View Full Version : Max Crz Alt - B738

Jet Man
29th Oct 2007, 20:59
My understanding of the maximum cruise altitude on the cruise page is on the ground it shows maximum altitude at anticipated TOC weight and in the air maximum cruise altitude for actual weight.
Anyone know if this is correct and is it written anywhere?

Kit d'Rection KG
29th Oct 2007, 21:20
It's written in Bill Bulfer's excellent FMC Guide. You just have to pay for it and read it. :)

30th Oct 2007, 17:24
And you'll have to take his word for granted ...:rolleyes:
Does the max cruise change at lift off? ..you got your answer then.

Lucky Angel
30th Oct 2007, 20:06
As some colleagues of mine found out this is not the case. On the ground they entered the DOW instead of the ZFW which gave a weight of around 13T less in the FMC. When trying to climb to FL380 whilst showing MAX ALT of FL385 i believe ,the a/c reverted to LVCH and started to descend.After investigation they realised the wrong entry. So i beleive its the good old saying garbage in garbage out!!

30th Oct 2007, 21:33
The boxes will almost ALWAYS show max crz altitude for current conditions and figures. The only reason i say "almost always" is due some NNC's.
Of course this does'nt negate the last statment.....

jonny dangerous
31st Oct 2007, 17:09
The boxes will almost ALWAYS show max crz altitude for current conditions and figures.

One other small, very small, caveat: the Max Alt may be off by, ohhh, 100-200 feet if you're heavy and fly into warmer air than was input on the PERF INIT page TOC OAT field while on the ground.

This airplane's FMC doesn't seem to receive updated SATs automatically.

Additionally, the FOB indication increases by tonne or so on departure. It might appear you're "making fuel" initially. My assumption is that this is due to the deck angle. Not sure of the impact on MAX ALT display, but another assumption would be that it would initially lower that output from the pre-takeoff number.

31st Oct 2007, 17:53
Max Alt will also be affected by the entered CRZ C of G. This defaults to 5% which is the worst cases scenario and thus gives the lowest Max Alt. For a more accurate estimation the real CRZ C of G needs to be entered when at CRZ FL.

31st Oct 2007, 18:59
Like I said......"Current conditions and figures"..........:hmm:

PPRuNe Towers
31st Oct 2007, 19:14
News has consistently been reaching us over the last couple of northern hemisphere summers and from both sides of the pond.

NG's, both 700 and 800 have not behaved as advertised at or close to FMC max level. It's equally interesting how the airframes have stayed out of the news as well as the intended flight level.

Advice from us at the Towers is to confirm updated, real, environmental conditions/weight and check the QRH cruise tables before the final part of the climb.

That's QRH/Flight manual not the FMC.


31st Oct 2007, 19:23
.........or, in these 'greener days', stick with 'Optimum' or lower and be safe - and green:)

31st Oct 2007, 19:39
Things changed a bit with the new FMC update. If the perf init page on the ground is asking for the actual temp at the cruise leveg eg. -56 iso isa deviation, then following will happen:

On the ground:

FMC does not know actual conditions at the cruise FL, so its calculating with the given (anticipated) figures (weight, temp, cg)

In the air:

FMC calculates with actual temp and weight and the entered CG, this way taking care of changes of temp along the route

If you want to be even more precise you can calculate your actual cg...

(Only for new FOs in line training..)

I Just Drive
2nd Nov 2007, 01:01
One day I would like to father healthy children therefore I stay away from anything beginning with a 4. On top of that (or under it I should say), on sectors less than 3 hrs, the savings from 370+ are (in my opinion) not worth the stress so why test the computer and go so high?

FCS Explorer
2nd Nov 2007, 01:35
during preflite i set CG to 18, so the box stops b#tching about "MAX LVL xy"
the TOC isa i leave blank. why? tried to do it perfect with temp for wpt nearest to TOC from fliteplan. didn't work. and it's dahes, not boxes.
with final loadsheet we set MAC@TO as CG. works fine. only very minor changes on CRZ page (opt/max) during climb due to fuel burn.

Jet Man
2nd Nov 2007, 08:42
MAX ALT does changes after take off - it lowers.
Our FMCs default to 26.2 in CRZ CG on PERF INIT page - I modify this by adding 1.5 units to T/O MAC (we always have centre tank fuel on take-off). I also add about 5 degrees to forecast top of climb ISA dev (slightly conservative).
Any other recommendations?
A foward CG and high (ISA dev above +15) sends the MAX ALT way down.

wee one
2nd Nov 2007, 10:45
Use the qrh to check. That is thrust maneuvere limited. Otherwise You can sit fat dumb and happy at max alt until you have an upset, hit warmer air, tailwind etc.then ure fecked

PPRuNe Towers
2nd Nov 2007, 14:02
And wee one has it correct.

The wing isn't the prime issue.

Thrust, or lack thereof, is the real problem.

Aircraft are genuinely falling out of the flight levels. There is an amazing use of the old boy network, sheer luck and commercial cowardice working to ensure you aren't reading about this in Aero or the crash comics.

Danny and I do not speak with forked tongue. Ain't enough thrust at max alt to keep you out of trouble. Happened to me in a light, 27k aircraft over Hungary - it can happen to you.

I file no level ever above 380 for every trip regardless of weight as SOP


jonny dangerous
2nd Nov 2007, 16:07
Mr Towers, just out of curiosity---was that a wingletted 800 or not?

Our outfit had some probs when we started receiving the wingletted 800's and still following Boeing's step climb recommendation of climbing to 2000 feet above OPT ALT etc.

Our SOP for step climbing is now (no more than) 1000 feet above OPT ALT. The 800 seems to be much less happy climbing thusly, so I'll most times delay until within 500 feet of OPT.

Personally I find lots of colleagues wanting to get up there as soon as we can: i.e. the instant the OPT says 380, guys/gals will want 390. I have taken to pulling out pen and paper and recording the vitals prior to the climb (Wind Component, G/S, TAS, FF) and then comparing them to the new vitals at altitude.

It sometimes, actually often surprises these colleagues that SGR has actually decreased (depending on winds etc).

Most colleagues also surprised when doing the Step Climb evaluation via the FMC and the New Cruise Evaluation via the FMC. Sometimes they differ by half. Who knows what the variables are in the algorithms employed, but in either case, what is 0.1% savings exactly? Not accounting for rounding error, if 5T of fuel will be burned between present position and destination, saving 0.1% fuel looks to me like a savings of 5 kilos of fuel (if the modelling assumptions are correct: a dubious proposition IMHO). And this on engines burning 1000 kilos an hour, over the following two hours. That's quite a level of precision...

My take? No rush to step climb. If the ride's good, and unless there's a terrific wind differential, the savings might be marginal, or non-existant.

However, my outfit's mantra is pretty much follow the flight plan from dispatch. And with no structured educational program to highlight these issues, we're at 410 much more often than is warranted, I think.

PPRuNe Towers
2nd Nov 2007, 23:20
Winglet equipped indeed.


2nd Nov 2007, 23:38
Over the years, whiling away the hours I used to try and back-figure what wetted area, roughness, drags, etc were. Goodly enough to blag my way into working for the makers rather than the flyers when the time came. Here is a strange thing... or maybe not so strange. On a wide spread of makes and marques including the B737-400, B767-300, B747-200, B747-400, A330-200 and A319-100 a common factor emerged. It is a rare bird that performs well aerodynamically in cruise (and mated to the engine) above lift coeffts around 0.56

I really ought to have got out more often but I ended up with wodges of comparative data that I rendered into range factor = (V/c)*(L/D)

Of course to know "c" you have to know "L/D" but it is not the individual values that matters so much as the product. All joking aside but I learned when to step and when not to step and usually got my fuel burn about right

Okay, c of g and trim and all that I know too. Just observing that as soon as the range factor next available level looks to be better and there are no penalties such as wind (pardon vicar) and you have at least an hour of cruise left, give it a shot. But if the lift coefft reqd would be high-ish and it is above FL390, I might be inclined to give it a rest.

What I mean by high-ish is hard to define other than I have given it a lot of thought in a lot of hotels. BTW I work for that other firm now. There is a Dog!

Best Rgds

The "E"

Jet Man
3rd Nov 2007, 02:47
We fly 26k 800Ws and QRH performance says you can't even reach optimum altitude at high ISA deviations (> +15) alot of the time! The machine seems to run out of puff very quickly. Besides, above optimum the yellow snake starts descending ready to bite!

Kit d'Rection KG
18th Nov 2007, 20:30
I hadn't realised that the forum toilet attendant was also now an aerodynamics, performance, and certification expert.

Many -800 pilots like to be within 1000ft of optimum altitude; few like to be at FMC-stated ceiling.

That said, the aircraft and its FMC meet the relevant certification standards in this regard, with any FMC update. Having had an upset at 410 in another type, well within its margins, I suspect that insufficiently-demanding standards and/or greater-than-predicted wind shifts are the genuine issue here. :D