News:

Precision Simulator update 10.173 (24 February 2024) is now available.
Navburo update 13 (23 November 2022) is now available.
NG FMC and More is released.

Main Menu

MAX & RECMD ALT

Started by simonijs, Fri, 27 Mar 2020 11:46

simonijs

Moin Hardy,

The variables for the calculation of Maximum Altitude are discussed in KLM's FCOM, but are described better in the (obsolete) AOM:

Maximum Altitude is the highest altitude to which the aircraft can climb for the corresponding weight, speed limit, Center of Gravity and rate of climb criteria.
The altitude is limited by:
•   1.3g load factor limited altitude based on Mach .86 and default or manual entered Centre of Gravity.
•   Rate of climb of 300 fpm, available at the planned climb speed and maximum climb thrust.
•   Rate of climb of 100 fpm, available at planned cruise speed and maximum cruise thrust.


The AOM contains tables – for both the PAX/Combi and Freighter – that make a distinction between the Maximum Altitude for 1,3G and Maximum Altitude for 100 fpm residual Rate of Climb [RoC], the latter being dependent of temperature at altitude. MAX ALT for 1,3G in these tables is always lower than MAX ALT for 100 fpm RoC.

I have two questions with regard to Maximum Altitude in PSX:

1.   Is the indicated MAX ALT the lower of the 1,3G MAX ALT and the MAX ALT for a residual 100 fpm RoC, or is MAX ALT always indicating the altitude at which it can still climb with a RoC, anywhere between 500 fpm and 100 fpm (company option)?

On several PSX flights at FL400, I noticed that the NG FMC is recommending an altitude very close to the MAX ALT: see screenshot below.

2.   Can you confirm that the FMC on the real plane is recommending altitudes, this close to the Maximum Altitude?

The suggested Step Climb on the FMC/ND – in this scenario - would occur where the OPT ALT reaches FL410. At that point, the MAX ALT had just changed to FL432. More screen shots and/or a situ.file are available if you like.

If the answer to question 1 would be: "100 fpm RoC"...: in the scenario for question 2, the PSX aircraft was still climbing happily at 1800 fpm passing through FL425 for FL430 with the MAX ALT indicating FL432.

To me, it looks like the FMC is calculating this step as if it is using a step size of 4000 ft (ICAO): when the OPT ALT is indicating 2000 ft below the next available altitude, it suggests to step climb to that next altitude. So: flying at FL310 with the OPT ALT indicating FL330, climb to FL350. MAX ALT in PSX then indicates FL358 (using situ Basic 009 and model PH-CKB). However, the spread in PSX between VMO and top of the amber band at FL350 is much wider (± 40 kts) than at FL430 (± 15 kts).

Regards,
Simon



Hardy Heinlin

Hi Simon.

Quote from: simonijs on Fri, 27 Mar 2020 11:46
1.   Is the indicated MAX ALT the lower of the 1,3G MAX ALT and the MAX ALT for a residual 100 fpm RoC, or is MAX ALT always indicating the altitude at which it can still climb with a RoC, anywhere between 500 fpm and 100 fpm (company option)?

As far as I recall, the data in PSX is based on a graph from a certain document. The output value is further fine-adjusted by the CRZ CG value entered on the PERF INIT page. The more aft the CG, the higher the MAX ALT.

Minimum RoC, CLB/CRZ thrust limit in CRZ, and minimum CRZ time data from the PERF FACTORS (AIRLINE POLICY) dataset are used for OPT ALT, not for MAX ALT.


Quote from: simonijs on Fri, 27 Mar 2020 11:46

2.   Can you confirm that the FMC on the real plane is recommending altitudes, this close to the Maximum Altitude?

I can't. I have no access :-)

According to various documents, the MAX ALT is the limit anyway. So why should it not be that close? RCMD ALT is the most economic altitude with regard to weather and cost index. When the wind at MAX ALT provides the cheapest conditions, then that's the recommended altitude. If the step size is as great as 4000, the cheapest conditions may actually be a bit lower; but you're forced to use RVSM levels obviously.


Regards,

|-|ardy

Hardy Heinlin

PSX uses these RVSM levels for the RCMD ALT function:

Westward
            rcmd[ 0 ].alt = 25000;
            rcmd[ 1 ].alt = 27000;
            rcmd[ 2 ].alt = 29000;
            rcmd[ 3 ].alt = 31000;
            rcmd[ 4 ].alt = 33000;
            rcmd[ 5 ].alt = 35000;
            rcmd[ 6 ].alt = 37000;
            rcmd[ 7 ].alt = 39000;
            rcmd[ 8 ].alt = 41000;
            rcmd[ 9 ].alt = 45000;

Eastward

            rcmd[ 0 ].alt = 24000;
            rcmd[ 1 ].alt = 26000;
            rcmd[ 2 ].alt = 28000;
            rcmd[ 3 ].alt = 30000;
            rcmd[ 4 ].alt = 32000;
            rcmd[ 5 ].alt = 34000;
            rcmd[ 6 ].alt = 36000;
            rcmd[ 7 ].alt = 38000;
            rcmd[ 8 ].alt = 40000;
            rcmd[ 9 ].alt = 43000;


No other altitude values are used for the RCMD ALT function when in RVSM mode.

And limited by MAX ALT, of course.