F
Fuel calculation
Calculation of fuel requirements ( especially trip fuel and reserve fuel ) is the most safety-critical aspect of flight planning .
This calculation is somewhat complicated :
• Rate of fuel burn depends on ambient temperature , aircraft speed , and aircraft altitude , none of which are entirely predictable .
• Rate of fuel burn also depends on airplane weight , which changes as fuel is burned .
• Some iteration is generally required due to the need to calculate interdependent values . For instance , reserve fuel is often calculated as a percentage of trip fuel , but trip fuel cannot be calculated until the total weight of the aircraft is known , and this includes the weight of the reserve fuel .
Considerations
Fuel calculation must take many factors into account .
• Weather forecasts The air temperature affects the efficiency / fuel consumption of aircraft engines . The wind may provide a head- or tailwind component , which in turn will increase or decrease the fuel consumption by increasing or decreasing the air distance to be flown . By agreement with the International Civil Aviation Organization , there are two national weather centres ( in the United States , the National Oceanic and Atmospheric Administration , and in and the United Kingdom , the Met Office ), which provide worldwide weather forecasts for civil aviation in a format known as GRIB weather . These forecasts are generally issued every 6 hours and cover the subsequent 36 hours . Each 6-hour forecast covers the whole world using grid points located at intervals of 75 nautical miles ( 139 km ) or less . At each grid point , the weather ( wind speed , wind direction , air temperature ) is supplied at 9 different heights , ranging from about 4,500 feet ( 1,400 m ) up to about 55,000 feet ( 17,000 m ). Aircraft seldom fly exactly through weather gridpoints or at the exact heights at which weather predictions are available , so some form of horizontal and vertical interpolation is generally needed . For 75-nautical-mile ( 139 km ) intervals , linear interpolation is satisfactory . The GRIB format superseded the earlier ADF format in 1998 – 99 . The ADF format used 300- nautical-mile ( 560 km ) intervals ; this interval was large enough to miss some storms completely , so calculations using ADF-predicted weather were often not as accurate as those that can be produced using GRIB-predicted weather .
A - Z of Flight
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