I posted in PPRUNE and got the answer below:

 

"I´m having a discussion on what entries one makes in the 777 FMC when flying for instance a trans-atlantic route which has legs where the FL is set, such as NATs legs. As I understand it from Bulfers Guide any specified step points entered into the LEGS page(s) overrides the FMC calculated ones. Say that the FMC indicates a step climb at point D, but your flight plan says to make that FL later, at point E. When you enter that FL370S the calculations will use that, not the previously calculated one at D?

Also, I assume you always enter something else than zero as STEP SIZE since you´ll still want the FMC to calculate for the parts of the flight that are not restricted vertically for various reasons? The argument could be that the flight plan from dispatch should be used completely and not make use of the FMCs calculations. I think that sounds wrong since the FMC mixes in measured data as opposed to the dispatch plan that only uses predicted data (wind/temp/weights etc). Bulfers guide also says somewhere that the STEP TO fields are blank when STEP SIZE is zero?"

 

Answer:

"Correct regarding inserting a step point- but there is a bulletin warning against entering steps down route at LOWER than the current cruising altitude.

Step size defaults to "RVSM", meaning it looks to the next appropriate RVSM level (It used to be "ICAO").

You change that to zero when you do not intend to make any more step climbs, and FMC predictions are then based on maintaining current level."

 

So, my take on it is that as long as step climbs, regardless of being calculated by the FMC or specified by the pilot, are to be planned/performed STEP SIZE should be set (RVSM most common). When, for whatever reason, no more step climbs of any kind are to be made, STEP SIZE = zero.

 

Still interesting to see how the PMDG FMC handles specified step points entered before/after calculated ones.

Linky to PPRUNE: http://www.pprune.org/tech-log/524114-entries-777-fmc-when-mixing-specified-step-points.html

 

Some more procedures surface in the thread.

 

 

I had FL350 set in the MCP, but that has automatically changed to FL380. I have not touched the Altitude selector at all since climb out from Dubai.

I am very surprised. My first instinct is to suggest this is a bug, for the reasons pcubine mentioned, as is climbing when the step is set to zero, but Iets wait and see what PMDG have to say.

STEP size only governs wether the FMC should do any step climb calculations. Think of it as an on/off switch where ON can have a number of settings. It has nothing to do with the aircraft actually performing those climbs or not. If you have a STEP size of for instance RSVM then the calculated fuel at destination is computed as if any step climbs calculated by the FMC are also done. In other words, if you have STEP size RSVM and it calculates a climb and you do not do that climb you will consume more fuel than what the FMC predicts.

 

IMO STEP size should have no direct impact in the AUTO STEP CLIMB feature behaviour. AUTO step climbs should only take into consideration the step climbs in the LEGS page, regardless if they are calculated by the FMC or specified by the pilot by entering FL370S at a certain point. I´ve yet to try out how the PMDG 777 FMC behaves when one injects specified step points. I´m curious to see if a calculated step point is removed when a specified step point is inserted after the calculated one.

I found this confusing the other day on a 2:20 EHAM-LROP flight carrying 57K lbs of fuel with Auto Step Climb disabled. Yes, I checked twice

 

During flight prep I set cruise to FL350 in PERF due to great enroute winds at that level. After reaching cruise I noticed that all of the altitude targets for waypoints at cruise level in the FMC were set to FL410. I looked at my CRZ page (which continued to show FL350 for the set cruise level) and it was suggesting a climb to FL410 ("NOW"), even though Max Altitude was shown to be FL396.

 

Don't know if it's relevant or not, but during flight prep I had also set cruise speed to M.840 in the CRZ page. I think this may be why the FMC (in the pic below) is showing increasing mach speeds as the a/c sheds fuel?

 

Two questions:

 

1. Is it normal in the 777 for the FMC to display the suggested Step Climb level in the FMC instead of the cruise altitude set on the PERF page and displayed on the CRZ page? I would think it would screw up VNAV descent planning for there to be a 6,000 feet difference between actual cruise level and what was being displayed in the FMC.

 

2. Why is the flight computer suggesting a Step Climb to a level above the displayed Max?

 

One final thing: Re-entering my cruise level in the CRZ page was not allowed: "Invalid Entry". So, I thought, why not enter it on the legs page for the next waypoint -- dumb idea, which caused me a world of hurt because it immediately changed my flight mode from CRZ to DES and I had a heck of a time getting back into CRZ mode. So, again, cruise level is set at FL350, MCP at 350, a/c is completely stable at FL350. I enter FL350 on the LEGs page and bam! I'm into DES mode. I'm having a hard time grasping the logic.

 

 

Jev, I´ll try to answer some things but this is a fairly complex topic. I´ll try to get back to you when I´ve had some more time looking at my books at home.

 

However, first of all the Auto Step Climb feature should be regarded as completely separate from the FMC. The FMC should not change its behaviour on ASC being on or off.

 

The ON/OFF switch for step climb calculations is the STEP climb size in the PERF INIT page. If you set that to zero the FMC will not do any calculations. If you set it to RVSM, 4000 or something else valid it will do step climb calculations and, very importantly, will calculate fuel burn throughout the flight as if those step climbs will be done when the FMC calculates them. In other words, if the FMC calculates that you should climb to FL370 at waypoint D the Fuel at Dest reflects that. If you delay in making that step climb you will end up burning more fuel than what is being predicted.

So, if you have set everything up for FL350 with Step Size RVSM and the FMC things that you should climb to FL410 that is what you´ll see in the LEGS pages. The FMC assumes that you will make that climb immediately. If you go to the VNAV page you will see STEP TO FL410 and AT NOW (or something like that, going from memory here).

 

The following is also from memory and may be less than 100% accurate! When you make the Step Climb you go to the CRZ page and enter the new FL there. This is how you tell VNAV what you want done. The altitudes in the LEGS pages are planning/calculations and restrictions and the FL set in CRZ is the ordered value. Then set the new FL in the MCP and push the button (I belive you can just set the new value in the MCP, push the button and this will enter into the FMC CRZ 1L field). If the aircraft is in VNAV it should now perform that climb. As you see it´s perfectly normal to see differences between the CRZ FL value (which lets VNAV know what you want it to do right now) and the calculated FL values in LEGS and VNAV STEP TO.

 

" I would think it would screw up VNAV descent planning for there to be a 6,000 feet difference between actual cruise level and what was being displayed in the FMC." No, I don´t think so. You´d simply enter into the descent later. The problem though is that all the fuel calculations will be wrong as I previously mentioned. Therefore this is not a situation that one should leave alone.

 

"Why is the flight computer suggesting a Step Climb to a level above the displayed Max?" Well, this is something that I want to check first. I remember reading explanations for what those OPT/MAX/RECMD FL actually mean and why they can differ in various ways, including what you´re seeing. OPT shows the optimum altitude based on gross weight and speed shown on the speed line (2L). MAX shows maximum cruise altitude based on current gross weight, engine out operation (not sure what this means), selected speed option, disregards any altitude or speed constraints and a residual climb rate of 100 ft per minute. So, my interpretation is the OPT is Optimum right now, MAX is How high can I go with a slow climb starting right now and what you see in the LEGS pages is what is doable in the future according to the FMC predictions.

 

I´d guess that the FMC thinks that by the time you get to FL410 from the current FL350 you will have burned off enough fuel to be safe and sound there.

 

Work calls, I´ll get back later.

The real 777 FMC does display the predicted values taking steps into account - once again, this is not the NGX. (sigh)

Ryan, are you talking about the predicted values for FUEL AT DEST? That´s precisely what I was saying, perhaps hidden in my wall of text

 

I don´t know how the NGX behaves so I´m not sure what you mean with your reference to that aircraft, but I assume from your comment that there are differences? Perhaps best to leave that topic out of this thread

Tord, thanks for that detailed explanation, and for clarifying that this has nothing to do with the Auto Step Climb setting. I apologize then for this being an off-topic post.

 

I understand a bit more now about why the LEGS page was displaying altitudes different than my entered cruise level, but I'm still a bit confused because I've not seen this behavior on other flights of similar length, loads, etc. As best I can recall the altitudes in LEGS have always been consistent with cruise altitude set in PERF and as displayed on the CRZ page.

 

So, to be clear, if I don't want to see Step Climb predicted altitudes in LEGS I should enter 0 (zero) into Step Size on the PERF page? I wonder if this can somehow be set as default?

" if I don't want to see Step Climb predicted altitudes in LEGS I should enter 0 (zero) into Step Size on the PERF page?" Correct.

 

Doing that, If you have a flight plan that includes step points that you intended to follow you can, and should, enter those as specified step points using "/FLnnnS" on the appropriate waypoint. Apparently different airlines have different SOPs on how they want to handle the whole step climb thing, but this is your sim so do what you want. You can let the FMC do its thing, you can mix that with specified step points or you can turn off the FMC prediction and only use specified climb points. Just be aware of the resulting changes in the predicted values for fuel/time @ Destination.

 

I´ve no idea of STEP SIZE can be set to default to zero.

 

I´ve only had time to do a quick test of mixing in specified step climbs and got mixed results. I won´t go more into them before doing more testing.

Tord-

 

Thank you for your extensive explanations on step climbs.  I've found over a series of flights since my last post that, with reference to the Auto Step Climb behavior which is an ingenious "cheat," for lack of a less negative word, that PMDG gave us, setting the STEP SIZE to 0 causes the leg-programmed step climbs (/370S on the LEGS page) to be ignored.  Not sure if this is intentional.  However, leaving STEP SIZE set to RVSM will cause Auto Step Climb to recognize leg-programmed step climbs (370S on the LEGS page).

 

When leaving the STEP SIZE set to RVSM, if the FMC wants a step climb *sooner* than the manually-programmed step, the FMC will disregard it's own calculations on waypoints prior to your manual LEGS page entry.  For example: on waypoints A, B, C, D, and E the FMC is calculating a step climb from FL310 to FL330 between waypoints A and B.  My flight plan says the step climb to FL330 should be at waypoint D (and I will personally follow the flight plan which uses a further-reaching wind matrix than the FMC which is only using data it's experiencing at the current time).  When I enter /330S on waypoint D on the LEGS page, all earlier waypoints will update to the previously compliant lower level; waypoint A will stay at FL310 while B and C bump down to FL310.  The STEP SIZE logic will resume after my last manually-entered step climb.  

 

Also, based on your PPRUNE responses, a common procedure is to change the STEP SIZE to 0 after your last step climb.  For example, my flightplan from RJAA to KORD left me at FL370 as a final altitude over much of the US all the way to TOD to take advantage of winds while the FMC, unaware of the winds ahead, indicated that at my speed and weight I could (and should) get to FL410.  So after my last step climb to FL370, I changed STEP SIZE to 0.  ...Incidentally, this could complicate some flights that don't reach maximum altitudes where my intention is to use AUTO STEP CLIMB all the way to PAUSE AT TOP OF DESCENT.  Guess I'll actually have to pay attention sometimes.  

 

Interesting stuff!

 

-Tony Fiore