747-8: RCMD ALT calculation in FMC

[Woozie 732]

Not sure if this a 747-8 "quirk" or possibly even a bug, but the RCMD ALT calculation in the 748 FMC seems to be a bit weird sometimes. While the 747-400's calculation was always quite close to the OPT ALT value, on the 747-8 it tends to be closer to the MAX ALT

One example from my flight yesterday: 

OMDB to EDDM, with a ZFW of 289'300kg, FOB 76'400kg and TOW 365'000kg. Simbrief calculated an initial CRZ ALT of 340 with a step climb to 360. 747-8 FMC shown something like 350 as OPT ALT, but RCMD ALT shown as 380 while MAX ALT was around 390 during preflight. Winds were loaded from AS. When setting the step size to 2000 it calculated a step climb to 360.

Once airborne and after I performed the step from 340 to 360, no further step was calculated by the FMC but RCMD ALT was still 380. About halfway through the flight, the RCMD ALT dropped down to 360 with a MAX ALT of 400/410

Is this behavior to be expected and is there any reason why its apparently different to how the 747-400 calculates the recommended value? Has anyone else seen this as its not the first time I by the ALT calculations on the 747-8, so it doesnt seem to be a on off occurrence. 

[Ephedrin 418]

20 minutes ago, Woozie said:

Not sure if this a 747-8 "quirk" or possibly even a bug, but the RCMD ALT calculation in the 748 FMC seems to be a bit weird sometimes. While the 747-400's calculation was always quite close to the OPT ALT value, on the 747-8 it tends to be closer to the MAX ALT

One example from my flight yesterday: 

OMDB to EDDM, with a ZFW of 289'300kg, FOB 76'400kg and TOW 365'000kg. Simbrief calculated an initial CRZ ALT of 340 with a step climb to 360. 747-8 FMC shown something like 350 as OPT ALT, but RCMD ALT shown as 380 while MAX ALT was around 390 during preflight. Winds were loaded from AS. When setting the step size to 2000 it calculated a step climb to 360.

Once airborne and after I performed the step from 340 to 360, no further step was calculated by the FMC but RCMD ALT was still 380. About halfway through the flight, the RCMD ALT dropped down to 360 with a MAX ALT of 400/410

Is this behavior to be expected and is there any reason why its apparently different to how the 747-400 calculates the recommended value? Has anyone else seen this as its not the first time I by the ALT calculations on the 747-8, so it doesnt seem to be a on off occurrence. 

I‘ve seen this too. I then checked the winds up there and they gave me an advantage in EFOB at the destination. I think the FMC takes this into consideration. While a lower flight level might have been better in calm conditions the 30kts more 4000 feet higher was a good argument to climb further in comparision to the higher required fuel burn. On an 11h flight this is quite a bit of fuel..

I am not sure why it wasn‘t displayed earlier on the ground but I guess the airplane, while knowing the winds already on the ground, still can calculate closer when it‘s actually in the air.

[Woozie 732]

Good point actually, thanks Marc!

I'll check winds higher up next time it happens, lets see if it makes any sense then

[Woozie 732]

Just did another flight yesterday, KSFO to EDDF, and the RCMD value was just below MAX ALT, and about 3500 feet higher than OPT ALT. I did check the winds on higher FL's and they weren't favorable at all. The B747-400's suggestion was much closer to the actual OPT FL, so i'm still wondering if this is a 747-8 thing

Anyone from PMDG or real world 747 pilots care to elaborate?

Edited October 15, 2018 by Woozie

[downscc 1,327]

I've seen the RECMD value explained a couple of times by PMDG and I still don't understand it.  It is as if the FMS derives a recommended value based on a small specific set of parameters and the value is not the optimum and has nothing to do with CI.  I ignore it.  The key value is the optimum value and in your case where you are at FL350 OPT FL355 the recommended step to FL370 is going to occur when the OPT is FL360... that is what I pay attention to.

[Woozie 732]

Thats right, according to the 747-8 FCOM, OPT ALT calcs are based on GW, CI, selected speed and pressure altitude, while RCMD ALT is calculated based on GW, selected speed, pressure altitude and forecast winds and temps for the next 500nm. So the RCMD value seems to be the one to be followed from an economical point of view. But i still dont quite get why its that high, compared to how the 777 and 747-400 calculates this value.

 

[downscc 1,327]

2 hours ago, Woozie said:

So the RCMD value seems to be the one to be followed from an economical point of view. But i still dont quite get why its that high, compared to how the 777 and 747-400 calculates this value.

"From an economic point of view," ignoring the value of time and simply based on fuel consumption (or LRC).  Comparisons with the B744 in this respect cannot be made because the -8 has significantly different wing and engines.... and the 777 is just an altogether different airplane.

[Woozie 732]

Ignoring CI but taking enroute winds into consideration, that still sounds more economical to me 😉

I am aware that the -8 is a different plane compared to the 744 and the 777, i‘m simply trying to understand why the calculation is so different. Its a „study level“ airplane after all, isnt it?

But as long as no one from PMDG or any real 748 pilots chime in it will remain a mistery i‘m afraid

Just to get things straight, i‘m not trying to critizise PMDG here, i‘m just trying to satisfy my thirst for knowledge

[downscc 1,327]

5 minutes ago, Woozie said:

i‘m simply trying to understand why the calculation is so different

I'm not sure you have established that the calculation is "so different."  You mention that you observe a RECMD that appears to be close the MAX than OPT than is the case for the B744.  Your TOW is 42 T below MTOW because you are relatively light on fuel (OMDG EDDM) for this aircraft... and compared to the B744 which would be much close to MTOW give then same block fuel load.  So yeah.... still lots of loose ends to make a conclusion that the calculation is "so different."

The one thing about the -8F that surprised me and has had an effect on flight planning is just how much fuel and how long the legs are for this lady given a hefty load.  She is impressive, real real global freighter.  And this is what is "so different" between her and her sister the -400.  I flew a KSDF OMDB mission then compared it to what I could do with the -8F which is what UPS uses on that route and the difference was impressive.  I suspect this has as much to do with what you are perceiving with the recommended verses optimum and maximum cruise altitudes as anything else.

[Woozie 732]

Yes, in my first example i was rather light, but see my second example, its from a KSFO-EDDF flight with 120 tons of fuel onboard and my TOW was pretty close to MTOW. I took the FMC screenshot about 1 hour after TO, so it was still pretty heavy. RECMD is 200 feet lower than MAX ALT. 

And again, i'm not concluding, i'm just trying to understand!

Never mind, I think I better ask this question in the pprune forum...

[downscc 1,327]

5 minutes ago, Woozie said:

I think I better ask this question in the pprune forum...

Great.. please elucidate here when able.

[skelsey 765]

Hi Lars,

I found some notes on the NGFMC RCMD and STEP TO functions which I think will answer many of your questions -- I'll write them up later this evening!

[Woozie 732]

Brilliant! Thanks Simon, i really appreciate that!

[skelsey 765]

No worries -- here goes!

The VNAV CRZ Page

The information on the VNAV Cruise page is used to fly the vertical profile and the FMC will recommend a cruise flight level, together with when it thinks you should "STEP TO" the next (higher) flight level from the current FMC CRZ alt.

STEP TO
This specifies when to climb above current cruise altitude, with the interval defined by STEP size (e.g. RVSM = 2000ft, ICAO = 4000ft, or pilot-entered value).

STEP TO will only ever recommend a climb above the current FMC CRZ altitude in accordance with the STEP size, and never a descent. The FMC uses forecast winds, temperatures and the specified STEP SIZE to calculate flight levels which give the lowest 'cost function' over a rolling (up to) 500nm in front of the aircraft. In other words, the FMC looks up to 500nm in front of the aircraft and if it can't see any advantage to climbing it continues to incrementally look beyond that distance until it can see an advantage in climbing. This then becomes the STEP TO point. Because of this incremental looking ahead of the aircraft, step predictions are regarded as 'stable'.

Factors which affect the STEP TO calculation include:

SPEED - changing the CI or entering a selected speed may affect ground speed and therefore cost function

STEP SIZE - the FLs recommended are constrained by the specified step size. Entering a step size of 0 tells the FMC that you wish to remain at your current cruise altitude for the remainder of the flight. This can be useful as a short term 'fix' to suppress nuisance FMC Scratchpad/EICAS messages if ATC prevent you from climbing BUT it will also suppress all STEP TO calculations which may affect ETA and fuel predictions, so be mindful of this.

OPT
This is only a function of aircraft weight for a given speed setting based on still wind conditions and should therefore not be used.

MAX
This takes in to account the current gross weight, temperature and speed schedule and is influenced by CG. It is limited to the maximum certified altitude of the aircraft. You may see a future STEP TO FL above the current MAX FL.

RECMD
This is the FL currently recommended by the FMC. Confusingly, this can be below the current FL. This is the only aspect of FMC predictions that requires a degree of flight crew interpretation (so say the notes!).

Even when the STEP TO FL becomes the RECMD FL you should still wait until you actually get to the STEP TO point before climbing.

RECMD uses the same fundamental algorithm as STEP TO and again uses forecast winds, temperatures and STEP size. However, it only ever looks 500nm in front of the aircraft and considers where it is best to be right now. As such, these predictions are regarded as being less stable than the STEP TO calculations.

Additionally, RECMD will also look below the aircraft and recommend descending if it determines a short term benefit from more favourable winds at lower levels.

This is where a degree of pilot interpretation is required -- i.e. "over what distance am I going to get better winds at lower levels and if I do descend will other traffic block me from climbing again later" -- using the forecast wind data on the paper OFP may be useful here.

Hope that helps!

[downscc 1,327]

2 hours ago, skelsey said:

it only ever looks 500nm in front of the aircraft and considers where it is best to be right now. 

Does the RECMD algorithm include the cost of the step?  I was under the impression that the STEP TO did but not RECMD.

UPDATE:  The FMS for 747-8 manual I have from Honeywell only says that RECMD display the most economical altitude for the next 500 miles based on GW, speed option (CI) and constant altitude based on entered winds and temperatures.  I infer that the difference between optimum and recommended is that optimum is based on trip values where recommended is based on a 500 nm segment.  Also recommended is based on winds/temps as entered by crew where the optimum is based on standard atmospheric conditions at the current gross weight.

Regardless, as the OP hypothesis that something is different about the -8 RECMD calculation I have found that the FMS is pretty uniform across the wide body line in how recommended is determined.  Any perceived differences are due to a different wing, weights and possibly winds given in the cited examples.

Edited October 17, 2018 by downscc