see close to 1000ft change and we would all be well aware of this.
I think that the update 10.8 might be different to the way it used to work but I'm not sure.
Start a timer and watch it on the climb out, I'l do the same. The surface temperatures are rarely high where I'm based but I'l keep an eye on that too.

[Sciolistes]

The optimum alt on the cruise page stays the same throughout the climb until you are 1000ft from cruise alt. Thats not from a book, just from observation.

From what I have seen, the opt alt changes most during climb as the ISA deviation changes, regrdless of what is on the perf page.

[framer]

Hmmmm, I wonder if we're running different updates or something, I've been keeping a pretty close eye on it lately and never seen it move, from take-off until levelling out. Thoughts?

[Oakape]

Just speculating, but perhaps when on the ground & during climb, the optimum altitude on the cruise page is a 'predicted' value, based on the FMC calculated burn during climb & therefore the predicted TOC weight. And then, after TOC, it goes back to real time calculations. It would make sense for it to be that way.

That being said, I have noticed that the predicted crossing altitudes for waypoints during climb can be out by a few thousand feet, although this may have more to do with changes in track miles, temperature & wind during the climb.