FMC ICAO step size and RVSM

[Hardy Heinlin]

Good morning,

as you all know, on the PERF INIT and VNAV CRZ pages a STEP SIZE can be entered. If ICAO is entered, flight level separation above FL290 used to be in 4000 feet steps.

I'm wondering ... is that still the default in the FMC, or has it changed to 2000 feet steps since the introduction of RVSM? But then, how should the FMC know whether it is currently in RVSM airspace or not?

I assume the FMC still uses the old rule. And when using RVSM, the crew has to enter 2000 in the STEP SIZE line manually.


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

More likely, they don't enter STEP SIZE at all, as everything has been painstakingly planned ahead of time. I don't think (think, I am not a dispatcher or pilot) that pilots have much opportunity to make this kind of operational (step climb) decisions any more. They request and climb at predetermined waypoint X (unless something prevents them from doing it).

I hope I am wrong though.


Jeroen

[IefCooreman]

Doesn't the FMC have the "RVSM" option? Haven't seen a 74 collegue in a long time, but from the moment I see one I'll ask. On the 737 we used to get a lot of FMC software updates, sometimes new functionalities would be included so maybe the 747 one has evolved as well. We had "RVSM" as the standard on our classic 737's, and on the 777 as well. We can select "ICAO", "RVSM", the step size in 100ft increments, or "0" if no steps are planned.

Dispatch provides a flight plan with step climbs but these are "estimates" based on estimate weights and winds. Sometimes dispatch doesn't even consider state regulations.

The way I use it is to keep "RVSM" initially, with an initial cruise flight level based on actual weight. Once levelled off, get a wind update from dispatch, have the FMC calculate step climbs and then put the step climbs on the legs page (/FL340S, /FL360S,...) and put the step size to 0 afterwards. If you don't put the 0 the FMC will continuously hope for a step climb and calculate fuel estimates accordingly.

I think there is more RVSM airspace being flown through than non-RVSM at the moment :-)

http://www.jasma.jp/pic_rvsm_plan_lrg_new.gif

Cheerz

[Michel Vandaele]

Hi Hardy,

I checked with a friend flying the B744/8's for a major cargo airline.  Your hypothesis is correct.  Their standard is stil ICAO and if they fly RSVM its 2000ft.

But in practice (not really an SOP), they are mostly following the S/C from the flightplan, which has been calcullated by the dispatchers in function of head/tailwinds and etc.
In that case they are setting the S/C's in the LEGS page and in PREF INIT or VNAV CRZ 0 (zero), even in RVSM conditions.

Hope this help clarifying

B. Rgds
Michel

[Hardy Heinlin]

Thanks guys,

conclusion: in the 737 FMC (Smith's) one can enter thousands or hundreds or the words ICAO or RVSM. The 747 FMC (Honeywell) still allows thousands only or just the word ICAO.

And from the pic I conclude: The whole world is now RVSM. The last white space was Russia until late 2011.


Cheers,

|-|ardy

[jonb]

conclusion: in the 737 FMC (Smith's) one can enter thousands or hundreds or the words ICAO or RVSM. The 747 FMC (Honeywell) still allows thousands only or just the word ICAO.

Or it also allows 0 (zero) to be entered to indicate no further step climb.

Cheers

Jon Bunting

[Hardy Heinlin]

Hi Jon!

Or zero, yes.

While you're here ... can I ask you another one?

If I enter a step climb altitude in a CRZ LEG, say, 360S, is this an advice that I shall capture FL360 at that waypoint, or that I shall start climbing to FL360 at that waypoint?

I haven't found any manual that answers this particular "S" question. It's always the same text everywhere. Bulfer or Honeywell doesn't even mention the "S", if I'm not overlooking something.


Cheers,

|-|ardy

[jonb]

Morning Hardy,

Yes, just passing through checking on your progress ;-)

My understanding is that the FMC will take it that you are level at the new flight level from the waypoint which has the step climb inserted.I also understand that the FMC doesn't recognise if you don't actually do the climb or not, so if you are at FL340 with a step 360S inserted at the next waypoint but remain at FL340 past the waypoint the FMC still thinks that you've done the climb for it's fuel predictions.
You get around that by re inserting 340 into the VNAV crz alt window 1L to tell it you are still at FL340 ( the VNAV CRZ ALT will still actually show FL340 if you haven't climbed, but the FMC will be working with FL360 winds ) I hope I've explained that in an understandable way?

Do you have access to the Honeywell FMC manual? I presume you have but if not we have a copy on board I can search for anything else you may need on my next trip.

Cheers

Jon

[Hardy Heinlin]

Thanks, Jon!

So, whenever I enter a CRZ ALT in 1L, any existing S altitudes in the LEGS will be removed and reset to small font predictions?

As far as I see, there's a "circular" hierarchy:

- A STEP TO entry (big) in 1R overrides the FMC's next recommended step alt
- An S entry in a LEG overrides the STEP TO entry in CRZ 1R (font remains big)
- A CRZ ALT entry deletes all S entries in the LEGs (1R font gets small)

Yes, I have that manual, too.


Cheers,

|-|ardy


By the way, if you have an altitude constraint in a SID/STAR/APP, and the altitude is predicted to be captured somewhere along a leg (not directly at the waypoint), then is there a green donut on the ND for that capture point? E.g. if I have 250/4000 on the leg to XY and the FMC predicts the capture will be 2 nm before XY, will the ND show something like O 250/4000 ... i.e. like a T/C or E/D, just not with "T/D", but with "250/4000"?

[jonb]

Hardy,

Your first 2 hierarchy points are correct yes.

The last point about entering a CRZ ALT deleting step climbs on the legs, I don't think that would happen no, it shouldn't delete anything entered in the legs page with regard step climbs.Entering a higher CRZ ALT than your present ALT at 1L on the VNAV CRZ page when in VNAV PATH may well switch you into VNAV ALT, I'll check that on the plane and get back to you Friday Morning.

If you have for example a step climb of 360S next to a waypoint on the legs page and climb up to FL360 prior to that waypoint, the 360S will still remain in bold font on the legs page.
If you are at FL340 and ATC wont let you climb at the way point which has 360S entered next to it the FMC will start using it's  FL360 wind/temp data from that passing that waypoint even though you haven't climbed.
In order to tell the FMC to keep using the data for your present FL 340 you need to manually enter 340 into 1L on the VNAV CRZ page.(which of course will already show FL340 anyway, but it still needs to be manually over written)
The honeywell manual explains  it a bit better than I can.

[Jeroen Hoppenbrouwers]

I think you can summarize it as follows:

Re-entering a cruise alt in VNAV CRZ 1L does not delete anything ahead of the current position, but kicks the FMC out of believing that you will any moment now climb to the planned step climb altitude posted at a waypoint already passed.

Should there be more step climbs planned ahead, they won't be touched. It is just the already passed planned step climb that is canceled.

Can you put step DOWN altitudes in a waypoint ahead of current position?


Jeroen

[Hardy Heinlin]

The Honeywell pilot's guide I have says that a CRZ ALT entry deletes HOLD constraints greater than the entered CRZ ALT; all other constraints not lower than the entered CRZ ALT are deleted.

However, an S altitude may not be considered a "constraint" here. So, the text remains vague.

Re FMC winds: Say, I have this in the LEGS:

FL390
FL390
FL410S
FL410
FL410

It's clear to me that the FMC will use FL390-winds for the first 2 wpts, then FL410-winds thereafter.

I can't imagine that re-entering the CRZ ALT will keep that FL410S entry and then apply FL390-winds for those FL410 wpts. I assume the FMC will always use the winds at the predicted altitudes.

In other words, I'd expect that the CRZ ALT re-entry will cause this:

FL390
FL390
FL390
FL390
FL390

And then it makes sense if these waypoints indeed use FL390-winds. But I may be wrong, of course.


Cheers,

|-|ardy

Can you put step DOWN altitudes in a waypoint ahead of current position?

Yes.

[jonb]

Hardy, your example looks correct after passing the FL410S waypoint the fmc will use FL410 wind data, unless you then enter FL390 into the CRZ ALT then as you say you'll get the FMC to continue using FL390 data. I only read that in the manual when I was bored one trip, it's not something many pilots would actually know about. Also you are quite correct an "S" altitude isn't considered a constraint, it wouldn't for example trigger an "unable next alt" scratchpad message if you put in a silly high number in there. (having said that, I'll have a play and see what happens eg FL650S !)

Jon

[Jeroen Hoppenbrouwers]

Take care that re-entering a cruise alt may happen AFTER passing the step climb altitude. There is nothing to delete then, but the FMC still assumes you will soon be at the (passed, not executed) step climb altitude.

[Hardy Heinlin]

(having said that, I'll have a play and see what happens eg FL650S !)

Theoretically, valid altitude entries range from -1000 to 45100 only.

Jon, when you're playing ... :-) -- also interesting would be if you can enter an S altitude below the active CRZ ALT. The books say you can. But it could be a misunderstanding.


Cheers,

|-|ardy

[IefCooreman]

One small remark: I'm pretty sure a pilot entered step climb on the legs page is an indication that the step climb will be commenced at that waypoint, not that the airplane will reach the step altitude at that specific waypoint.

The way I understand is, the "FLXXXS" function on the legs page is a pilots tools to create the most accurate vertical profile during cruise and get the best fuel prediction for the arrival. Reasons for step climbs can be performance, but also expected ATC FL restrictions during cruise. Example would be that over certain countries certain flight levels are expected to be unavailable.  So step climbs can be higher or lower (ie certain flight levels not available) and are not treated as constraints by definition.

Cheerz

[Hardy Heinlin]

One small remark: I'm pretty sure a pilot entered step climb on the legs page is an indication that the step climb will be commenced at that waypoint, not that the airplane will reach the step altitude at that specific waypoint.

Thanks Ief. A small remark, but a big step in the programming work :-)

What you're saying is what I originally expected to be true. But I have already programmed it as a capture waypoint and the algorithm for this is rather complicated because it includes reversal extrapolation. So, before I disable that part of the code in order to make it a climb start waypoint, I'd like to be 100% sure this is the correct way.

So, how sure are you on a scale from 0 to 100? :-)

99?


Cheers,

|-|ardy

[Hardy Heinlin]

My understanding is that the FMC will take it that you are level at the new flight level from the waypoint which has the step climb inserted.I also understand that the FMC doesn't recognise if you don't actually do the climb or not, so if you are at FL340 with a step 360S inserted at the next waypoint but remain at FL340 past the waypoint the FMC still thinks that you've done the climb for it's fuel predictions.

Jon, by "next" waypoint do you mean the active waypoint? Are the two waypoints quoted in blue the same in your statement? If so, I don't understand the logic. I assume if the active waypoint is sequenced out (removed from the LEGS), the constraint of this sequenced waypoint will no longer restrict the flight plan, and the FMC computed optimum step profile will be displayed again, starting at the current aircraft altitude.


Cheers,

|-|ardy

[IefCooreman]

I guess if you're programming it, it can't be 99, but has to be 100% :-). I've send an e-mail to a 744 collegue and waiting for the answer.

Although it is perfectly possible, it would be a strange functionality difference between 2 Boeing airplanes (777 vs 747). But I don't know anything about different FMC versions and software they use,... 777 is 100% sure: /FL360S is start of the step climb at the specified waypoint. Boeing has a strange way of thinking here because in the manuals they actually call it an "insert as an altitude constraint and the letter S" :-).

[Autopilot]

Hi. Sorry have been away for a while.

We still fly around with the older FMC (memory capacity of around 800kb..... yes you read that right kb !!)

We have to upload the winds manually and tend to do just one column from the flightplan. i.e. the one you intend to fly.

As you know, if you upload the winds manually and there is a step climb, there is no facility to alter the FL's in the wind page for subsequent waypoints.

i.e. initial cruise is at FL320. and you put a level 320 (for the wind) in the windpage for that waypoint, it will be there for all other waypoints. Therefor the FMC calculated optimum stepclimbs are not always very acurate.

We tend to stick with the Flightplan stepclimb calculations, rather than the optimum calculations from the FMC.

In our flightplan we get the winds for 4 FL's for each waypoint, (i.e. 4 columns... e.g. FL280, FL320, FL360, FL380) with the 3rd column being the level you are planned at for that waypoint.

There doesn't seem to be a set guidance in the difference between levels published on the winds page. (i.e. sometimes its 2000ft sometimes 4000 and other times 5000ft.

As an example at

                             260     280    300    350
AAAAA  300 M39 30080 30079 30078 30090
BBBBB  300 M39 30073 30072 30071 30086
                         260    280     320    350
CCCCC   320 M44 30064 30062 30063 29071
DDDDD  320 M44 30065 30063 30063 29069
                             280     310     340    390
EEEEE   340 M49 29067  29064 30060 29067

really sorry for the formating.. can't get the levels to show above the winds to show correct columns, but hopefully you get the drift

So at waypoint AAAAA you get the temp (= FL300 temp -39 (M39)) for that planned cruise level (which then goes into the winds page for that waypoint).

And the winds at AAAAA are given for the levels FL260, FL280, FL300, FL350

So entering the winds for EEEEE are then going to be a problem if in the FMC you have entered all the FL's (260, 280, 300 & 350), because now at EEEEE you want (280, 310,340 & 390)
               
And to get a more accurate fuel prediction (becaus that's what you're trying to achieve by doing all this) what we do is:

Enter the wind (and dont update the FL)for the actual FL that you are planning to fly for each waypoint from the flightplan. (i.e. the 3rd column) and accept that the level given for that wind waypoint is not correct.

So in the FMC it would look like:

WPT AAAAA
300.... 300/78
300/-39

WPT BBBBB
300.... 300/71
300/-39

WPT CCCCC
300.... 300/63 even though you are planning to fly at FL320 here
320/-44

so the accurate FL in the temp section.

What we then do is put the accurate Stepclimbs in the legs page
i.e. CCCCC...... 0.85/320S

and then go to the VNAV page and put the Stepclimb to 0 !!

This way you get an accurate fuel prediction for landing.... = the most important information you want..

What has happened in the past is that someone got say FL280 for the atlantic crossing i.s.o. say FL360, but didn't set the VNAV stepclimb to 0. The FMC then still assumes optimum stepclimbs and the fuel prediction is a LOT more optimistic..

Hope all of this makes sense