Producing an accurate optimised flight plan requires millions of calculations, so commercial flight planning systems make extensive use of computers (an approximate unoptimised flight plan can be done by hand in an hour or so, but more allowance must be made for unforeseen circumstances). When computer flight planning replaced manual flight planning for eastbound flights across the North Atlantic, the average fuel consumption was reduced by about 1,000 pounds per flight, and the average flight times were reduced by about 5 minutes per flight. [1] Some commercial airlines have their own internal flight planning system, while others employ the services of external planners.