View Full Version : FMC MAX ALT Calculation
Apart from ZFW, CRZ CG and the PERF FACTORS on the MAINTENANCE page of the FMC, what else affects the MAX ALT calculation?
I'd always thought once airborne, MAX ALT would always go up assuming you didn't fiddle with any of the above factors...
However, there I was climbing away from CWL yesterday watching the MAX ALT, and having touched nothing, it went down.
:confused:
What might be the reason for that? Perhaps a VNAV performance iteration that resulted in a computed ECON TGT SPD that was a little higher, with the corresponding effect on MAX ALT? What other factors affect the MAX ALT calculation?
Cheers.
:ok:
Seat1APlease
29th Dec 2004, 10:42
On the 734 the speed is certainly a prime candidate.
You can sometimes get it to give 350 rather than 349 by altering the cruise mach number very slightly. I am wondering whether the actual OAT will have an effect also. I don't know how the FMC takes this into account but on occasion I noticed the max alt varying during the climb as it is recalculated.
spoilers yellow
29th Dec 2004, 12:18
On the 757,767. the CRZ page Mach No./ speed entry aswell as the ALT/OAT entry on the legs RTE DATA page will all change the calculated max alt, not too sure why it would go down if none of these were manually changed though.:confused:
Alaskan Timber
29th Dec 2004, 20:58
After T/O max altitude in the FMC indeed shows lower. Maybe because the pitch is different than on the ground, resulting in a different fuel quantity indication = different GW.
Lackof747
30th Dec 2004, 18:53
That may differ between old FMCīs and Pegasus on the 757/767. The older ones do not have cg input, but...thinking while writing... I donīt think that even the Pegasus has direct cg reading. Itīs put in manually, at least where I know.
Sorry...
Should have read the previous post...
Darn lllysdddexia!
I read the GW as CG.....
FullWings
30th Dec 2004, 22:25
Yes Wino, hit the nail on the head!
Did a N. Atlantic crossing about 18 months ago. Planned at FL360. Took off from LHR several tons lighter due some freight not making it.
Asked by ATC about 1/2hr after takeoff if could take FL380 now and for the crossing. Checked MAX ALT and it was FL392 so no problem and the oceanic entry point was at least an hour away giving an even greater margin.
About three hours into the flight, in the vicinity of Iceland, I noticed that the thrust levers had been well forward for some time. I looked down from my newspaper ;) and saw MAX ALT FL368 on the scratchpad and the yellow lines almost meeting in the middle of the speedtape!
It was a rather interesting situation, sitting there with full chat for about 15mins - what made it even more novel was another aircraft 1000' lower, directly underneath. We held our breath for a bit but luckily it was flat calm at 380 and the air slowly cooled off over the next 1/2hr and that combined with the weight reduction brought us back inside the envelope.
At FL380 the TAT was +3, i.e. nearly ISA+30. TAS was well over 500Kts, which was nice, but I would have traded that for a little more margin...
Techman
31st Dec 2004, 02:16
As any graph on a piece of paper will tell you, it is speed, weight and temperature that determines your maximum altitude.
But a graph on a piece of paper, would be too old fashioned to use I suppose.
FlareArmed
2nd Jan 2005, 13:11
Our policy is to enter the forward limit CoG and the forecast cruise temperature. This generates a conservative MAX ALT.
The problem of the MAX ALT dropping (sometimes considerably) can be due to the cruise temperature being warmer than expected. The FMC defaults to ISA. If nothing else has been entered, the FMC will calculate MAX ALT on a predicted cruise temperature of ISA. When cruise altitude is reached, the MAX ALT and amber bands are then calculated on ACTUAL temp.
Therefore, if the actual temperature is ISA +10 or 15, the MAX ALT will drop and the gap between the upper and lower amber bands will reduce.
fire wall
2nd Jan 2005, 18:15
In VNAV path and CRZ page selected to ECON CRZ....fly into increasing headwind and CRZ Mach will increase resulting in reduced max alt
Cheers gentlemen.
As Wino & Techman allude to, you can find graphs of max altitude versus weight or temperature, but I haven't seen many parametric graphs with both on...
None in our performance manuals that I've come across....(must look harder).
The FMC Guide (http://www.fmcguide.com) doesn't indicate the variables that affect the calculation.
Its not that I wasn't aware of the fact that temperature affects maximum altitude capability but I wanted to know how the FMC works.
If FlareArmed is correct, I wouldn't have expected MAX ALT to have changed in the climb in the absence of changing any other variables.
In this case, I would expect the predictive ability for MAX ALT to behave as per the ECON TGT SPD (in the CLB and CRZ phases of flight) in the sense that, as the FMC Guide indicates, these speeds will take into account wind if a CRZ WIND is entered after PERF INIT has been executed, but, subsequently, the TGT SPD's will not adjust for wind until you either enter a new CRZ ALT or you reach the CRZ whereupon the CRZ TGT SPD speed is adjusted depending on the actual wind.
In my case, MAX ALT changed early in the climb.
What might be the reason for that? Perhaps a VNAV performance iteration that resulted in a computed ECON TGT SPD that was a little higher, with the corresponding effect on MAX ALT? What other factors affect the MAX ALT calculation?
I'm not sure whether the above statement is gobbledigook?
I've not actually noticed whether the ECON CRZ TGT SPD changes during the climb phase. I hazard a guess, not, for the reason already suggested.
However, it may be that the relatively simple problem of determining MAX ALT is done real time via the FMC using a look-up table that depends on actual instantaneous OAT as contributors suggest. Can anyone confirm this?
My next question concerns the statement on Pg 11.8 of the above FMC Guide which alleges that the use of OPT ALT when flying in ECON is mis-guided.
Why is this?
Sure enough today, at M=0.741 in ECON, OPT ALT = 354. Entering a fixed Mach cruise of M=0.741 gave OPT ALT = 361.
Whilst I understand that ECON flying seeks to minimize cost per nautical mile and LRC/fixed Mach cruise seeks to maxmimize nautical miles per pound of fuel, I'm not sure I understand the above discrepancy.
Cheers.
:ok:
oldebloke
2nd Jan 2005, 20:01
On the Airbus fooling around with the 'cost index' changes the MAX ALT..If planned at M78 @CI +50,once in cruise note the M<ALT then reduce CI(speed) to +10 note the 'higher MaxALT due to reduced speed demands for the Aircraft...Helps when approaching a Line o'WX....
Cheers..
:ok:
FlareArmed
3rd Jan 2005, 21:39
At the 2004 Flight Operations Symposium, there was a B737 presentation on high altitude turning. I understand an aircraft entered low speed buffet at high altitude when turning in heading mode. In LNAV, the FMC will take in to account the thrust available whereas in HDG mode, the angle of bank is only limited by pilot input.
It was presented that the FMC can only calculate the MAX ALT properly if it is given an accurate weight and cruise temperature. My recollection was that the point at which it transfers from predicted to actual temperature was at TOPC but it may well be more complex than that.
777boeings
6th Jan 2005, 10:13
Boeing recommend a flight level no greater than 2000 ft above FMC indicated optimum level.
Bearing in mind that optimum level will also change depending on cost index, speed and a/c weight, it does seem to be a much more consistent method of flight level decision making and will avoid the dangers of reduced margins.
Also a reminder that FMC recommended level is only valid if winds are accurately entered. It is not a figure to be followed blindly.
SR71,
You need to get your hands on the FMC supplementary data document for the Boeing Jet transport model xxx-xxx with whatever engines
This document contains detailed instructions on how the FMC calculates the Maximum Altitude. The document for the B777 has a chart showing Thrust limited maximum altitude based upon a standard day with 100 fpm. There are also correction charts for Temperature and Residual Rate of Climb.
If you are unable to find the appropriate document for your aircraft, I will scan the B777 pages for you.
The Optimum Altitude based on a fixed speed provides the greatest nautical air miles per amount of fuel based upon the weight and temperature.
The Optimum Altitude based on an ECON speed is calculated for the complete route using the wind and cost index value.
Both of them are Optimum Altitudes :):)
Mutt.
Cheers Mutt,
Very helpful as usual!
I approached my Chief Tech Pilot a couple of days ago with a request for any specific FMC literature he has at his disposal. I presume our Tech Manual was written with reference to actual Smiths documentation....
Hopefully it'll include the document you make reference to.
If not, I'll drop you a PM for that 777 material.
Cheers.
:ok:
dartman
10th Jan 2005, 22:47
without digging into the books on this. When you are on the ground, the Max Alt, performance numbers are based on the manually input ZFW, GTOW, and C of G. This will give you a Max Alt of xxx.x Once airborne, the C of G defaults to a conservtive forward position, and will give a different lower figure for Max Alt. Assuming you haven't fiddled with anything else, as in the original question, you can override this cruise default C of G, with the actual C of G based on ZFW C of G, and FOB. This will give, all other things aside the Max Alt. And yes, changing CI will affect the MaxAlt.