Flight planning
Flight planning is the process of producing a flight plan to describe a proposed aircraft flight. It involves two safety-critical aspects: fuel calculation, to ensure that the aircraft can safely reach the destination, and compliance with air traffic control requirements, to minimize the risk of midair collision. In addition, flight planners normally wish to minimize flight cost through the appropriate choice of route, height, and speed, and by loading the minimum necessary fuel on board. Air Traffic Services (ATS) use the completed flight plan for separation of ACFT in ATM services, including tracking and finding lost ACFT, during search and rescue (SAR) missions.
Flight planning requires accurate weather forecasts so that fuel consumption calculations can account for the fuel consumption effects of head or tail winds and air temperature. Safety regulations require aircraft to carry fuel beyond the minimum needed to fly from origin to destination, allowing for unforeseen circumstances or for diversion to another airport if the planned destination becomes unavailable. Furthermore, under the supervision of air traffic control, aircraft flying in controlled airspace must follow predetermined routes known as airways (at least where they have been defined), even if such routes are not as economical as a more direct flight. Within these airways, aircraft must maintain flight levels, specified altitudes usually separated vertically by 1000 or 2000 feet (305 or 610 m), depending on the route being flown and the direction of travel. When aircraft with only two engines are flying long distances across oceans, deserts, or other areas with no airports, they have to satisfy additional ETOPS safety rules to ensure they can reach some emergency airport if one engine fails.
Overview and basic terminology
A flight planning system may need to produce more than one flight plan for a single flight:
summary plan for air traffic control (in FAA and/or ICAO format)
summary plan for direct download into an onboard flight management system
detailed plan for use by pilots
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:
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)
Complete routes
There are a number of ways of constructing a route. All scenarios using airways use SIDs and STARs for departure and arrival. Any mention of airways might include a very small number of “direct” segments to allow for situations when there are no convenient airway junctions. In some cases, political considerations may influence the choice of route (e.g., aircraft from one country cannot overfly some other country).
Airways from origin to destination. Most flights over land fall into this category.
Airways from origin to an ocean edge, then an ocean track, then airway(s) from ocean edge to destination. Most flights over northern oceans fall into this category.
Airways from origin to an ocean edge, then a free-flight area across an ocean, then airways from ocean edge to destination. Most flights over southern oceans fall into this category.
Free-flight area from origin to destination. This is a relatively uncommon situation for commercial flights.
Even in a free-flight area, air traffic control still requires a position report about once an hour. Flight planning systems organise this by inserting geographic way points at suitable intervals. For a jet aircraft, these intervals are 10 degrees of longitude for eastbound or westbound flights and 5 degrees of latitude for northbound or southbound flights. In free-flight areas, commercial aircraft normally follow a least-time-track so as to use as little time and fuel as possible. A great circle route would have the shortest ground distance, but is unlikely to have the shortest air distance, due to the effect of head or tail winds. A flight planning system may have to perform significant analysis to determine a good free-flight route.