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 (ATS) use the completed flight plan for separation of aircraft in air traffic management services, including tracking and finding lost aircraft, 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 , 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 (an approximate unoptimised flight plan can be produced using an E6B and a map in an hour or so, but more allowance must be made for unforeseen circumstances). 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 (e.g., US FAR §121, Canadian regulations). 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:
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:
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 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 (e.g., ) 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 (e.g., a Pacific island) 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.
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 (ExTended range OPerationS). 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 (typically 1–3 hours).
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 (ICAO) has recommended a unification of units of measurement within aviation based on the International System of Units (SI). 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:
Distance units.
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 (the runway was one of two at a former airport then being used as a dragstrip).
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.
Height units.
The altitude of an aircraft is based on the use of a pressure altimeter (see flight level for more detail). The heights quoted here are thus the nominal heights under standard conditions of temperature and pressure rather than the actual heights. All aircraft operating on flight levels calibrate altimeters to the same standard setting regardless of the actual sea level pressure, so little risk of collision arises.
In most areas, height is reported as a multiple of , i.e. A025 is nominally . When cruising at higher altitudes aircraft adopt flight levels (FLs). Flight levels are altitudes corrected and calibrated against the International Standard Atmosphere (ISA). These are expressed as a three-figure group e.g., FL320 is ISA.
In most areas, the vertical separation between aircraft is either .
In Russia, China and some neighbouring areas, altitudes are measured in metres. The vertical separation between aircraft is either 300 metres or 600 metres (about 1.6% less than 1,000 or 2,000 feet).
Until 1999, the vertical separation between aircraft flying at high altitudes on the same airway was . Since then there has been a phased introduction around the world of reduced vertical separation minimum (RVSM). This cuts the vertical separation to between flight levels 290 and 410 (the exact limits vary slightly from place to place). Since most jet aircraft operate between these heights, this measure effectively doubles the available airway capacity. To use RVSM, aircraft must have certified altimeters, and autopilots must meet more accurate standards.
Speed units.
Aircraft cruising at lower altitudes normally use knots as the primary speed unit, while aircraft that are higher (above Mach Crossover Altitude) normally use Mach number as the primary speed unit, though flight plans often include the equivalent speed in knots as well (the conversion includes allowance for temperature and height). In a flight plan, a Mach number of "Point 82" means that the aircraft is travelling at 0.820 (82%) of the speed of sound.
The widespread use of global positioning systems (GPS) allows cockpit navigation systems to provide air speed and ground speed more or less directly.
Another method of obtaining speed and position is the inertial navigation system (INS), which keeps track of a vehicle's acceleration using gyroscopes and linear accelerometers; this information can then be integrated in time to obtain speed and position, as long as the INS was properly calibrated before departure. INS has been present in civil aviation for a few decades and is mostly used in medium to large aircraft as the system is fairly complex.
If neither GPS or INS is used, the following steps are required to obtain speed information:
Mass units.
The weight of an aircraft is most commonly measured in kilograms, but may sometimes be measured in pounds, especially if the fuel gauges are calibrated in pounds or gallons. Many airlines request that weights be rounded to a multiple of 10 or 100 units. Great care is needed when rounding to ensure that physical constraints are not exceeded.
When chatting informally about a flight plan, approximate weights of fuel and/or aircraft may be referred to in tons. This "ton" is generally either a metric tonne or a UK long ton, which differ by less than 2%, or a short ton, which is about 10% less.
Describing a route.
A route is a description of the path followed by an aircraft when flying between airports. Most commercial flights will travel from one airport to another, but private aircraft, commercial sightseeing tours, and military aircraft may do a circular or out-and-back trip and land at the same airport from which they took off.
Components.
Aircraft fly on airways under the direction of air traffic control. An airway has no physical existence, but can be thought of as a motorway in the sky. On an ordinary motorway, cars use different lanes to avoid collisions, while on an airway, aircraft fly at different flight levels to avoid collisions. One can often see planes passing directly above or below one's own. Charts showing airways are published and are usually updated every 4 weeks, coinciding with the AIRAC cycle. AIRAC (Aeronautical Information Regulation and Control) occurs every fourth Thursday, when every country publishes its changes, which are usually to airways.
Each airway starts and finishes at a waypoint, and may contain some intermediate waypoints as well. Waypoints use five letters (e.g., PILOX), and those that double as non-directional beacons use three or two (TNN, WK). Airways may cross or join at a waypoint, so an aircraft can change from one airway to another at such points. A complete route between airports often uses several airways. Where there is no suitable airway between two waypoints, and using airways would result in a somewhat roundabout route, air traffic control may allow a direct waypoint-to-waypoint routing, which does not use an airway (often abbreviated in flight plans as "DCT").
Most waypoints are classified as compulsory reporting points; that is, the pilot (or the onboard flight management system) reports the aircraft's position to air traffic control as the aircraft passes a waypoint. There are two main types of waypoints:
Note that airways do not connect directly to airports.
Special routes known as "ocean tracks" are used across some oceans, mainly in the Northern Hemisphere, to increase traffic capacity on busy routes. Unlike ordinary airways, which change infrequently, ocean tracks change twice a day, so as to take advantage of favourable winds. Flights going with the jet stream may be an hour shorter than those going against it. Ocean tracks may start and finish about 100 miles offshore at named waypoints, to which a number of airways connect. Tracks across northern oceans are suitable for east–west or west–east flights, which constitute the bulk of the traffic in these areas.
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).
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 waypoints 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.
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:
Considerations.
Fuel calculation must take many factors into account.
Calculation.
The weight of fuel forms a significant part of the total weight of an aircraft, so any fuel calculation must take into account the weight of any fuel not yet burned. Instead of trying to predict the fuel load not yet burned, a flight planning system can handle this situation by working backward along the route, starting at the alternate, going back to the destination, and then going back waypoint by waypoint to the origin.
A more detailed outline of the calculation follows. Several (possibly many) iterations are usually required, either to calculate interdependent values such as reserve fuel and trip fuel, or to cope with situations where some physical constraint has been exceeded. In the latter case it is usually necessary to reduce the payload (less cargo or fewer passengers). Some flight planning systems use elaborate systems of approximate equations to simultaneously estimate all the changes required; this can greatly reduce the number of iterations needed.
An alternative approach to fuel calculation is to calculate alternate and holding fuel as above and obtain some estimate of the total trip fuel requirement, either based on previous experience with that route and aircraft type, or by using some approximate formula; neither method can take much account of weather. Calculation can then proceed forward along the route, waypoint by waypoint. On reaching the destination, the actual trip fuel can be compared with the estimated trip fuel, a better estimate made, and the calculation repeated as required.
Cost reduction.
Commercial airlines generally wish to keep the cost of a flight as low as possible. There are three main factors that contribute to the cost:
Different airlines have different views as to what constitutes a least-cost flight:
Basic improvements.
For any given route, a flight planning system can reduce cost by finding the most economical speed at any given altitude and by finding the best altitude(s) to use based on the predicted weather. Such local optimisation can be done on a waypoint-by-waypoint basis.
Commercial airlines do not want an aircraft to change altitude too often (among other things, it may make it more difficult for the cabin crew to serve meals), so they often specify some minimum time between optimisation-related flight level changes. To cope with such requirements, a flight planning system must be capable of non-local altitude optimisation by simultaneously taking a number of waypoints into account, along with the fuel costs for any short climbs that may be required.
When there is more than one possible route between the origin and destination airports, the task facing a flight planning system becomes more complicated, since it must now consider many routes in order to find the best available route. Many situations have tens or even hundreds of possible routes, and there are some situations with over 25,000 possible routes (e.g., London to New York with free-flight below the track system). The amount of calculation required to produce an accurate flight plan is so substantial that it is not feasible to examine every possible route in detail. A flight planning system must have some fast way of cutting the number of possibilities down to a manageable number before undertaking a detailed analysis.
Reserve reduction.
From an accountant's viewpoint, the provision of reserve fuel costs money (the fuel needed to carry the hopefully unused reserve fuel). Techniques known variously as reclear, redispatch, or "decision point procedure" have been developed, which can greatly reduce the amount of reserve fuel needed while still maintaining all required safety standards. These techniques are based on having some specified intermediate airport to which the flight can divert if necessary; in practice such diversions are rare. The use of such techniques can save several tons of fuel on long flights, or it can increase the payload carried by a similar amount.
A reclear flight plan has two destinations. The "final destination" airport is where the flight is really going to, while the "initial destination" airport is where the flight will divert to if more fuel is used than expected during the early part of the flight. The waypoint at which the decision is made as to which destination to go to is called the "reclear fix" or "decision point". On reaching this waypoint, the flight crew make a comparison between actual and predicted fuel burn and check how much reserve fuel is available. If there is sufficient reserve fuel, then the flight can continue to the final destination airport; otherwise the aircraft must divert to the initial destination airport.
The initial destination is positioned so that less reserve fuel is needed for a flight from the origin to the initial destination than for a flight from the origin to the final destination. Under normal circumstances, little if any of the reserve fuel is actually used, so when the aircraft reaches the reclear fix it still has (almost) all the original reserve fuel on board, which is enough to cover the flight from the reclear fix to the final destination.
The idea of reclear flights was first published in "Boeing Airliner" (1977) by Boeing engineers David Arthur and Gary Rose. The original paper contains a lot of magic numbers relating to the optimum position of the reclear fix and so on. These numbers apply only to the specific type of aircraft considered, for a specific reserve percentage, and take no account of the effect of weather. The fuel savings due to reclear depend on three factors:
Filing suboptimal plans.
Despite all the effort taken to optimise flight plans, there are certain circumstances in which it is advantageous to file suboptimal plans. In busy airspace with a number of competing aircraft, the optimum routes and preferred altitudes may be oversubscribed. This problem can be worse in busy periods, such as when everyone wants to arrive at an airport as soon as it opens for the day. If all the aircraft file optimal flight plans then to avoid overloading, air traffic control may refuse permission for some of the flight plans or delay the allocated takeoff slots. To avoid this a suboptimal flight plan can be filed, asking for an inefficiently low altitude or a longer, less congested route.
Once airborne, part of the pilot's job is to fly as efficiently as possible so he/she might then try to convince air traffic control to allow them to fly closer to the optimum route. This might involve requesting a higher flight level than in the plan or asking for a more direct routing. If the controller does not immediately agree, it may be possible to re-request occasionally until they relent. Alternatively, if there has been any bad weather reported in the area, a pilot might request a climb or turn to avoid weather.
Even if the pilot does not manage to revert to the optimal route, the benefits of being allowed to fly may well outweigh the cost of the suboptimal route.
VFR flights.
Although VFR flights often do not require filing a flight plan, a certain amount of flight planning remains necessary. The captain has to make sure that there will be enough fuel on board for the trip and sufficient reserve fuel for unforeseen circumstances. Weight and centre of gravity must remain within their limits during the whole flight. The captain must prepare an alternate flight plan for when landing at the original destination is not possible.
In Canada, however, the regulations state that "... no pilot-in-command shall operate an aircraft in VFR flight unless a VFR flight plan or a VFR flight itinerary has been filed, except where the flight is conducted within 25 NM of the departure aerodrome."
Additional features.
Over and above the various cost-reduction measures mentioned above, flight planning systems may offer extra features to help attract and retain customers:
See also.
Flight planning providers