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zachlog

Fuel Forecast at Destination vs. Alternate

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After setting-up the FMC fully for a flight KJFK à KLAX, the fuel qty. and DTG for KLAX in FMC | PROG was 4.2 and 2168 NM respectively.  When I checked these same parameters for the ALTN DEST airport KLGB (Long Beach, about 20-30 NM from KLAX), fuel qty. was 14.2 and DTG 2136 NM.

 

I would think that the fuel qty. difference between KLAX and KLGB would be minimal since the difference in DTG is small.  What are possible causes for this larger than expected difference ?

 

Thanks,

Zachlog


Zachary Kapsiotis

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Full names in the PMDG forums please.

 

You are looking at the fuel required from current location to alternate, you are expecting fuel from destination to alternate and the difference as you note is big.  Alternate predictions are always from current location based on safest assumption that you are diverting to alternate now.


Dan Downs KCRP

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It could also be a winds issue. The alt prediction is based on your current winds (I think) and the destination is based on all the leg winds.

 

I had a DCA-SEA flight with a BFI alt last week. When I checked the alt fuel prediction, it came up similar to your situation.

 

The FMC had winds for each leg to SEA, but only one lower wind for BFI.


Matt Cee

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Full names in the PMDG forums please.

 

You are looking at the fuel required from current location to alternate, you are expecting fuel from destination to alternate and the difference as you note is big.  Alternate predictions are always from current location based on safest assumption that you are diverting to alternate now.

Dan,

 

Not the case !  The numbers in my post are current position to destination and current position to alternate with minimal difference in DTG and a significant difference in fuel QTY prediction, thus my question.  Also, the two airports are very close so the path to each from current position is almost identical.

 

Zach K.

It could also be a winds issue. The alt prediction is based on your current winds (I think) and the destination is based on all the leg winds.

 

I had a DCA-SEA flight with a BFI alt last week. When I checked the alt fuel prediction, it came up similar to your situation.

 

The FMC had winds for each leg to SEA, but only one lower wind for BFI.

Matt,

 

What you say makes sense.  I specified winds in PERF INIT (single value).  Where in the FMC would I find:

 

1. The winds for each leg to the destination airport ?

2. The one lower wind to the alternate ?

 

Thanks,

Zach K.


Zachary Kapsiotis

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What you say makes sense. I specified winds in PERF INIT (single value). Where in the FMC would I find:



1. The winds for each leg to the destination airport ?

2. The one lower wind to the alternate ?

 

On the LEGS pg RTE DATA you can enter the winds for each leg. The FMC then can compute more accurate ETA and  FUEL.

 

I'm not sure which wind the FMC uses for its prediction for the ALT. It might be the current wind or the Top Of Climb wind from the Perf Init.

 

Great question that stumped me a few days ago, too!


Matt Cee

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Zach K.

 

Full names in the PMDG forums please. Thank you.


Dan Downs KCRP

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Also a read of Tutorial #2 might be useful here. Ryan goes into quite some detail about using the FMC for fuel planning with some great explanations about how the FMC predicts fuel usage.

 

Dave


Dave Paige

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