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 minimise the risk of midair collision. In addition, flight planners normally wish to minimise flight cost through the appropriate choice of route, height, and speed, and by loading the minimum necessary fuel on board. Air Traffic Services use the completed flight plan for separation of aircraft in air traffic management services, including tracking and finding lost aircraft, during search and rescue missions. Flight planning typically includes route selection, fuel calculation, alternate aerodrome planning, weight and balance considerations, and an assessment of meteorological conditions.
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, 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, 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 an emergency airport if one engine fails.
Producing an accurate optimised flight plan requires millions of calculations, so commercial flight planning systems make extensive use of computers. When computer flight planning replaced manual flight planning for eastbound flights across the North Atlantic, the average fuel consumption was reduced by about per flight, and the average flight times were reduced by about 5 minutes per flight. Some commercial airlines have their own internal flight planning system, while others employ the services of external planners.
A licensed flight dispatcher or flight operations officer is required by law to carry out flight planning and flight watch tasks in many commercial operating environments. These regulations vary by country but more and more countries require their airline operators to employ such personnel.

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
  • 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 airway 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, then for 45 minutes thereafter at normal cruising speed
  • percentage of time: typically 10%
  • percentage of fuel: typically 5%
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. 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 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 ceilings and 3 statute miles of visibility; however, the 45-minute reserve at normal cruising speed still applies.
It is often considered a good idea to have the alternate some distance away from the destination so that bad weather is unlikely to close both the destination and the alternate; distances of up to are not unknown. In some cases the destination airport may be so remote that there is no feasible alternate airport; in such a situation an airline may instead include enough fuel to circle for 2 hours near the destination, in the hope that the airport will become available again within that time.
There is often more than one possible route between two airports. Subject to safety requirements, commercial airlines generally wish to minimise costs by appropriate choice of route, speed, and height.
Various names are given to weights associated with an aircraft and/or the total weight of the aircraft at various stages.
  • Payload is the total weight of the passengers, their luggage, and any cargo. A commercial airline makes its money by charging to carry payload.
  • Operating weight empty is the basic weight of the aircraft when ready for operation, including crew but excluding any payload or usable fuel.
  • Zero fuel weight is the sum of operating weight empty and payload—that is, the laden weight of an aircraft, excluding any usable fuel.
  • Ramp weight is the weight of an aircraft at the terminal building when ready for departure. This includes the zero fuel weight and all required fuel.
  • Brake release weight is the weight of an aircraft at the start of a runway, just prior to brake release for takeoff. This is the ramp weight minus any fuel used for taxiing. Major airports may have runways that are about 2 miles long, so merely taxiing from the terminal to the end of the runway might consume up to a ton of fuel. After taxiing, the pilot lines up the aircraft with the runway and puts the brakes on. On receiving takeoff clearance, the pilot throttles up the engines and releases the brakes to start accelerating along the runway in preparation for taking off.
  • Takeoff weight is the weight of an aircraft as it takes off partway along a runway. Few flight planning systems calculate the actual takeoff weight; instead, the fuel used for taking off is counted as part of the fuel used for climbing up to the normal cruise height.
  • Landing weight is the weight of an aircraft as it lands at the destination. This is the brake release weight minus the trip fuel burned. It includes the zero fuel weight, unusable fuel, and all alternate, holding, and reserve fuel.
When twin-engine aircraft are flying across oceans, deserts, and the like, the route must be carefully planned so that the aircraft can always reach an airport, even if one engine fails. The applicable rules are known as ETOPS. The general reliability of the particular type of aircraft and its engines and the maintenance quality of the airline are taken into account when specifying how long such an aircraft may fly with only one engine operating.
Flight planning systems must be able to cope with aircraft flying below sea level, which will often result in a negative altitude. For example, Amsterdam Schiphol Airport has an elevation of −3 metres. The surface of the Dead Sea is 417 metres below sea level, so low-level flights in this vicinity can be well below sea level.

Units of measurement

Flight plans mix metric and non-metric units of measurement. The particular units used may vary by aircraft, airline, and location across a flight.
Since 1979, the International Civil Aviation Organization has recommended a unification of units of measurement within aviation based on the International System of Units. Since 2010, ICAO recommends using:
However, a termination date for completion of metrication has not been established. While SI units technically are preferred, various non-SI units are still in widespread use within commercial aviation:
Distances are nearly always measured in nautical miles, as calculated at a height of, compensated for the fact that the earth is an oblate spheroid rather than a perfect sphere. Aviation charts always show distances as rounded to the nearest nautical mile, and these are the distances that are shown on a flight plan. Flight planning systems may need to use the unrounded values in their internal calculations for improved accuracy.

Fuel units

Fuel measurement will vary on the gauges fitted to a particular aircraft. The most common unit of fuel measurement is kilograms; other possible measures include pounds, UK gallons, US gallons, and litres. When fuel is measured by weight, the specific gravity of the fuel used is taken into account when checking tank capacity.
There has been at least one occasion on which an aircraft ran out of fuel due to an error in converting between kilograms and pounds. In this particular case the flight crew managed to glide to a nearby runway and land safely.
Many airlines request that fuel quantities be rounded to a multiple of 10 or 100 units. This can cause some interesting rounding problems, especially when subtotals are involved. Safety issues must also be considered when deciding whether to round up or down.