T
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 mos t common methods are:
•
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
•
percentage of time: typically 10% (i.e., a 10-hour flight needs enough reserve to fly for
another hour)
•
percentage of fuel: typically 5% (i.e., a flight requiring 20,000 kg of fuel needs a
reserve of 1,000 kg)
Except for some US domestic flights, a flight plan normally has an alternate airport as well as a
destination airport. The alternate airport is for use in case the destination airport becomes
unusable while the flight is in progress (due to weather conditions, a strike, a crash, terrorist
activity, etc.).
This means that when the aircraft gets near the destination airport, it must still have enough
alternate fuel and alternate reserve available to fly on to the alternate airport.
Since the aircraft is not expected at the alternate airport, it must also have enough holding
fuel to circle for a while (typically 30 minutes) near the alternate airport while a landing slot is
found.
United States domestic flights are not required to have sufficient fuel to proceed to an
alternate airport when the weather at the destination is forecast to be better than 2,000-foot
(610 m) ceilings and 3 statute miles of visibility; however, the 45-minute reserve at normal
cruising speed still applies.
A - Z of Flight
July 2017
www.alliance-airways.net
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