I planned a flight from Pisa,Italy to Rio D.J Brazil and it was around 4200 miles. I was reading online that the 747-400 can fly up to 8,000 miles yet I get insufficient fuel on FMC on the ground. Passengers are near 1/3 full. 366,000 lbs were loaded through PMDG config. How is that not enough fuel?

I use TOPCAT to initially estimate fuel required

 

LIRP to SBGL (TOPCAT says) 5260nm... with about 18,000 pax + 55,000lbs cargo requiring 296,000lbs of fuel... so your fuel load should be "way more" than enough. I have no idea why "insufficient fuel" unless you have plugged in some very strong (head) winds or your reserve fuel number is quite high.

Check for any issues in your flight plan, sometimes an incorrect waypoint will add thousands of miles to your flight.

FMC insufficient fuel msg on the 747-400 doesn't necessarily mean you don't have enough fuel to get to your destination, it means the minimum arrival fuel you have set (in case of a diversion ) is not going to be met. Have a read of your FCOM.

 

Regards

I took an image for you to see that is shows 0 fuel over 1,000 miles away still.

 

http://s18.postimage.org/tdl9j201l/2012_7_10_21_58_18_954.png