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 The GRIB format superseded the earlier ADF format in 1998 – 99.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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