The continuing saga of fuel remaing at destination as predicted by the 777F FMC. I was informed by PMDG Tech Support that this issue was caused by the "limitations of FSX". I would imagine the MD-11 is subject to the same limitations. Several days ago I did a RJBB-KOAK flight in the MD-11 with the TOW 2000 lbs. below the limit. On the ground at RJBB the remaing fuel at KOAK was predicted to be 26500 lbs and that is what is was on landing at KOAK. The ETA was 1139Z and that is when arrival was. No average winds were used. For each waypoint the PFPX forecasted winds were entered by altitude, direction and speed. So the predicted remaining fuel for a large heavy aircraft can be accurate.

 

What I am doing for 777F now is using the correct fuel policy, re-dispatch, and a bizare fuel bias of around 90.5%. This way I arrive at the destination with fuel remaining that is close to the reserves required. But the fuel reserves are not correct since are they based upon a fuel bias of 90.5%. I just completed EEDP-KLAX. The ETA was 1501Z and the actual arrival was 1505Z. Not bad for an 11 hour flight. However, the remaning fuel per the progress page went from 0 lbs. at takeoff to 13500 lbs at KLAX. I usually spend about half of the flight with no reserves and almost all the flight with an insufficent fuel warning on the upper EICAS.

I just ran a series of tests to compare actual vs PFPX predicted fuel flow rates and arrived at an average bias of 101.5% for the B77L.  Fuel bias should not be used to adjust fuel remaining or reserve requirements. The only thing it does is adjust the predicted fuel flow during cruise.  I've been trying to understand why PFPX estimates a certain value remaining that is much lower than what I experience on most flights but not all.  I have noticed the step climb and optimum altitudes are usually wrong, that there is a lot of cruise data missing from the tables too. For example, my last test run was at FL360 GW 549.2 having just step climbed per FMS from FL340 but PFPX thinks max GW at FL360 is 530.0 klbs.  None of this explains the fuel remaining mystery. Still probing.

 

As we've shared before, the PMDG FMS also has its quirks in the fuel remaining prediction.  Lately I've notice more of an error on the dangerous side with tail winds, but even with head winds the value starts out too low then eventually converges on an expected value.

 

Lots of moving pieces.

I meant to follow up on this myself quite a while back, and unfortunateky forgot to do so.

 

As discussed previously there are various PFPX performance profiles (from different sources) available.

 

The most commonly used are probably the -300ER and -200LR provided by the fefault PFPX installation. Followed by the far more accurate and FPPM sourced profiles by "FlyPrecisely" which among others, myself and IIRC Michael also use.

 

Indeed there is a range of fuel burn bias when flying the PMDG 777 between flights. This is however a bias, based solely on figures outputted by PFPX. Why is it that on a DXB-DFW with a 15h36m trip time, I am requiring a 102% bias (vs. the profile default 100%), while on a subsequent DXB-DFW with a 15h and 3m trip time, the 100% cruise burn bias is spot on?

 

Static Air Temperature or SAT. As found on PROG page 2/4.....

 

The PFPX performance profiles for the B777 max out at a ISAequivalent of -60C SAT. The longer of my two DXB-DFW flights took place in the winter months, with a Polar routing via ABERI. The latter and more recent one, in April with a more southern routing, via the North Atlantic.

 

The lowest SAT on the latter flight was -51C (within the PFPX performance file data range). The previous flight had a lowest SAT of -66C (requiring a temporary step down to a lower flight level, as is common on Polar routings to remain within the FCOM limitation fuel temp/time range). At -66C the B777 will of course have a higher fuel flow, due to the cooler and thus denser air.

 

This below -60C SAT fuel flow, is entirely outside of the range of temperature/fuel flow values in the very performance files we are using in our PFPX calculations. Thus, on specific flights with OAT/SAT's outside of the PFPX aircraft performance files data range, PFPX is significantly under estimating TRIP fuel burn in its computation. A 2% error is no laughing matter with 120,000kgs of TRIP fuel...

 

Countless comparisons using weight/speed/FL/TAT, show a fuel flow delta within .05% between PMDG's B777 and the latest FPPM. Both for the GE90-110B1L1 powered -200LR, and the GE90-115BL1 powered -300ER. As a result, the only significant error I see, is in the incomplete temperature/FL/fuel flow data range in the available PFPX profiles.

 

The OPT/ MAX CRZ alt values are also slightly off in the available PFPX aircraft performance files. (more for some weights than others). The result being PFPX either plans for too early step climbs when the FMC reports a lower MAX crz value, or step climbs that are computed too late by PFPX. This depending on which aircraft performance file is used in PFPX (default being the worst). There is no delta between the PMDG weight/temp/MAX crz values and the FPPM values. They truly are spot on and straight out of the book!

 

I would like to point out that after having looked at countless OFP's of a very big B777 operator, the fuel burn bias of a specific -300ER, can continuously vary from from a low 1% to over 2% within a few months. Small variations between flights being possible and automatically tracked and adjusted by the dispatch software as a result of fuel burn data downlinked via ACARS and monitored by the operator. This being a requirement of statistical based contingency fuel!

 

As a result, I duplicated my FlyPrecisely based -200LR/-300ER PFPX profiles, and have a range of % cruise bias from a low of 99% to 102% on the -300ER/-200LR (in 1% increments). As a result, even on my longest sectors, I find myself within ~500 to 1000kg of expected TRIP burn and fuel at destination. Easily covered by planned contingency fuel.

 

In addition this TRIP bias is also always attributable in part, to the PFPX planned vs. actual encountered wind/ISA conditions outputted by ASN. Which are of course to be expected and thus planned for to begin with!

calzonister, on 08 May 2016 - 10:13 PM, said:

As discussed previously there are various PFPX performance profiles (from different sources) available.

I am using a "Fly Precisely" profile in PFPX. That is the profile that I had to change the fuel bias to 90.5% to have a reasonable reserve at the end of flights. For a flight I just started, EDDF-KIAH, I ran two OFPs. The 90.5% fuel bias for release fuel and another one with 100% fuel bias for the correct reserves. Right now I am 46 minutes into the flight and the PROGRESS page shows 0 fuel remaining at KIAH. I believe every thing you say applies to the 777LR and the 300ER. However the 777F at a high TOW is a different bird. In this situation it is the FMC that is the culprit not PXPX. All though PFPX recommended an initial cruise of FL320 when in fact the FMC on page 2 of VNAV showed the optimum altitude as 30100 ft. But since I am going west bound I could only use even FLs. So I used FL300. At TOC is was 645 nm before I could climb to FL320. As you say, sometimes PFPX is optimistic in its calculations. When this flight is complete I will be back with more including the results of this flight

The EDDF-KIAH flight was completed a couple of hours ago. The PROGRESS page showed fuel remaining of 11100 lbs. which was 400 lbs. below the reserves of 11500 calculated by PFPX using a 100% fuel bias. I need to change the OFP #1 fuel bias to maybe 91%. My concern is that this percentage will only apply to EDDF-KIAH and no other city pairs. Oh well, I will find out tonight on a VHHH-EDDP flight. It is amazing to start a flight with no reserves on the PROGRESS page and end up with 11100 lbs. at the destination. Something out of nothing. The 777F could become another oil producer.

 

Speaking of FMC issues, I wanted to bring your attention to this post by Rob Robson which I will quote.  

“We used to have to do this for the longest time on the real 777 as well.
Something in the approach of the destination could cause a problem on the SID during departure.
Dont remember waht the problem was....not an OOM though LOL.
So there was a Boeing bullitin out back then to not program an approach untill after the SID was flown.
An FMC version update solved the problem ocne and for all after a while.....but it took....gosh....I think more than a year.”

I got a real laugh out of that post. You may have seen it already. It is in the thread you started on VAS leakage.

 

 

I believe several months ago I wrote that at VHHH, 07L/25R  was the landing runway and 07R/25L was the takeoff runway and you indicated there were some landings on 07R/25L. From watching a lot of YouTube videos of real world operations at VHHH, I get the impression that generally 07L/25R is for passenger planes and 07R/25L is for cargo planes. So now I can start deleting the missed approaches for 07R/25L just like I do now with 26L at EDDP.

 

 

As we've shared before, the PMDG FMS also has its quirks in the fuel remaining prediction.

In so many words, that's what PMDG support says also. It has been so long since I have flown the 744 I don't remember how it was with estimated remaining fuel or if it even reported it. I see no problems with the NGX or the MD-11. Both are fairly accurate at predicting the estimated remaining fuel.

 

Have you encountered any issues with the PFPX re-dispatch or is it still okay. If there was a problem, I would even know it if I saw it. If you say it's okay, it's  okay and  use it.

I didn't start using planning software until the 777, didn't need it with the NGX or MD-11 which was always spot on.

 

I do use PFPX Redispatch often, whenever applicable, to reduce the fuel carried to destination.  It provides a good or best RDP redispatch point but you need to pick your own airport.  For example, on the EGLLKLAX trip it may pick a small airfield in Nebraska when you really want to go land at KDEN with lots of infrastructure and the common constraint is landing weight at the RDP airport so in this case selecting a further distance to landing is working for you. It will allow more payload.

For example, on the EGLLKLAX trip it may pick a small airfield in Nebraska when you really want to go land at KDEN

Dan

I see what you mean. On VHHH-EDDP flight RD picked the well know UMOO at Mogilev in Belarus. Navigraph does not even have charts for the airport. But to me re-dispatch is just a click of the button to save fuel/increase payload.

I am using a "Fly Precisely" profile in PFPX. That is the profile that I had to change the fuel bias to 90.5% to have a reasonable reserve at the end of flights. For a flight I just started, EDDF-KIAH, I ran two OFPs. The 90.5% fuel bias for release fuel and another one with 100% fuel bias for the correct reserves. Right now I am 46 minutes into the flight and the PROGRESS page shows 0 fuel remaining at KIAH. I believe every thing you say applies to the 777LR and the 300ER. However the 777F at a high TOW is a different bird. In this situation it is the FMC that is the culprit not PXPX. All though PFPX recommended an initial cruise of FL320 when in fact the FMC on page 2 of VNAV showed the optimum altitude as 30100 ft. But since I am going west bound I could only use even FLs. So I used FL300. At TOC is was 645 nm before I could climb to FL320. As you say, sometimes PFPX is optimistic in its calculations. When this flight is complete I will be back with more including the results of this flight

Understood Michael,

 

My last B77F flight is a few months back. 11h sector, departed at 343t. 4t short of MTOW.

 

I dont remember seeing anything out of the ordinary with regards to fuel at destination prediction,, but I certainly believe your eyes arent lying to you either.

 

How about we set up a controlled test scenario and compare what we are seeing on our systems. Of course keeping in mind, that I am running the latest P3D release (3.2.2), and you are running FSX SP2.

 

0 wind, default clear clies, identical zfw, calc fuel, weights, step climbs and so on.

 

I will run a PFPX plan and share via google drive. Like that we can compare fuel at destination prediction on the ground, at TOC and at a given point in flight.

 

Ill report back in 48 hours latest.

 

 

I will run a PFPX plan and share via google drive. Like that we can compare fuel at destination prediction on the ground, at TOC and at a given point in flight.

Ill report back in 48 hours latest.

Do you want me to do anything prior to you reporting back?

VHHH-EDDP fuel remaining was 13400 lbs. Required reserves 13000 lbs. Using 91% fuel bias for release fuel and 100% bias for required reserves.