Flight Planning - Overview and Basic Terminology

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 altitude 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:

• USA 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 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. 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.

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 (970 km) 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 usable fuel.
• Zero fuel weight is the sum of operating weight empty and payload, i.e. 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 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 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 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, unusable fuel 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).

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.