<Li> detailed plan for use by pilots </Li> <P> The basic purpose of a flight planning system is to calculate how much trip fuel is needed in the air navigation process by an aircraft when flying from an origin airport to a destination airport . Aircraft must also carry some reserve fuel to allow for unforeseen circumstances, such as an inaccurate weather forecast, or air traffic control requiring an aircraft to fly at a lower - than - optimal altitude due to congestion, or the addition of last - minute passengers whose weight was not accounted for when the flight plan was prepared . The way in which reserve fuel is determined varies greatly, depending on airline and locality . The most common methods are: </P> <Dl> <Dd> <Ul> <Li> US domestic operations conducted under Instrument Flight Rules: enough fuel to fly to the first point of intended landing, then fly to an alternate airport (if weather conditions require an alternate airport), then for 45 minutes thereafter at normal cruising speed </Li> <Li> percentage of time: typically 10% (i.e., a 10 - hour flight needs enough reserve to fly for another hour) </Li> <Li> percentage of fuel: typically 5% (i.e., a flight requiring 20,000 kg of fuel needs a reserve of 1,000 kg) </Li> </Ul> </Dd> </Dl> <Dd> <Ul> <Li> US domestic operations conducted under Instrument Flight Rules: enough fuel to fly to the first point of intended landing, then fly to an alternate airport (if weather conditions require an alternate airport), then for 45 minutes thereafter at normal cruising speed </Li> <Li> percentage of time: typically 10% (i.e., a 10 - hour flight needs enough reserve to fly for another hour) </Li> <Li> percentage of fuel: typically 5% (i.e., a flight requiring 20,000 kg of fuel needs a reserve of 1,000 kg) </Li> </Ul> </Dd>

Which control activity involves relating different sets of data to one another