Published: April 1998

One potential solution is a concept called Future Air Navigation System, or FANS. FANS offers a space-based method for handling increased air traffic, allowing operators to obtain maximum revenue from their operations while ensuring safe conditions for their passengers.

The air transport industry has developed a new concept for air traffic management that involves significant changes to airplanes, infrastructure, and ground systems. Known as Future Air Navigation System, this system is becoming increasingly attractive as an option for coping more efficiently with current traffic levels, as well as with the increased traffic levels anticipated in the future.

The current air traffic management system is based on ground navigational aids, radar, and voice communications, and will eventually be unable to cope with predicted air traffic growth. In response, Boeing has been working with the industry since 1983 to create FANS, which relies on space-based navigation and communication. ("What is FANS?", describes the elements of FANS and the changes required for its implementation.)

What is FANS? The concept of Future Air Navigation System (FANS) is based on using satellite technology to manage air traffic. The space-based FANS would be less expensive and less dependent on ground infrastructure than the current air traffic management system. Using FANS, flight crews and air traffic controllers would communicate through data links based on satellite-based networks and a global positioning system. Airplanes would also send data link position reports using satellite communication networks. Several changes would be necessary to implement FANS, including changes in the operational concept used by states, airspace operational procedures used to coordinate how traffic controllers and flight crews communicate, and ground and airplane equipment.

Operator benefits offered by FANS include reduced fuel burn and flight time through direct routing, and increased payload capability for takeoff-weight-limited flights. If FANS were implemented, operators would be able to take advantage of several needed improvements:

1. Reduced separation between airplanes.
2. More efficient route changes.
3. Satellite communication.
4. No altitude loss when crossing tracks.
5. More direct routings.

1. Reduced Separation Between Airplanes

In non-FANS procedural airplane separation, errors in navigation and potential errors in voice communication between the flight crew and air traffic controller are considered when determining the necessary airspace separation between airplanes. The uncertainties of traditional voice position reporting and the delay associated with high-frequency relayed voice communications (20 to 45 min to make a high-frequency voice position report) require the air traffic controller to allow a tremendous amount of airspace between each airplane, typically 100 nm laterally and 120 nm longitudinally. This computes to 48,000 mi2 of airspace to protect one airplane, and means that airplanes often operate at less-than-optimal altitudes and speeds.

However, through a satellite data link, airplanes equipped with FANS can transmit automatic dependent surveillance reports with actual position and intent information at least every five minutes. The position is based on the highly accurate Global Positioning System (GPS).

Digital data communication between the flight crew and the air traffic controller drastically reduces the possibility of error, and allows greatly reduced airplane separations. The combination of improvements in communication, navigation, and surveillance allows authorities to reduce required separation distances between airplanes, which in turn allows airplanes to fly at their optimum altitude and burn less fuel.

2. More Efficient Route Changes

Oceanic operations currently are based on weather data that are 12 to 18 hr old. By using the satellite data link that is part of FANS, however, the latest weather data can be transmitted to an airplane while it is en route. Flight crews can then use these data to develop optimised flight plans, or those plans can be generated on the ground and transmitted to the airplane. Such dynamic re-routing may allow airlines to consider reducing discretionary fuel, which further reduces fuel burn or allows an increase in payload.

3. Satellite Communication

Satellite communication can reduce to a few minutes the response time for an airplane requesting a step climb to a new, optimum altitude to reduce fuel burn. Response time is currently 20 to 60 min.

4. No Altitude Loss When Crossing Tracks

To avoid potential conflict, an airplane that is approaching crossing tracks must be separated by altitude from any traffic on another track. As a result, one of the two airplanes can be forced to operate as much as 4,000 ft below optimum altitude.

But if the air traffic controller has timely surveillance data, including projected intent, and the airplane is able to control its speed so that it reaches the crossing point at a given time, altitude separation would be required less frequently.

5. More Direct Routings

In many cases, current air traffic routings are compromised to take advantage of existing navigation aids and radar coverage, resulting in less-than-optimum routings. Taking advantage of space-based navigation and communication would allow more direct (shorter) routes.

With FANS in place, operators could benefit from reduced fuel burn and flight time as well as increased payload capacity for takeoff weight-limited flights. As a result, costs associated with crew and engine maintenance could be reduced, allowing operators to apply the money saved toward implementing and operating new routes.

Airplanes must be equipped for several functions to support implementation of FANS (see "Status of FANS 1" for airplanes equipped for these functions):

Status of FANS 1 Approximately 15 airlines have purchased 350 ship-sets of FANS 1 upgrades for the 747-400. The 777 includes FANS 1 as a basic feature, and as of press time, Boeing anticipated certification of its 757, 767, and MD-90 FANS 1 airplanes during first-quarter 1998. FANS 1 will be an option on the 717. Certification dates are being determined for FANS programs for the MD-11 and MD-10F (a two-crew modification to the DC-10 freighter under development for Federal Express).

• Airline operational control (AOC) data link.
• Automatic dependent surveillance (ADS).
• Air traffic control (ATC) data link.
• Global Positioning System (GPS) integration.
• Required navigational performance (RNP).
• Required time of arrival (RTA).

The elements of the Current Air Traffic Management System operating in most of the world differ significantly from the potential air traffic management system based on the concept of FANS ("What is FANS?").

Current Air Traffic Management System The current air traffic system contains communication, navigation, and surveillance elements. Communication is through voice contact with an air traffic controller using high-frequency or very-high-frequency radio. Typically, high-frequency contact is handled through a third party who transcribes the messages on teletype and sends it to the controller. Over the continental United States and Europe, navigation is handled through ground-based navigational beacons. In oceanic airspace and airspace over less-developed countries, inertial-based navigators are used. In airspace over developed countries, surveillance is conducted using ground-based radar; in oceanic airspace or in airspace over less-developed countries, surveillance uses high frequency voice reports.

Figure 1 shows existing and potential sites of FANS installation.

Airline Operational Control Data Link.
The AOC link gives airline data systems the ability to transmit new routes, position reports, and updated winds through the data link network.

Automatic Dependent Surveillance.
The ADS function reports the current flight position via satellite or VHF data link to the air traffic controller or to the airline. This improves the surveillance of en route airplanes.

Air Traffic Control (ATC) Data Link.
This function replaces the tactical communication between the flight crew and air traffic controller, allowing the flight crew to request deviations to, or replacements of, the filed flight plan. The air traffic controller also has the ability to directly request tactical changes to the airplane flight plan.

Global Positioning System.
This improvement provides a more accurate position for en route operations and some approach operations. The navigation system must demonstrate that it can meet the required navigational performance criteria.

Required Navigational Performance.
RNP criteria address accuracy, integrity and availability as set forth in FANS. The actual navigation performance is constantly monitored; if it exceeds the required navigational performance, the flight crew is alerted so that they can compensate for a situation in which they have less accurate information than the route requires.

Required Time Of Arrival.
This gives the flight crew the ability to assign a time constraint to a way point, allowing the airplane to cross a latitude or longitude at a specified time. The cruise speed is automatically adjusted to achieve that time, plus or minus 30 seconds. If the RTA is not possible, the flight crew is notified with a visual alert.

Summary FANS represents a potential solution to the growing need for an air navigation system with greater capability. If all elements of the system were implemented, operators could expect such benefits as reduced fuel burn and flight time as well as increased payload and cargo. Possible flight operations improvements resulting from FANS include reduced space between airplanes, more efficient route changes based on updated wind models, satellite communication, no altitude loss when crossing tracks, and more direct routings.

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