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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 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 height than optimum due to congestion, or some last-minute passengers whose weight was not allowed 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 USA 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 from there 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. It is often considered a good idea to have the alternate some distance away from the destination (e.g. 100 miles) so that bad weather is unlikely to close both the destination and the alternate; distances up to 600 miles are not unknown. In some cases the destination airport may be so remote (e.g. 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. 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 fuel. Zero fuel weight is the sum of operating weight empty and payload, i.e. the laden weight of an aircraft, excluding any 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 take-off. This is the ramp weight minus any fuel used for taxiing. Major airports may have runways which are about two miles (3 km) 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 take-off clearance, the pilot revs up the engines and releases the brakes to start accelerating along the runway in preparation for taking off. Take off weight is the weight of an aircraft as it takes off part way along a runway. Few flight planning systems calculate the actual take-off 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 burnt. It includes the zero fuel weight and all alternate, holding, and reserve fuel. When twin-engine aircraft are flying across oceans, deserts, etc. 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 Twin-engine Operational Performance Standards). 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 for how long such an aircraft may fly with only one engine operating (typically from 1 to 3 hours). Units of measurement Flight plans use an unusual mixture of metric and non-metric units of measurement. The particular units used may vary by aircraft, by airline, and by location (e.g. different height units may be used at different points during a single flight). Distances are always measured in nautical miles, as calculated at a height of 32,000 feet, with due allowance 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 which are shown on a flight plan. Flight planning systems may need to use the unrounded values in their internal calculations for improved accuracy. There are a variety of ways in which fuel can be measured, depending mainly 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 must be taken into account when checking tank capacity. Specific gravity may vary depending on the location and the supplier. 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 airport 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. The actual height of an aircraft is based on 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 100 feet, i.e. FL320 is nominally 32,000 feet. Vertical separation between aircraft is either 1000 or 2000 feet. In China and some neighbouring areas, height is handled using metres. Vertical separation between aircraft is either 300 metres or 600 metres (about 1.6% less than 1000 or 2000 feet). Up until 1999, the vertical separation between aircraft flying on the same airway was 2000 feet. Since then there has been a phased introduction around the world of Reduced Vertical Separation Minimum (RVSM). This cuts the vertical separation to 1000 feet between about 29,000 feet and 41,000 feet (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. Aircraft with propellers normally use knots as the primary speed unit, while aircraft powered by jet engines 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 820 means that the aircraft is travelling at 0.820 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. If GPS is not used, the following steps are required to obtain speed information: An airspeed indicator is used to measure indicated airspeed (IAS) in knots. IAS is converted to calibrated airspeed (CAS) using an aircraft-specific correction table. CAS is converted to equivalent airspeed (EAS) by allowing for compressibility effects. EAS is converted to true airspeed (TAS) by allowing for density altitude, i.e. height and temperature. TAS is converted to ground speed by allowing for any head or tail wind. 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 often do a circular or out-and-back trip and land at the same airport from which they took off. Components Worldwide, there are a large number of named official airways, along which aircraft fly 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 heights to avoid collisions. Charts showing airways are published by various suppliers and are usually updated once a month coinciding with the AIRAC cycle. AIRAC (Aeronautical Information Regulation and Control) occurs every fourth Thursday when every country publishes their changes, which are usually to airways. Each airway starts and finishes at a waypoint, and may contain some intermediate waypoints as well. 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, i.e. the pilot (or the onboard flight management system) reports the aircraft 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 any favourable winds. Flights going with the jet stream may be an hour shorter than those going against it. Ocean tracks often start and finish perhaps a hundred 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 can't 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 east-bound or west-bound flights and 5 degrees of latitude for north-bound or south-bound 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 do quite a lot of analysis in order 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. The air temperature affects the efficiency/fuel consumption of aircraft engines. The wind may provide a head or tail wind component which in turn will increase or decrease the fuel consumption by increasing or decreasing the air distance to be flown. By agreement with the International Civil Aviation Organization, there are two national weather centres (in U.S.A. and U.K.) which provide worldwide weather forecasts for civil aviation in a format known as GRIB weather. These forecasts are generally issued every 6 hours, and cover the next 36 hours at intervals of 6 hours. Each 6-hour forecast covers the whole world using gridpoints located at intervals of 75 miles or less. At each grid point the weather (wind speed, wind direction, air temperature) is supplied at 9 different heights ranging from about 4,500 feet up to about 55,000 feet. Aircraft seldom fly exactly through weather gridpoints or at the exact heights at which weather predictions are available, so some form of horizontal and vertical interpolation is generally needed. For 75-mile intervals, linear interpolation is satisfactory. GRIB format superseded the earlier ADF format in 1998/9. The ADF format used 300-mile intervals; this interval was large enough to miss some storms completely, so calculations using ADF predicted weather were often not as accurate as those which can be produced using GRIB weather. The particular route to be flown determines the ground distance to cover, while winds on that route determine the air distance to be flown. Each inter-waypoint portion of an airway may have different rules as to which flight levels may be used. Total aircraft weight at any point determines the highest flight level which can be used. Cruising at a higher flight level generally requires less fuel than at a lower flight level, but extra climb fuel may be needed to get up to the higher flight level (it is this extra climb fuel and the different fuel consumption rate which cause discontinuities). Almost all the weights mentioned above in 'Overview and basic terminology' may be subject to minimum and/or maximum values. Due to stress on the wheels and undercarriage when landing, the maximum safe landing weight may be considerably less than the maximum safe brake-release weight. In such cases, an aircraft which encounters some emergency and has to land straight after taking off may have to circle for a while to use up fuel, or else jettison some fuel, or else land immediately and risk having the undercarriage collapse. Also, the fuel tanks have some maximum capacity. On some occasions, commercial flight planning systems find that an impossible flight plan has been requested. The aircraft can't possibly reach the intended destination, even with no cargo or passengers, since the fuel tanks are just not big enough to hold the amount of fuel needed; it would appear that some airlines are over-optimistic at times, perhaps hoping for a (very) strong tailwind. The rate of fuel consumption for aircraft engines depends on: air temperature, height as measured by air pressure, aircraft weight, aircraft speed relative to the air, and any increased consumption as compared with brand-new engines due to engine age and/or poor maintenance (an airline can estimate this degradation by comparing actual and predicted fuel burn). Note that a large aircraft such as a jumbo jet may burn up to 80 tons of fuel on a 10 hour flight, so there is a substantial weight change during the flight. | |||||||||
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