According the FMS Operating Manual (FMS.40.44) there is a "[..] capability to constrain the aircraft to an altitude at a waypoint cruise. [..] The flight level must be equal to one [..] on the F-PLN INIT page. [..] Entry of an altitude constraint during cruise flight will relocate the associated S/C to the constrained waypoint [..]".When I'm trying to input a S/C before the optimum position calculated by the FMS everything works as stated but when delaying it I get the predictions related to the optimum S/C position instead of the constrained one. Some pics will probably make everything easier to understand.1. a/c cruising at FL360 with a S/C at FL380 in the F-PLN INIT CRZ LEVELS2. typed /S380 at KUKUN........shows that regardless of the entered constrain, the FMS hasn't relocated the constrained S/C and is making predictions on the assumption that the a/c will overfly NANER and KOSIV steady at FL380. Is this the way it's supposed to work?Regards,Olmo Matteo DI CARLO

Salve,you're trying to constrain so that you S/C after KUKUN, right?What you need to do is enter the constraint as S360 at KUKUN.You were thinking Boeing way. With the MD11 the logic is different.Cheers,Kris

Ciao Kris,

[..] you're trying to constrain so that you S/C after KUKUN, right?

From what is written in the FMS Operating manual, my understanding is that the a/c will complete his step climb over KUKUN...

"[..] capability to constrain the aircraft to an altitude at a waypoint cruise. [..]".

What you need to do is enter the constraint as S360 at KUKUN.

????? The a/c is already flying at FL360 and the S/C has been constrained on KUKUN by typing /S380 and confirmed by SFL380 on the ALT column

2. typed /S380 at KUKUN.....

The point is that notwithstanding the entered constrain, the FMS is still planning to S/C at FL380 at the optimum calculated position about 9' before KUKUN.Regards,Olmo Matteo DI CARLO

Olmo,you keep thinking the Boeing way...The logic in the MD11 is different. By entering S360 on KUKUN vertical revision you tell the FMC to maintain your current level of 360 to KUKUN and S/C to the next level after KUKUN.Try it. She'll do what you are requesting.Cheers,Kris

The logic in the MD11 is different. By entering S360 on KUKUN vertical revision you tell the FMC to maintain your current level of 360 to KUKUN and S/C to the next level after KUKUN.

Hm. From reading page FSM.40.44 (which is pretty hard to understand) I would have expected it to work the way Olmo is trying. Kris, do you have knowledge of how the real-world FMS behaves, or are your comments based on experience from flying the PMDG model?The paragraph that I find most relevant is this one:"Entry of an altitude constraint during cruise flight will relocate the associated S/C to that constrained waypoint. For example, entering S330 (S is the operative prefix) will move the optimum FL330 S/C from the optimum position in the F-PLN to the newly constrained waypoint".

No, I don't fly the MD11 in RW.My statement is based on the paragraph you quote and experience on the PMDG MD11.

"Entry of an altitude constraint during cruise flight will relocate the associated S/C to that constrained waypoint. For example, entering S330 (S is the operative prefix) will move the optimum FL330 S/C from the optimum position in the F-PLN to the newly constrained waypoint".

Look at the FMC in Olmo's pic. There is the S/C line and the T/C line.The sentence is tricky, but is telling is that you move the S/C to the waypoint of choice. You are constraining the S/C line to a waypoint.PMDG has done a fantastic job with the MD11s FMC so I assume this behavior should be similar on the real bird, but someone else with RW MD11 experience needs to confirm...Cheers,Kris

Hi, if you want to delay the S/C you have to enter the start altitude of the S/C This means you have to enter /S360 at KUKUNOn the MD-11 it works as described.Jan-Paul

PMDG has done a fantastic job with the MD11s FMC so I assume this behavior should be similar on the real bird, but someone else with RW MD11 experience needs to confirm...

Indeed but it isn't a certified training simulator.

The sentence is tricky, but is telling is that you move the S/C to the waypoint of choice. You are constraining the S/C line to a waypoint.

Right but lets discuss about the indications provided by the legs page. Using /S380 or /SFL380 you are telling the FMS where to end the step climb. By doing so it will disregard his optimum climb profile and recalculate the S/C (Step Climb) in order to have the T/C (Top of Climb) exactly over the constrained fix.Example:We are flying inbound A, B, C at F360 and the optimum S/C calculated by the FMS is located between A and B. Legs page would be like this....A 1500 .82 FL360 ---> We left A at 1500UTC at Mach0.82 FL360(S/C) 1510 .82 FL360 ---> FMS suggesting to start the Step Climb at 1510UTC at Mach0.82 while we are still at FL360(T/C) 1512 .82 FL380 ---> FMS predicts that at 1512UTC at Mach0.82 we will reach FL380 (Top of Climb)B 1523 .82 FL380 ---> FMS predicts that we will overfly B at 1523UTC at Mach0.82 at FL380C 1547 .82 FL380 ---> FMS predicts that we will overfly C at 1547UTC at Mach0.82 at FL380Now lets insert a S/C constrain over C ---> /S380 or /SFL380. Back to the legs page.....A 1500 .82 FL360 ---> We left A at 1500UTC at Mach0.82 FL360B 1522 .82 FL360 ---> FMS predicts that we will overfly B at 1522UTC at Mach0.82 at FL360(S/C) 1547 .82 FL360 ---> FMS is suggesting to start the Step Climb at 1547UTC at Mach0.82 in order to achieve the T/C over C(T/C) 1549 .82 FL380 ---> FMS predicts that at 1549UTC at Mach0.82 we will reach FL380C 1549 .82 SFL380 ---> FMS predicts that at 1549UTC, while reaching our T/C we will overfly C at 1549UTC at Mach0.82 at FL380When you try to anticipate the optimum climb profile provided by the FMS, the a/c behaves by the book but when I try to postpone the S/C nothing changes although the FMS Ops Manual clearly states that....

[..] Entry of an altitude constraint during cruise flight will relocate the associated S/C to the constrained waypoint [..][/i]"

Looking at the posted screenshot....

and keeping in mind the above description we can see that even with a constrain over KUKUN (SFL380) the FMS will start and end his climb at FL380 well before KUKUN (9' before) and completely ignore the constrain command.

if you want to delay the S/C you have to enter the start altitude of the S/C. This means you have to enter /S360 at KUKUN

Jan-Paul you are right but as shown on the previous screenshot....1. The constrain as been entered (SFL380);2. The S/C and T/C haven't been recomputed accordingly.Regards,Olmo Matteo DI CARLO

you are telling the FMS where to end the step climb

This is wrong here. Its just the other way around. With SC you are telling the FMC where to start the SC. If you want to start the SC at KUKUN from FL 360 to FL 380 you have to enter FL 360 at KUKUN. If you want to be at FL 380 at KUKUN you have to place a bearing waypoint before KUKUN where you start the SC to reach FL380 at KUKUN. Jan-Paul

This is wrong here. Its just the other way around. With SC you are telling the FMC where to start the SC. If you want to start the SC at KUKUN from FL 360 to FL 380 you have to enter FL 360 at KUKUN. If you want to be at FL 380 at KUKUN you have to place a bearing waypoint before KUKUN where you start the SC to reach FL380 at KUKUN. Jan-Paul

Actually, that is not quite correct either as Olmo has shown, it does not force a step climb nor does it describe the start or end of one. All it is is a constraint that applies to the specified point. In Olmo's case, putting /SFL380 at KUKUN had no effect because the aircraft was already at FL380. Had the aircraft been at FL360, then this constraint would have forced a step climb to 380 (assuming this was still below max cruise), and had Olmo entered /S360, then the constraint would have delayed the earlier step climb until at (or after) KUKUN.

If you perform a planned earlier S/C before the calculated S/C the constrained waypoint will be reached at the entered (higher) altitude where you go to. - you are telling the FMC where to end the S/C. If you want to perform a planned delayed S/C after the calculated S/C the constrained waypoint is the start point of the S/C (altitude entered where you come from)- you are telling the FMC where to start the S/C.Jan-Paul

If you perform a planned earlier S/C before the calculated S/C the constrained waypoint will be reached at the entered (higher) altitude where you go to. - you are telling the FMC where to end the S/C. If you want to perform a planned delayed S/C after the calculated S/C the constrained waypoint is the start point of the S/C (altitude entered where you come from)- you are telling the FMC where to start the S/C.

I got it now, thanks for the patience to Jan-Paul and Kris. It seems that also real life MD11 drivers had discussed about it on another forum.

[..] If you want to be at FL 380 at KUKUN you have to place a bearing waypoint before KUKUN where you start the SC to reach FL380 at KUKUN.

Example to make sure that things are clear to everybody:1. go to ACT F-PLN 2/2 and note the distance between (S/C) and (T/C) --> 17NM;2. type in the scratchpad KUKUN/-17 and LSK over KUKUN to get 017KUK;3. Constrain 017KUK with the following /S360 (S/C will start here);4. (T/C) will match KUKUN position.Regards,Olmo Matteo DI CARLO

Hi Olmo, I do not think that is the correct way to think about step climbs and constraints. If I wanted to be certain the aircraft was at 380 at KUKUN then I would simply give it a hard constraint /380 at KUKUN, and if I wanted to force a change of altitude between two points, then again, I would ensure the points are in the flight plan and use hard constraints. Where I think step control comes into its own is on company routes where I will use the same basic flightplan day in, day out but with different loads, weather etc, and so different optimums. Then I could say /S380 at KUKUN and by that I would be telling the FMS that I want to be at FL380 at or before KUKUN but let the FMS chose when to do the steps based on current optimums, or I could say /S360 at KUKUN to say do not exceed FL360 at KUKUN even if that means delaying a step climb. Again, you are not forcing the step climb to happen, you are only constraining when they should or should not happen. Perhaps a better example would be when you have a a reporting point at 320 (perhaps joining an airway). Most days you are heavy and optimum would be 310 but some days you are light and optimum might be 340. By setting /S320 you tell the FMS to delay the step climb when you are light but push it early on the days when you are heavy. This way the plane can stay as close to optimum as possible and still match the altitude constraints where required.