FMC ICAO step size and RVSM

[Manumanu]

Hi Hardy,
many thanks for the warm welcome.

N61W030 is a crew-entered waypoint in the LEGS page :



I finally managed to dig out my 1996 old 747-400 FMS Pilot's Guide (yeah quite old I know but that's all I've got )

and page 3.4-4 reads :

If there are planned steps on the LEGS page, predicted values assume step climbs are made at the planned STEP AT points.

Then pages 3.4-24 and 25 gives us an interesting -imho- info :

A planned step point is made on the RTE LEGS page by entering the step altitude followed by "S" adjacent to the desired step point. The flight scenario requires a step climb to FL350 at FITES, so adjacent to FITES the pilot would enter /FL350S.
[...]
The FMC calculates a step climb from FL310 to FL350 on the KLAX to YSSY flight at a point 41NM prior to FITES. A second step climb is calculated from FL350 to FL390 at a point 120NM prior to 26S158.

(my bold emphasis)

then almost the same diagram as the 777 :



But the interesting info in my humble opinion is the line saying that after entering FL350S next to FITES... "The FMC calculates a step climb... 41NM prior to FITES". Which would mean that even when you enter a XXXS planned step climb in the LEGS page, the FMC calculates a S/C point in order to reach the new cruise altitude at the specified waypoint.

Hope this helps (although I can't help having the feeling it just complicates things. Sorry.) So don't delete your code just yet

I'll try and ask an Air France 747 FO (OPL as we call them in France) friend of mine about this.

Cheers

Emmanuel

PS : a little hello to Steve B. who taught me so much about the 744 and this place

[Hardy Heinlin]

Well, that sounds convincing.

If there are planned steps on the LEGS page, predicted values assume step climbs are made at the planned STEP AT points.

I've read this sentence before; the question was whether "are made" means "are started" or "are completed".

In the Honeywell book I bought I couldn't find the other stuff you quoted. Your manual is obviously more detailed. Edit: Found it now. I'm so blind.

I guess I'll have to re-activate my previous step-to model ...


Cheers,

|-|ardy

[Hardy Heinlin]

Or ... were there any changes in the FMC software recently? Here's an opposite statement again :-)

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" :-).

[Hardy Heinlin]

Sigh. I have doubts again :-)

Now that I've found these pages in my manual as well, looking at the whole context, this STEP TO before FITES doesn't seem to be an S altitude based step but an FMC suggested step. Also, the step in that image seems to end 10 or 20 nm before FITES.


|-|

[Manumanu]

Sorry  :oops: I knew I would just be adding doubts more than solving the problem.

this STEP TO before FITES doesn't seem to be an S altitude based step but an FMC suggested step.

The only thing that's weird is the sentence above :

adjacent to FITES the pilot would enter /FL350S

which seems to suggest it's a crew-entered step.

As soon as I have the answer from my friend I'll let you know.

[Hardy Heinlin]

No need to sorry :-) I think you're right.

I was confused again when I re-read in my manual what you already quoted: "The FMC calculates a step climb from FL310 to FL350 on the KLAX to YSSY flight at a point 41NM prior to FITES."

By "the FMC calculates a step climb" I was suspecting the text doesn't refer to a crew entered step climb.

Ief, if you read this, do you remember what your source was? My Honeywell manual is from 1996.


Cheers,

|-|ardy

[Hardy Heinlin]

Confirmed by 744 guy: FL360S in legs page means step climb is supposed to start at the waypoint.

I'm lost :-)

What is right, the book or the observation?

Ief and your 744 guy: Are your source the aircraft or a manual?

We all know, manuals are wrong sometimes.


Cheers,

|-|ardy

[IefCooreman]

The life of the FMC programmer, interesting :-).

My personal _777_ source is the 777 FCOM, but don't ask me what FMC version or software provider we use... The airplanes are brand new, max 18 months old, we even discovered dreamliner flight display functionalities so these things do evolve in time. Our 747-400's are max 5 years old.

The FCOM states literally:

A legs page inserted step climb like "XXX/FL350S" indicates: "the FMC assumes the step climb starts at the waypoint. Accomplish the step climb at the waypoint with the steps described in the cruise climb".

My 747 source: no idea what reference he used. We run into eachother because we have layovers in the same hotel together fairly often, but since I'm having a vacation now, it might take a while before I see him.

I'm still having difficulties accepting the FMC would internally assume a step climb before the waypoint. Ie: an every-day-real-life example is the initial NAT track cruising level. We are cruising FL330, inbound to our oceanic entry point. We might ask for a slightly higher level (say FL350) than optimum for our initial oceanic clearance only to prevent being stuck all the way at a lower level (step climbs along the way end quite often end up "unable due traffic"). If we get FL350 as our initial level, it is a requirement to be AT the entry point AT FL350. And this is the only example where we don't use FL350S for that particular reason: it is a step climb that starts at the waypoint, and we need a step climb that ends at the waypoint. So we have to estimate the start of our climb ourselves.

Now actually I'm lying, we do put ie .83/FL350S at our entry point, but only to have more correct fuel estimates than without, and to remind us of the entry level we are required to be at (local ATC often doesn't know our oceanic clearance).

I'll try to experiment a bit once back in the air, and see if I can catch me a 747 instructor :-).

[Manumanu]

Hi IefCooreman

Do you have a specific page in your 777 FCOM so that I can check in my several editions and revisions ?
edit : found it page 11.42.22 (2006 edition, I'll have a look in a more recent edition but no really important) in the Flight Management, Navigation / FMC Cuise chapter.

In order to have a more "practical" reference, I asked to 2 777 FO, and one 747 FO at Air France and 1 777 FO and 1 747 CPT at BA. I didn't want to start writing the answers before having them all or at least a few of them, but since things are moving, let's go

So far I only had one answer (I only sent my emails yesterday) from a 777 AF guy and it was pretty clear : a step climb is considered as any altitude constraint, meaning the plane will plan to BE at the new altitude at the waypoint the altitude is associated to. Therefore planning a climb BEFORE said waypoint, hence the little green circle with S/C written next to it BEFORE the waypoint.

I'll come back as soon as I get more answers from my sources.

PS : For the moment, I'm trying to check in all my documents if in FLCH mode, the 747-400 does as the 777 and tries to reach the new altitude within 125 seconds. It's in part 22 of the 777 AMM but not in the 747 AMM...

[Manumanu]

Found in my copy of Cathay Pacific Cargo 747-8 SOPs (dated 31st of October 2011):

Step Climb
Step climbs must be entered as a Planned Step on the LEGS page. Once a Planned Step is entered, the CRZ page shows predictions to reach the altitude at the Planned Step point or after it if the Planned Step altitude cannot be attained by the waypoint.

Seems pretty clear here to : "predictions to reach the altitude at the Planned Step point". But it's 747-8 :/

edit : Found in the 787 FCOM, the exact same line as the 777 FCOM :

Planned Step Climb
When a step climb is planned to start at a waypoint, the data can be entered on the RTE LEGS page. The FMC performance predictions assume the airplane will start the climb at the identified waypoint.[...] Enter the cruise altitude as an altitude constraint and the letter S. The FMC assumes the step climb starts at the waypoint. Accomplish the step climb at the waypoint with the steps described in cruise climb.

One step further, one step backwards. Sorry  :?

Or is the word "start" possibly meaning something else ?  :?

[Manumanu]

All right, got some time to chat with my sources. Some even took pictures for me during their last flight.
There was a misunderstanding between me and one of my 777 sources so we cleared things up and the conclusion goes in the same way as what IefCooreman wrote earlier : in the 777 when you enter a step climb (not an altitude constraint) in the LEGS page as "FLxxxS" the FMS plans for you to START climbing AT the waypoint. So when the waypoint is sequenced, you are still at the old cruise altitude and only start climbing AFTER passing the waypoint. The little green circle labelled "S/C" is situated ON the waypoint.

Here are two pictures to illustrate :





In this situation they have entered a step to FL320 (oceanic entry point) at SIVIR and you can clearly see the S/C ON the waypoint.

Ok this is still 777 but things are getting clearer. I'm expecting news from my 747 sources shortly but I don't see why it should be otherwise.

[Hardy Heinlin]

Interesting. Thanks.

Ok this is still 777 but things are getting clearer. I'm expecting news from my 747 sources shortly but I don't see why it should be otherwise.

I believe the 744 is different and that the stuff you quoted from the 744 manual is correct. There are quite a few differences between the 777 and 744 EFIS/FMC. To name just one (it's also on the photos): In the 744 FMC, you can't enter Mach speeds in the LEGS, in fact you can't enter a speed at all in a 744 cruise leg. That .848 on that 777 photo would be in small font on a 744 FMC. Also, the 744 ND doesn't show speeds in the route DATA, only altitude and time.

(Unless there was a major software update recently.)


Cheers,

|-|ardy

[Manumanu]

I'm on the phone with one of my 747 sources at the moment. Plenty of info coming in as soon as the call's over.

[Manumanu]

First a little picture from the 747-400 :



Then a few info before getting more :

my buddy 747-400 FO at AF never uses the "S" in the legs page. Some of his colleagues only use step size and climb when the FMC tells them to, some others use their OCTAVE (now LIDO) CFPL to climb only when the flight plan says, but he prefers using a step size (2000 or 1000 if on a NAT track) and every time the FMC tells him to climb, he compares to the CFPL to see how winds are compared to planned ones. If winds are close to planned winds (which may be quite old as their system updates winds only twice a day. Although it's going to change soon to 4 times a day) he then climbs when his CFPL tells him to. Otherwise he looks at the wind trade charts to estimate when it would be better to climb between the FMC suggestion and the CFPL suggestion.

It will also depend on Cargo or passenger flight (they do both at AF 747) and turbulence : if previous aircraft tell them on 123.45 that it is turbulent they will stay lower longer (provided fuel permits) to avoid getting to close to the margin, and if not turbulent go higher earlier. During cargo flights obviously comfort is of less importance.

So no definitive answer for now. He just came back from Cairo and only got my message now so couldn't take any pictures. He's off to Kuweit on next monday and has promised to take a film for us. Should settle the matter.

Cheers everybody

Emmanuel

[Manumanu]

Another 747-400 one showing clearly the S/C ON the waypoint (or is it because the scale is max 640 ?) :

[Hardy Heinlin]

Another one showing clearly the S/C ON the waypoint

Clearly, indeed.

(or is it because the scale is max 640 ?)

I don't think so. Even if the green donut were only 5 nm from the white star center, the offset of the donut would be clearly visible.


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

May we conclude that the designers of the S/C function had a strategic fuel planning feature in mind, not a tactical feature that can be used to comply to ATC clearances?

It makes some sense. If it were to be used tactically, it should work by popping up RESET MCP ALT so you can ask ATC for clearance ahead of time, and when you get clearance, dial the altitude up and sit there until the FMS decides to climb (or descend). Does it? If not, better stay with just for fuel planning, where climb point does not matter too much.

A S/C is not an altitude constraint.


Jeroen

[Hardy Heinlin]

That has been my conclusion through the past 20 years until I got confused recently.

I think, at the moment, -- 13 NOV 2012, 1300z --, Honeywell needs to correct the text in the 744 FMS Pilot's Guide.


|-|ardy

[IefCooreman]

It can not be tactical because it's not ATC's job and they don't have the tools for it. Even when you send a flightplan with step climbs in it at specified waypoints, all the cruise FL's are subject to "requested by pilot on the spot and approved by local controller if possible" and hence the definition of a step climb cannot be a constraint (which is the other way around: ico no news you have to comply with it).

It is informative only, a "suggestion" from the FMC for you to play with. The fact that it is written like a constraint in the FMC confuses a lot of people apparently.

There are differences between 777 and 747 FMC's (from what I've seen jumpseating on 744's). 777 has extra functionalities. The most interesting here is the "recommended" cruise FL next to opt and max, which includes winds from the legs data page into calculations. So if dispatch is kind enough to beam the correct ones up to your FMC, it will tell you all you need to know :-). Besides that, it's faster, has a bigger memory and is multi-color! And it's more silent... oh well... the whole 777 is :-)

[Manumanu]

Final word on this : the manual planned step entered in the LEGS page in the 747-400 FMC means the climb STARTS at the waypoint it is associate with (photo taken a few days ago but didn't have time to post here before today) :



Here we were cruising at FL310, we entered the "FL390S" planned step, and before EXECuting the FMS automatically updates the following planned altitudes and we see that he plans the next altitude below FL390, hence climb STARTING at the waypoint which is associated to the constraint.
Same as the 777 in the end.

Sorry I haven't been able to come back earlier on this.

Cheers