Redispatch

[downscc 1,327]

I've had PFPX about as long as the PMDG 777 has been out and I am just now trying to understand the intricacises of flight planning with redispatch. My motivation is a flight I just finished DL17 KLAX-YSSY using same route and times. My planned fuel remaining at YSSY was 33,764 lbs (a lot of fuel to haul for all the whatnots that happen) and it looked like I was going to get there closer to 38 klbs which puts me at maximum landing weight (492 klbs) so I am now motivated to learn about redispatch.

 

I've tried it before, PFPX has a button you push and it's automatically included, but didn't understand what was happening. My fuel remaining went down but so did the max ZFW so it was confusing for me that saving fuel required me to dump payload. It turns out that this seems to happen because the landing weight is the critical constraint on this long flight, and any intermediate redispatch point must allow a landing below that constraint.

 

I tried a few alternatives to the PFPX automatic redispatch and found it was selecting the optimum solution. For example:

                  No DP   Auto DP Manual

ReDisp Location    na     NWWW   PHTO   NFFN
Fuel Release     287.8   258.5  221.3  247.3
Fuel Remaining    33.8    17.6   22.5   18.7
ZFW              457.1   444.8  347.7  416.9
Constraint        LDW     LDW    LDW    LDW
Fuel/Weight      0.5557  0.5414 0.5720 0.5484


All weights are lbs x 1,000

Fuel/Weight is trip fuel / ZFW yields pounds of fuel required
 to carry a pound of aircraft and payload. Smaller is better.

The first column is the flight without redispatch. Second column is PFPX autoselection of redispatch location and DP fix and the last two columns are my attempt to find a better solution trying locations that allowed for a DP fix closer to the midpoint of the trip. I tried this because all the literature I've read about redispatching says that the midpoint is optimum because the reserves for each segment are equal and at minumum.  I left this paradox last year and came back this morning to try to sort it out.

 

The problem is that the midpoint may achieve the optimum solution but only if there are not other constraints. In this case the maximum landing weight. For example, selecting PHTO on the big island means much less payload and fuel to allow a landing under MLW 492 klbs.

 

Just to prove my hunch that it was the MLW constraint, I reset the PFPX aircraft properties for my B77L to have a MLW equal to MTOW of 766 kbls.

 

Now my trip without redispatch has similiar profile except ZFW becomes max ZFW which is now the constriaint but the big difference is the redispatch case, choosing same location and DP fix the ZFW on release increases from 444.8 to max ZFW of 460.9 klbs and the fuel/weight ratio decreases to 0.5390 which is significant.

 

The decrease in payload when selecting redispatch option does remove money making weight but that is simply due to the maximum landing weight of the aircraft, and even with the lighter payload the flight will generate more income due to improved fuel/weight efficiencies. Very interesting.

[BlueStar 323]

Dan,

 

I have a good friend who flies this trip for DL. 

 

One thing to realize that landing over MGLW is not the end of the world.  The aircraft is certified to land at MGLW with 10/feet/sec rate of descent and at MGTOW at 6/feet/second. 

 

Is this cost analysis part of a VA or program that you are using?

[Les Parson 99]

Greetings, 

 

I worked with Pacific redispatch  in the real world first as an oceanic controller in Australia, and some years later as an international dispatcher for Continental. We did things a little differently than you've described; however, this was 25 to 30 years ago so things have obviously changed. 

 

I could elaborate if you are interested. 

 

In the FS world, I don't go near the heavy iron and stick exclusively with GA. 

 

Les Parson

[downscc 1,327]

 

 

Is this cost analysis part of a VA or program that you are using?

 

Nope, nothing other than PFPX... just rerun different scenarios using same path and weather and keep track with ..... get this: pen and paper. Yup, I am so old I still use pen and paper.

 

 

 

I could elaborate if you are interested.

 

Of course, we still do a lot of things about the same way as "way back then," only the tools have changed. Better tools might change procedures but the problem is the same.

[BlueStar 323]

I can remember the days when we would figure TOD as Altitude/1000 +10.  Would add 10 if strong downwind and subtract 10 if strong headwind.  We would estimate fuel flow on the JT3Ds was 3,000/hour/engine.  Not perfect, but close enough.

[Richard McDonald Woods 168]

Hi Dan,

I feel that you will get the expert answer to your questions re re-dispatch by posting instead in the PFPX forum. There, there are current practitioners to answer in detail.

HTH

Cheers, Richard

[downscc 1,327]

Howdy Richard, thanks but my post was actually just a sharing rather than an inquiry. I think I now have successfully scratched the surface, and now understand why payload must decrease when employing redispatch in cases where MLW is the critical constraint.

[Les Parson 99]

From the dispatch side at Continental, we could never plan an overweight landing. The object was to maximize payload thru fuel reduction utilizing ICAO Flag reserves with the redispatch procedure. The bottom line was Fuel Over Destination (FOD); roughly 18,000 LBS for B747-200; 15,000 LBS for DC-10-30. 

 

Les Parson

[DHG750R 0]

What a redispatch does is reduce the amount of contingency fuel that needs to be carried.  Using your LAX-SYD as an example, without a redispatch flightplan you would have to carry 10% reserves for the entire 14hr 25mins or so, in this example it adds up to carrying a total of 17300lbs for  vs 4041lbs.  You have to burn fuel in order to carry the extra weight .   It costs an additional 3400lbs burn over the entire trip to carry the additional weight of the fuel

 

 

        FUEL    CORR    ENDUR

TRIP         219028  ........  14:25

10 PCT        17300  ........  01:26

ALTN YMDG      6142  ........  00:22

INTL HOLD      6197  ........  00:30

CONT           1000  ........  00:05

MIN T/O      249667  ........  16:43      

EXTRA             0  ........  00:00      

TAXI            570  ........  00:10      

RELEASE      250237  ........  16:53

 

 

               FUEL    CORR    ENDUR

TRIP         215628  ........  14:27

10 PCT         4041  ........  00:21

ALTN YMDG      6081  ........  00:22

INTL HOLD      6089  ........  00:30

CONT           6042  ........  00:05

MIN T/O      238881  ........  15:40      

EXTRA             0  ........  00:00      

TAXI            570  ........  00:10      

RELEASE      239451  ........  15:50  

 

 

Hope this helps 

 

Darrell

 

[downscc 1,327]

Thanks Darrell (full names in the PMDG forums please). The decrease is fuel carried to destination is obvious, what I am trying to explain is why the payload also has to decrease. A decrease in payload to save money is counter intuitive (less payload is less income), so I dug deeper and presented my conclusion; apparently, not very clearly. Oh well.

[DHG750R 0]

At my company, one of the benefits of using a redispatch  is increased payload since you are carrying less fuel.  The trick when flightplanning is to find the redispatch destination and alternate which balances the lower required fuel while not limiting payload due to a landing weight restriction.  Using the same city pair LAX-SYD and the 77L , PFPX will calculate max payload for the trip if you specify.  Of course some aircraft types benefit more for a redispatch than others. 

 

Max payload 144500 using redispatch

               FUEL    CORR    ENDUR

TRIP         253745  ........  14:10

10 PCT         4770  ........  00:21

ALTN YMDG      6744  ........  00:22

INTL HOLD      7317  ........  00:30

CONT           7506  ........  00:04

MIN T/O      281082  ........  15:23    

EXTRA             0  ........  00:00      

TAXI            570  ........  00:10      

RELEASE      281652  ........  15:33

 

 

Max payload 140542 lbs without redispatch

               FUEL    CORR    ENDUR

TRIP         255397  ........  14:09

10 PCT        19810  ........  01:25

ALTN YMDG      6776  ........  00:22

INTL HOLD      7372  ........  00:30

CONT           1000  ........  00:04

MIN T/O      290355  ........  16:26      

EXTRA             0  ........  00:00      

TAXI            570  ........  00:10      

RELEASE      290925  ........  16:36  

 

 

Thanks

Darrell Gordon   

[downscc 1,327]

I don't see the redispatch calcs above, there should be a 10 PCT value for each segment and an ALTN value for the redispatch alternate. The problem is maximum landing weight at the redispatch airport. For example, the return trip that I am flying now the limitation is MLW at PHTO, that limits ZFW from maximum for no redispatch to about 436000 lbs with redispatch. This is using PFPX.

[pcubine 143]

Dan

I get the impression that re-dispatch in PFPX pretty much works automatically with no user intervention required. Is this correct?

[downscc 1,327]

That is an option, and it's a good place to start. Press a button and a decision point and airport is automatically selected to minimize fuel remaining.  I've played with it trying to find better but haven't yet. I may alter the results just because I don't think the airport is realistic... there's more to consider than just runway length.

[pcubine 143]

That is an option, and it's a good place to start. Press a button and a decision point and airport is automatically selected to minimize fuel remaining.  I've played with it trying to find better but haven't yet. I may alter the results just because I don't think the airport is realistic... there's more to consider than just runway length.

 

Thanks for the information.