Long-haul international ops question: enroute descents

[Will]

Every now and then, PFPX gives me a route with descents in it. For example, I built an eastbound route from Walvis Bay, Namibia to Tokyo Narita, and it has me climbing initially to FL330, then descending very soon, while still in Namibian airspace and still eastbound, to FL310. We stay at FL310 all the way to the Seychelles, where there's another climb to FL330, then a while later, a step to FL350.

Then, entering Indian airspace (but still off-shore, still eastbound), we descend to FL290 until midway through India (over Bengaluru), where we cliimb back up to FL370. At Dhaka (Bangladesh), we descend again, briefly, only to climb up to FL390 when back over India.

Later, there's a step climb to FL410, with another descent (still eastbound) to FL390 midway over Japan. The TOD is reached while at FL390.

Here's the route, with the altitude changes in it:

N0501F330 DCT APLIN DCT IMLIK DCT UVKIK UT945 VEDRY UM437
IBKIN/N0507F310 UM437 AVOGU DCT EGTIG DCT EPMED UM439 VLS UL431 DV
DCT ANVUN DCT AVIGO UT940 ANVET UG424 MITCH/N0504F330 UG424
ITPOK/N0500F350 UG424 VUTAS/N0516F290 L875 BIA/N0500F370 W47 VVZ
A465 CEA A462 DAC/N0516F310 W1 LATIM/N0472F250 W1 SYT/N0497F390 DCT
VENUM DCT LIGSA DCT MALNO DCT PT/N0496F390 DCT LEVBA DCT FJC DCT GAO
DCT DAX DCT DSY DCT ML DCT KAMDA DCT ATVIM DCT IVPIP/K0907S1250 DCT
DALSU DCT SAMUL V543 SAPDI DCT PSN Z84 KALEK Y206 YAKMO DCT SAKYU
Y45 KMC Y515 ZUWAI Y517 HAKKA/N0490F390 Y517 YAGAN DCT YAITA Y887
DAIGO DCT SWAMP SWAMPN


So, my question is this: how often do these descents occur in real life?

I would find it totally plausible that various airways have restrictions making them usable only at certain altitudes, so I'm not so much debating whether this is plausible, just asking how common it is.

The one that seems most strange is climbing to FL330 in Namibia only to descend to FL310 after 250 miles while essentially on the same heading.

[cagarini]

I'd say it doesn't make much sense?

Climbing, until TOD, that's ok, and the higher you go the less you'll have to mess with airspace restrictions, which would, at most, send you higher ( ? ) ...

Strange that route ....  Where does PFPX fetch it from ?

[Will]

Well, PFPX can't auto-build this route, so I built it myself within PFPX by picking waypoints and airways to approximate the great-circle route.

Building it myself of course opens up the possibility that I'm linking implausible airway combinations together, like one airway with a hypothetical max altitude of FL330 and another airway with a hypothetical max altitude of FL290.

Except that PFPX gives altitude ceilings for the airways, and these cruise altitudes are all below the ceilings.

For example, the first descent happens while on airway UM437, max ceiling FL460 throughout. You pass waypoint VEDRY, then IBKIN, and while still on UM437, at IBKIN, PFPX shows a descent to FL310. Next come waypoints IMPIG and GEPOP, all on the same airway, all while flying heading roughly 065 degrees.

My point above is that PFPX plans a descent while staying on the same airway... So the issue can't be fully due to linking up implausible combinations of airways.

[cagarini]

Would be interesting to check how it goes using Simbrief ...

It gave me this:

http://www.hoppie.nl/forum/var/2020-07-05_16.00.37_FYWBRJAA01_.route

ROUTE ID: DEFRTE
FYWB/09 DCT UVKIK UT945 VEDRY UM439 TIKOK UT358 VLS UA407 GIPVO
UL431 DV UL437 MOGDU UM306 ORLID G450 BBB Q19 SEBMA A791 CEA B465
CTG A599 SGM A581 WHA R343 HFE W163 VEMEX W73 NOBEM V14 PIMOL G330
PIKAS W109 NTG V8 HSH G455 LAMEN A593 FUE V40 KUE Y40 KAZMA V40
WOODY Y40 MYE V40 KTE Y33 LABEL Y121 SWING Y88 CHINO DCT RJAA/16R

After Departure initially FL290, then step to 310 @ APGIK while on UA407, then 330 @ ETOPS entry while on UL437, then 350 @ DONSA, 370 @ KILKATA, 390 @ SADLI, 410 @ POTET till TOD...

[Hardy Heinlin]

Does your route planer take forecast headwinds into account so that the calculated CRZ ALT profile doesn't refer to OPT ALT but to RECOMMENDED ALT?

[DougSnow]

Sounds like its chasing tailwinds.  My personal rule is I dont let FPS at work do more than a climb every 2 hours or so, unless airspace or airway restrictions require it.   

Lets say I am working a westbound flight across the Atlantic and its going to go into the band of Datalink exempt airspace across Southern Greenland and Iceland. My airplane is No GPS (MEL'ed), thus no ADS-B. To operate in that airspace you must be GPS equipped as ADS-B is proving the surveillance FL290-FL410. So I'll have to restrict my altitude to FL280 until I am out from underneath the airspace. I'll put a remark on the OFP as to why.

[Will]

Chasing tailwinds.... very plausible explanation. Thanks.

[Jeroen Hoppenbrouwers]

... and its going to go into the band of Datalink exempt airspace across Southern Greenland and Iceland. My airplane is No GPS (MEL'ed), thus no ADS-B. To operate in that airspace you must be GPS equipped as ADS-B is proving the surveillance FL290-FL410.

Today this is still ground-based ADS-B, right? Aireon is not formally operational yet, as far as I know.


Hoppie

[DougSnow]

Yes, several receivers in Southern Greenland and Iceland...

[andmiz]

As a regular in some of that airspace, my response is 'nah'.  It's the disconnect between planning requirements vs reality.  We have similar occur in Vietnamese airspace; we're flight planned down to FL250.  It never happens though.

The only place it 'may' occur by ATC regularly is China; and in those circumstances we're usually carrying the fuel preemptively anyway.

[Britjet]

As Andmiz says - reality wins. There would rarely be an occasion when ATC would let you go back down anyway, and then you probably wouldn't get back up again.
It might mean that the arrival fuel would be affected - but it's rarely an issue.
Once up - stay up!
Peter

[dhob]

Typical flight plans are built on winds aloft data from NOAA/NWS. These winds are updated every 6 hours, starting at 0000Z. New winds data is available to uplink into the FMC at 0400Z, 1000Z, 1600Z and 2200Z. Available Flight Levels are FL240, FL300, FL340, FL390, FL450, and FL530. From this one can evince the wind data used for flight planning to build the flight plan can be very old by the time the flight departs. Thus the flight plan is a more of a strategic plan.
The NGFMC has enhanced capabilities to aid the flight crew in making tactical changes once airborne that may differ from the flight plan. The NGFMC has both OPT and RECMD altitudes on the VNAV cruise page. The OPT altitude is the same as the Legacy FMC and is a function of weight, CG and Cost Index/LRC etc. However OPT does not consider temperature or winds in its calculation. RECMD does incorporate both temp and winds, and will analyze these both above and below the current airplane altitude, up to 9000' below the current altitude. RECMD altitude calculation will look up to 500 NM in front of the airplane as well, though this distance is a function of the step size. With RVSM (2000) in for step size, I believe this distance is 300 NM.
Additionally, for the NGFMC with BP3.0 and later, the Wind-Trade Steps (WTS) is available. As RECMD will analyze winds out to 500 NM, the Wind-Trade Step function will continue to analyze the winds (and temps) from where RECMD ends, to the Top-of-Descent. However, WTS will only consider wind/temps at altitudes above the airplane.
For crews, they uplink the winds in preflight, then at the 0400/1000/1600/2200Z intervals request new winds. These FMC winds are then blended with the IRS winds to give the time and fuel calculations. At the nose of the airplane, the FMC uses 100% IRS wind data for calculations. At 200 NM in front of the airplane, the FMC uses a blend of 50% IRS winds and 50% FMC RTE WINDS. At 800 NM in front, the FMC uses 100% FMC RTE WINDS for time/fuel calculations.
From this, we train our crews to use RECMD altitude, and follow the WTS climbs (if there are any). If RECMD is displaying an altitude below the current airplane altitude (limited by the step size interval), then its analysis must assume the lower altitude with tailwinds is more efficient than the current altitude and winds. If so, it may be beneficial to descend, again depending on several factors.
Bottomline, the flight plan is just that, a plan. But with RECMD and WTS, and regular wind data updates, the crew can use these to make tactical decisions to fly more efficiently than the original flight plan.

[Will]

Thanks, that's helpful. How often do you receive flight plans from the dispatch office that call for en-route descents for purposes of winds/efficiency?

[Bluestar]

Typical flight plans are built on winds aloft data from NOAA/NWS. These winds are updated every 6 hours, starting at 0000Z. New winds data is available to uplink into the FMC at 0400Z, 1000Z, 1600Z and 2200Z. Available Flight Levels are FL240, FL300, FL340, FL390, FL450, and FL530. From this one can evince the wind data used for flight planning to build the flight plan can be very old by the time the flight departs. Thus the flight plan is a more of a strategic plan.
The NGFMC has enhanced capabilities to aid the flight crew in making tactical changes once airborne that may differ from the flight plan. The NGFMC has both OPT and RECMD altitudes on the VNAV cruise page. The OPT altitude is the same as the Legacy FMC and is a function of weight, CG and Cost Index/LRC etc. However OPT does not consider temperature or winds in its calculation. RECMD does incorporate both temp and winds, and will analyze these both above and below the current airplane altitude, up to 9000' below the current altitude. RECMD altitude calculation will look up to 500 NM in front of the airplane as well, though this distance is a function of the step size. With RVSM (2000) in for step size, I believe this distance is 300 NM.
Additionally, for the NGFMC with BP3.0 and later, the Wind-Trade Steps (WTS) is available. As RECMD will analyze winds out to 500 NM, the Wind-Trade Step function will continue to analyze the winds (and temps) from where RECMD ends, to the Top-of-Descent. However, WTS will only consider wind/temps at altitudes above the airplane.
For crews, they uplink the winds in preflight, then at the 0400/1000/1600/2200Z intervals request new winds. These FMC winds are then blended with the IRS winds to give the time and fuel calculations. At the nose of the airplane, the FMC uses 100% IRS wind data for calculations. At 200 NM in front of the airplane, the FMC uses a blend of 50% IRS winds and 50% FMC RTE WINDS. At 800 NM in front, the FMC uses 100% FMC RTE WINDS for time/fuel calculations.
From this, we train our crews to use RECMD altitude, and follow the WTS climbs (if there are any). If RECMD is displaying an altitude below the current airplane altitude (limited by the step size interval), then its analysis must assume the lower altitude with tailwinds is more efficient than the current altitude and winds. If so, it may be beneficial to descend, again depending on several factors.
Bottomline, the flight plan is just that, a plan. But with RECMD and WTS, and regular wind data updates, the crew can use these to make tactical decisions to fly more efficiently than the original flight plan.

Hardy,

Does the NGFMC for PSX perform the "wind-trade-steps" as discussed in the above statement?

[Hardy Heinlin]

I don't think so.

[Bluestar]

I don't think so.

It looks interesting.  Would it be possible to get it in one of your updates?

[Jeroen Hoppenbrouwers]

I wonder whether there are any actual operators out there that use this tactical option based on more recent wind data than what is in the holy flight plan?

I'd rather think the operator's dispatcher would recalculate a flight plan at the office and see whether there is an advantage possible, and only if so, contact the crew with a proposal and let the crew contact ATC to see whether they can get it.

Hoppie

[Hardy Heinlin]

Would it be possible to get it in one of your updates?

Probably not. The first problem is: I have no exact documentation of this function.


|-|ardy

[boeing747430]

Hi guys!
Just wanted to say that while it is rare, it sure does happen that you will get an OFP with cruise descents. As in-flight performance software is becoming better and better, one might get an OFP with all climbs, but the in-flight recalculation may recommend a descent for some portion of the flight. In the olden days, I was taught that a step makes sense when it is for roughly 25 minutes or longer.
Some fellow pilots think that cruise descents don't make sense and ignore them when they are recommended by either OFP or in-flight performance software. I don't and believe that it truly can save fuel.
Also, few pilots consider that only 5kts of more favorable TWC/HWC can alter your optimum altitude by 1800'.

And as a response to Hoppie:
Our Dispatch office never ever recalculates when a flight is enroute unless we specifically ask them to. But as I wrote, we do have this quite sophisticated in-flight performance software, which allows us to do most recalculations ourselves. It is constantly fed with the latest wind data, is able to show you veeery accurate turbulence predictions (even vertically), and you can recalculate your cost index for passenger connecting flight requirements. It's been quite a game-changer and actually, we do not look at the FMC level recommendations anymore, because the mentioned software is so much more accurate. It even gives you an exact CG at any time during the flight, which you can then enter on the FMC performance page, in order to give you a more exact maximum altitude readout.
And before this, we always used the FMC with the latest winds for tactical decision. The OFP on a 12 hour flight may look very old 8 hours after briefing.

Best,
Kim.

[boeing747430]

A few screenshots from the mentioned flight optimising software. Unfortunately there is no step descend on these. The pics are taken from the pilot's EFB device, not from the aircraft device. On the latter, you will be able to see exactly where you are.

The first picture shows the altitudes suggested by the software with the current turbulence forecast. Also, the maximum altitude is shown. As stated before, the turbulence forecast is a very helpful tool and quite accurate. It is definitely much more accurate than the significant weather charts one uses at dispatch and it gets constant automatic updates during the flight. The suggested altitudes are based on the actual aircraft weight, the CG, the suggested speed, and the actual winds and temperatures, all of which may be different from the operational flight plan. This is the page of the software I mostly use during cruise, in order to see the suggested cost index or fixed mach number, upcoming turbulence and suggested level changes at the same time. Of course, all the data is constantly being updated during the flight.

The second pic shows the temperature and tropopause forecast. I rarely look at it.

The third pic shows the connecting flights of the passengers, how many of which class to where, and the amount of fuel which it is worth to wast extra in order to make any given connection. Of course, there is an incredible amount of data being taken into account for this data. On this screen, everything is theoretical because we were early. The software optimises the suggested CI and speed based on this data.

The fourth pic shows the OFP calculated altitudes in white, and again the altitudes suggested by the software. This shows the difference between data available at the time of the calculation of the OFP and the changes to it during flight progress. Actual weight, CG and winds are the biggest players, here.

On the fifth pic you see the wind components of the most current wind prediction. The greener, the better.

On the sixth pic you see an example of the data I can change. On this page I check that the airborne time matches and sometimes play around with LTOP (latest time on position) and cost index.

The gist is, that this baby is so much more accurate than the FMC, especially when it comes to winds and altitude predictions. Rare is the opportunity to follow the suggestions exactly, but when you do, you realise how accurate the fuel and time prediction at the destination is, even 12 hours in advance. So my experience with the software is, that I don't bother with the FMC altitude suggestions at all. The only things I do are, that I take the latest CG from the software, which is very accurate, and enter it in the FMC, so that it can show the "rec max alt" more accurately, and enter the CI or Mach number suggested by the software. And of course I order and load new winds into the FMC, when they come out. But it's really just for comparing one computer with the other and whether all of it still makes sense.

So you see, there really is no need for dispatch to do any recalculations during flight. Even a rerouting when entered into the EFB flight plan will be considered.

And concerning cruise descents: on the 744, 1700-1800' above or below optimum altitude with 5 kts better wind, you have roughly the same specific fuel consumption as in optimum altitude. 10 kts, equals roughly 2300' higher or lower. Higher than 2000' above optimum altitude will most likely be no option because of maximum altitude, but lower can easily make sense. As one can see on the fifth pic, the winds may change quite significantly with altitude. On this particular sector, when we were finally able to climb from FL340 all the way to FL400 (traffic kept us down), the first 2000' gave us a more that 20 kt ground speed increase. Sometimes, the same could be the case for descending, so it does make sense.









How do I do it, to let them actually show up in the post?


Cheers, Kim.