The debate about free-flight air navigation continues.

In the USA...


THE US FEDERAL Aviation Administration and the civil-aviation community "...stand at the threshold of a great opportunity to safely re-order the [nation's] air-traffic system". This statement supporting "free-flight" air navigation is taken from a recent Radio Technical Commission for Aeronautics (RTCA) task-force report recommending its phased introduction.

The report, delivered to FAA Administrator David Hinson in time for fiscal year 1996 budget plan, maps out an air-traffic-management (ATM) system which allows pilots and airlines to set their own flight paths. Air-traffic controllers would intervene only to prevent accidents.

The task force makes 45 recommendations which cover the near term up to 1997, the medium term (1998-2000) and the long term (2001 and beyond). The key proposal calls for Hinson to form a free-flight steering committee within six months to establish an implementation strategy and firm schedule.

The task force believes that free flight is an idea borne out of necessity, since the present ATM system is rapidly reaching saturation. Free flight does not, however, include a free ticket, and the FAA and airspace users will have to dig into their pockets to pay for the devices needed to make the idea a reality.

The US airline industry - the chief proponent of the concept - says that fuel savings alone could offset the investment in technology. American Airlines and the FAA are already working on a collaborative project known as "negotiated wind routes" which yields savings of $10-12 million a year. A NASA study estimates that annual cost savings could be more than $1 billion.

Free-flight would evolve ATM, from today's rigid and largely procedural analogue and ground-based system, to a flexible, collaborative, system using current and emerging technologies such as two-way data-links, satellite-based navigation, the automated dependent surveillance-broadcast (ADS-B) system, the traffic-alert and collision-avoidance system, and the aeronautical telecommunication network (ATN).

Reducing separation standards to improve the efficient use of airspace is a prime consideration. The RTCA's draft report says that "...improved surveillance technology is available today to provide more accurate position reports for use by controllers and decision-support systems in the task of separation assurance". It recommends that the FAA push development of the ADS-B, which provides current aircraft positions.


New displays, both in aircraft and at ATC stations, are needed to provide flight crews and controllers with accurate, real-time-situational awareness, the report notes. "There will be circumstances where separation assurance shifts to the cockpit," it adds.

Under the free-flight concept, which would come to fruition in the 21st century, the pilot, not the controller, would choose the route, speed and altitude. Free-flight is designed to accommodate all users, whether air carrier, air taxi, general aviation or the military. In the system, a flight plan would be available to the controllers to assist in flow management. It would no longer be the basis for aircraft separation.

The pilot's discretion would only be restricted to prevent unauthorised entry into special-use airspace and to ensure separation and safety of flight. Restrictions would, however, be imposed at busy airports and in congested airspace.

The free-flight concept is based on two airspace zones: protected and alert, the sizes of which are based on an aircraft's speed, performance characteristics and communications, navigation and surveillance equipment.

The smaller protected zone (the one surrounding an aircraft) can never meet the protected zone of another aircraft. The alert zone extends well beyond the protected zone and, upon contact with another aircraft's alert zone, a pilot or controller would determine if a course correction is required. In principle, until the alert zone is breached, aircraft can be manoeuvred freely.

Elements of the free-flight concept are part of the National Route Programme expansion which permits aircraft flying above 29,000ft (8,850m) to select their own routes as alternatives to published instrument-flight-rules routes. The FAA says that the programme saved the aviation industry about $40 million in 1994 by allowing pilots to fly more optimal routes.

Better conflict alert/resolution and data transmission/display systems are required if free flight is to work. Direct exchange among aircraft, airline-operations centres and controllers, are critical to a successful evolution to free-flight.

A precursor, to free-flight is the Future Air Navigation System (FANS). It provides accurate and direct pilot-to-controller information exchange over oceans and remote areas normally out of range of ground-based stations. The FAA and airlines are now running FANS-1 operational trials.

The near-term programme being recommended would involve no significant avionics changes, and relatively small investments in, or modifications of, buildings, equipment or procedures, either by the FAA or the user. Data-links and automated dependent surveillance systems which are affordable and which support reduced separation standards are critical, as are "decision-support systems" which assist the controller in the prediction and resolution of conflicts.

In Europe...

Julian Moxon/PARIS

THE EUROPEAN view of "free-flight" and the speed with which it might be introduced is substantially different to that of the USA. This is not least because Europe has not only the world's densest air-traffic environment, but also a highly developed ground-based infrastructure, which is already capable of performing many of the ATM functions required for at least the initial stages of FANS implementation.

Eurocontrol's development director for its European ATC Harmonisation and Integration Programme, Phil Escritt, points out that the US approach to the FANS is "necessarily different" to that of Europe, since Europe is concentrating on solving its capacity problems, whereas the USA is aiming to solve the problem posed by hundreds of small airports which are still without ground landing aids. Escritt adds, however, that " the long run, if you look in detail at what the USA is doing under the free-flight banner, it is very similar to what we're doing with the EATMS [European air-traffic-management system]".

Eurocontrol, the European Commission and the European Space Agency are working together to develop the global-navigation satellite system (GNSS), and the group, says Escritt, is maintaining "very close" contacts with the FAA to ensure compatibility.

"We're not being Luddite," says Escritt in response to criticism that Europe is dragging its feet over the FANS. "We recognise the future, but we must see that GNSS provides as good a service as the existing system before Europe gets rid of its ground-based ATC infrastructure," he adds.

Europe is proceeding "quietly, and without drama", getting on with the institutional arrangements, plugging the research and development gaps, and gathering the data needed for certification of new systems. "We're dealing with some fundamental issues that the USA hasn't even thought about," he says.

Eurocontrol's Roland Rawlings says that the EATMS "...will achieve initial operational capability around 2005". He foresees a ten-year transition to full EATMS, culminating in complete operational capability in 2015, which he says, "...will be characterised by the stepwise introduction of new features and benefits".

Rawlings is careful not to commit Europe to the free-flight concept so enthusiastically promoted by the USA, cautioning that it "...would demand significant airborne-system developments". Taking only the problem of aircraft separation in the crowded European airspace, he says that any significant developments "...are likely to demand high-integrity onboard monitoring and conflict-resolution capability demanding direct air-to-air data-links".

Experience with the problems of introducing the "...much more limited capability" aircraft collision-avoidance system indicates that "...the airborne tools enabling the realisation of free flight in Europe are not going to be achieved without significant development and substantial investments".


Rawlings confirms that the existing ground-based navigation infrastructure in Europe is sufficient to support near-term EATMS requirements. To ensure compatibility with the International Civil Aviation Organisation (ICAO) communications, navigation and surveillance/ATM timetable, Europe is ensuring that it will be able to move from ground-based navigation aids to satellite navigation. The initial augmented satellite-based system based around the GNSS "...will be part of the system architecture by 2005". Rawlings stresses, however, that the GNSS "...will not provide the integrity and availability for stand-alone input to area navigation", so that ground aids "...will continue to be employed for the period up to 2010-2015".

Satellite augmentation will be provided by The European Geostationary Navigation Overlay Service (EGNOS), and will employ two leased transponders aboard Inmarsat satellites covering Europe and other areas, which come under their footprint, such as Africa. The associated ground infrastructure is already funded, says Escritt, "...and we're talking to Africa and other nations about its possible use for them".

The EGNOS is comparable to the US wide-area augmentation system, says Escritt, "...except that it provides a European instead of a US monitoring service". The system will reach initial operational capability in 1999 (a year after the USA), with full operational capability in 2002. "By then, the system should be technically capable of achieving sole-means certification down to Category 1 precision approach over most, although probably not all, of the ECAC [European Civil Aviation Conference] area," he adds.

The debate over who controls the global-positioning system (GPS) and the GNSS is unresolved, says Rawlings, "...but should not hinder progress towards developing arrangements for complex systems such as EGNOS, where different states all provide components of the overall system". The deployment of several monitoring networks, he says, "...increases distribution and control of the global GNSS infrastructure".

Escritt notes that Eurocontrol is intent on developing the ATN high-speed communications datalink as soon as possible, and will meet the ICAO deadline for validation of the ATN-1 at the end of 1996 (clearing the way for its introduction). He is clear about the existing ARINC 622 system "...which is not designed for safety-critical operations". Escritt agrees that airlines are confused about the time scale for the arrival of ATN "...which is why we're now putting a huge effort into developing it".

He also points out that Europe does not have the same need for a local-area augmentation (GPS/GNSS) system, such as that being developed in the USA, since its ground-based infrastructure already provides extensive Category II and III landing-system capability.

Source: Flight International