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conchulio

step climbs hardcoded into flightplan in fmc

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hello everyone,

 

i recently watched the justplanes video Air Canada flight from vancouver to sydney and i realized that the flightcrew entered step climbs into the flightplan (in the fmc) manually at certain waypoints via a command (e.g. 350/S). 

My questions are now the following:

 

When do you "hardcode" the flightlevel change into the flightplan instead of climbing when the fmc wants you to, i.e. when reaching the s/c point?

 

Is it due to some restrictions along an airway or due to the reason they were flying over the pacific?

 

If the second question is not the case, wouldn't it be more fuel efficient to just climb when the aircraft says so instead of climbing at particular waypoints along the route?

 

I'm just curious  :ph34r:  and would be thankful if somebody could give me an answer (which is understandable for a not-real-life-pilot :unsure: ).

 

Bes regards

Dominik

 

 

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When do you "hardcode" the flightlevel change into the flightplan instead of climbing when the fmc wants you to, i.e. when reaching the s/c point?

 

You do this when you want to tell the FMC that you plan to step at that point, instead of on the OPT profile. As an example, when crossing the NATs, aircraft are required to be separated by much larger distances due to the lack of radar coverage out there. This means that, when traffic is heavy, climbs and descents are usually not approved. As such, you would usually want to climb to your MAX altitude just prior to entering the NAT segment, such that you'd hit your OPT altitude about half way across. To help ensure you have the proper amount of fuel, you can enter this on the legs page, and the predictions will take that into account.

 

For more on this, see Tutorial #1.5, on our website's Download > Documentation page.

 

Be careful, though...you may see simmers try and go overboard when they look at flight plans. Flight plans sent to ATC (like what you'd see in the PFPX flight plan formats) have the anticipated step climbs shown in them. This is only to give the ATC automation (like EUROCONTROL's CFMU) an idea of what the flight plans on doing, based on the dispatch program's prediction of when it anticipates the flight will need to climb to remain on the OPT profile. Unless you are sure this is because of an altitude restriction along the route, these are advisory only and should not be set into the FMC.

 

 

Is it due to some restrictions along an airway or due to the reason they were flying over the pacific?

 

Restrictions, yes, but over the Pacific, no. The Pacific Ocean, and its associated traffic is usually more spread out, and there are fewer restrictions, compared to the Atlantic. There are occasionally restrictions when flying on the PACOTS, though. Much like the NATs, if the track you're on is busy, then it's unlikely you'd get cleared higher to follow the OPT path.

 

 

 

If the second question is not the case, wouldn't it be more fuel efficient to just climb when the aircraft says so instead of climbing at particular waypoints along the route?

 

More fuel efficient, yes, but in the NAT example, above, you're sacrificing performance by entering the NAT at the OPT altitude, and then being stuck below the OPT altitude for the remainder of the NAT (again, climbs are rare when it's busy).

 

This might also be done for regional concerns, too. Certain airway segments have various altitude requirements (min/max). Conflicts with airliners are usually rare, but can occasionally be found. In those cases, you may want to force a step to get above that altitude earlier rather than later.

 

 

 

Bes regards

Dominik

 

Full names - first and last - in the forum, please.

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Thanks Kyle, that was very informative. So i guess the FL in the OFP is rather discretionary than obligatory in most cases (except for certain regions like north atlantic).

 

Dominik Teda

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Thanks Kyle, that was very informative. So i guess the FL in the OFP is rather discretionary than obligatory in most cases (except for certain regions like north atlantic).

Even on the North Atlantic, anything in the FP represents an aspiration rather than a firm commitment: ATC will clear you along a track at a level and speed which may or may not be the same as that which you planned or requested. Indeed, even the routing may be different, which is why there's a big emphasis in training for MNPS operations on ensuring that you fly the route, speed and level you are cleared on rather than that which you planned/expected to.

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Thanks Kyle, that was very informative. So i guess the FL in the OFP is rather discretionary than obligatory in most cases (except for certain regions like north atlantic).

 

You're welcome. From a dispatch perspective, that altitude is almost never obligatory (I can't think of a case in which it would be, honestly, but I don't like saying "never" unless I know that beyond a shadow of a doubt). It's really only there to tell ATC that it is likely what will be requested at or around that point by the crew so that the automation (traffic flow management tools) can run its numbers accurately, it wouldn't be accurate if it ran a flight projection only at its filed cruise level if that cruise level is planned to change.

 

If you were to climb at that point without clearance simply because the note is in the flight plan, your error would end up on my desk (literally). Additionally, you'd be wasting fuel, as the flight planning tools usually choose the closest fix to the planned step. In some cases, this may be well beyond, or well in advance of your OPT step point.

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Thank you very much guys, it is always amazing that one always learn something new! much appreciated.

 

Dominik Teda

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A good example for all this is Afghanistan.

 

When you cross Afghanistan from Bangkok to Europe you will be assigned an exact departure time (slot), an exact arrival time at the Afghan border and a FL.

 

This FL for Afghanistan overflight is stuck to by ATC quite strictly, so while your FMC might predict another FL to be more economic there, you would still enter your expected FL in the legs page at the Afghan entry point because most likely you will get that filed FL.

This way fuel prediction to destination is closer to what it will actually be as when you would leave optimum FMC predicted step climbs in there.

 

There are also areas in the world where you have no VHF contact but only horrible horrible hissing noisy HF communications.

With CPDLC these comm problems are getting fewer and fewer though.

It is been a while that I flew there, but Mumbai HF is one of those areas where you can just forget about climbing because you simply cant get through to make them understand what you want.

So since you then expect the next step climb to be over Oman (going west) you might as well enter it like that in the FMC for more accurate fuel prediction along the route and to destination.

 

Also, our airline claims that the OFP (OFP=printed Operational Flight Plan that you write on and work with) step climbs that are pre calculated by the dispatcher are more accurate than what the 777 FMC can do.

The dispatcher can calculate with many more variables (temp gradients and wind data) that are simply not available to the FMC!

So it is recommended to stick to OFP altitudes rather than FMC prediction, and so, you could enter those step climbs in the FMC manually.

I prefer to leave things flexible though and constantly compare OFP and FMC prediction and try to figure out what is the best time for the next step climb....what else to do on those long flights right :-)

 

When your TO weight varies a lot from the printed OFP, or when you can get enroute wind updates that are more accurate than the data the dispatcher used, then you would let the FMC do its thing again.

 

In FSX though, I dont know what is more accurate...a flight planning tool or the FMC?

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what would happen, if you cross the border to china? would you automatically change to metric system and climb to, e.g. FL331 from FL330 and adjust all future step climbs, i.e. instead of FL350 to the next approriate metric counterpart?

 

It's very interesting to hear how different certain airspaces in different countries are operated, thanks Rob!

 

Dominik Teda

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Your welcome :-)

 

The next ATC sector with the Metric system will tell you to change altitude upon first contact.

They will tell you to climb or descend to the metric altitude in meters and you have to convert that to a FL that you can put in the MCP.

 

Thereafter all step climbs are requested according the metric system (so you have to convert recommended FMC flight levels to metric altitudes and then you request that in meters).

 

Oh yes...so much fun ;-)

Even more fun flying a metric approach (but at least you will have conversion tables printed on the approach plate so you can easily change meters to feet)

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