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Nick Dobda

Fuel Planning with FMC

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A few questions for fuel planning in the FMC

 

Tutorial 2 pretty straightforward, but some a few what if questions I have-

 

What if you're at the gate and your parked with fumes in the tank? You're at the gate, and you program the route - you go into the FMC and PROG - but there's not enough fuel in the tank to get you to your destination. You can't then subtract the fuel at your destination from the fuel currently onboard. Is this true or did I miss something? Similar with the alternate fuel calculation. If this were the case how do we use the FMC to plan for fuel?

 

I would think fuel load would impact fuel burn. Say we do our fuel planning with 8,000 lbs onboard while sitting at the gate. After calcs we find we need to be at 16,000 lbs. So we then fill the tanks to 16,000 lbs. Do we have to run the calcs again with the FMC taking into account the additional 8,000 lbs of fuel we just loaded? Is this an endless iterative loop, or is the difference negligible?

 

We run TOPCAT after the fuel planning. I'm guessing this is because the impact on thrust settings must be minimal?

 

 

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What if you're at the gate and your parked with fumes in the tank? You're at the gate, and you program the route - you go into the FMC and PROG - but there's not enough fuel in the tank to get you to your destination. You can't then subtract the fuel at your destination from the fuel currently onboard. Is this true or did I miss something? Similar with the alternate fuel calculation. If this were the case how do we use the FMC to plan for fuel?

 

The FUEL line on the PERF INIT page has boxes you can use to write in the planned amount of fuel. You could easily put 20 in there just as a notional value to get you started.

 

I would think fuel load would impact fuel burn. Say we do our fuel planning with 8,000 lbs onboard while sitting at the gate. After calcs we find we need to be at 16,000 lbs. So we then fill the tanks to 16,000 lbs. Do we have to run the calcs again with the FMC taking into account the additional 8,000 lbs of fuel we just loaded? Is this an endless iterative loop, or is the difference negligible?

 

It does, yes, because fuel is weight. Change the fuel to whatever value that you think you will need, verify that you have enough. Get to the point where you have enough, or a little more, and stop. You could go on forever trying to find a perfect value if you continued long enough.

 

We run TOPCAT after the fuel planning. I'm guessing this is because the impact on thrust settings must be minimal?

 

No. It's because you want to run the calculations on the most accurate weight value. Fuel being weight, you want to be sure on the final fuel number before trying to calculate performance.

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Thank you. The tutorial talks about the PLAN / FUEL line on LSK2L on the PERF INIT page - I guess I misunderstood what it was saying.

I also worded my TOPCAT question confusingly - I meant that thrust settings must have a minimal effect on fuel consumption because we determine the thrust settings AFTER having done the fuel calcs.

The "good enough" approach -- Run the generic (distance) x 2200lbs + 5500lbs calculation and type that in LSK2L - then do the fuel calcs with the FMC, one iteration - then run TOPCAT, then done, correct?

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Thank you. The tutorial talks about the PLAN / FUEL line on LSK2L on the PERF INIT page - I guess I misunderstood what it was saying.

 

Note the fact that the line is also called PLAN. Interesting name for it if we were only concerned with the amount of fuel in the tank currently  :wink:

 

I also worded my TOPCAT question confusingly - I meant that thrust settings must have a minimal effect on fuel consumption because we determine the thrust settings AFTER having done the fuel calcs.

 

Reduced thrust will actually use more fuel over the course of the takeoff and climb, but it's not a huge difference.

 

The "good enough" approach -- Run the generic (distance) x 2200lbs + 5500lbs calculation and type that in LSK2L - then do the fuel calcs with the FMC, one iteration - then run TOPCAT, then done, correct?

 

Yep.

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