Harry Hopkins/LONDON
An unplanned-for side-effect of the fitting of the traffic-alert and collision- avoidance system (TCAS) to airliners - compulsory already in the USA, and shortly to be so in Europe - is that pilots can have a much greater awareness of the positions of other aircraft around them, even when those aircraft are not on potential collision courses. That enhanced situational awareness may now be exploited in a new approach to air-traffic management (ATM) in which pilots could take a much greater responsibility for plotting their own routes through crowded airspace. This new approach, known as the airborne separation-assistance system (ASAS), will substantially affect the roles of pilots and controllers.
The existing TCAS equipment has a feature which was not compulsory in the basic airborne-collision-avoidance-system (ACAS) specification set out by the International Civil Aviation Organisation (ICAO) - a plan-view display of relevant aircraft which are equipped with secondary-radar responders.
The TCAS traffic display shows all transponder-equipped non-threat traffic which is within ±8,700ft (2,600m) altitude vertically and within the range of the TCAS. Proximity traffic is defined as being within 1,200ft vertically and 11km (6nm) horizontally. Traffic Advisories identify those aircraft which are on potentially hazardous routes, and verbal and visual Resolution Advisories (RAs) are triggered where necessary.
Overall picture
This display, which additionally presents the crew with relative-altitude read-outs and vertical direction, gives pilots an overall picture of traffic in their immediate vicinity. Although such displays are advisory or supplementary in nature, they have incidentally enhanced the situational awareness which is so heavily promoted in human-factors terms.
Pilots could hardly be expected to ignore such useful information. Ken Carpenter, of the Air Traffic Control Systems Group of the UK Defence Research Agency at Malvern, Worcestershire, says: "It undermines the collision-avoidance role of the TCAS - where pilots seek to or actually take tactical decisions based on their TCAS displays - but a concept arose in response to this tendency, one which has the potential to increase airspace capacity. ICAO postulated the ASAS as a system designed to support tactical responsibility for separation on the part of the pilot."
Carpenter sees the ASAS as one response to the still-vague concept of free flight. "It is based on a more managed delegation of specific decisions to the pilot. It involves him taking on more responsibility for ensuring that his aircraft maintains separation from other aircraft - but pilots will need the technical and procedural basis for exercising this responsibility. The ASAS is not simply a piece of equipment, but more a way of life."
He says that the issue is whether free flight is meant to materialise as pilots simply doing their own thing, when and where they choose, or as a more managed delegation of specific decisions to the pilot, to improve traffic throughput in particular circumstances.
Put simply, the proposed ASAS can be regarded as a tool for flight under electronic flight rules. Pilots have a responsibility for keeping an effective look-out in visual meteorological conditions (VMC) - when they can see. The ASAS implies a similar responsibility, if they have the proper display, in instrument meteorological conditions (IMC) - when they cannot see.
The essential new tool proposed for the flight-deck is a cockpit display of traffic information (CDTI). The equipment producing the display would be kept independent of the safety-critical functional role of the ACAS, although it could well share some features. The display would have a maximum range in excess of five times that associated with TCAS.
Cultural change
Pilot and airline representative bodies have already performed a significant amount of work on the CDTI concept, with which goes a cultural change for pilots and controllers, who do not necessarily have the same conceptions of the air-traffic situation .
The basic idea is that, when conditions permit in a suitable ATC environment, pilots would be responsible for monitoring nearby traffic and maintaining their cleared separation from other aircraft. This role bears some similarity to the responsibility for circuit separation and approach positioning when adopting visual flight rules (VFR) within an airport zone, so it is not entirely novel. To date, two phases for the application of the concept have been identified:
The basic concept has already been tried in IMC, during the in-trail climbs operated by the US Federal Aviation Administration beyond the West Coast of the USA and in the Oakland and Anchorage flight-information regions. In these trials, aircraft have been guided through the flight levels of preceding aircraft by the use of the secondary-transponder returns shown on cockpit TCAS traffic displays, with identification effected by the transponder of the aircraft in front being switched off and on. Separation has remained the responsibility of the controller, however.
Broadcast
Work continuing at Malvern, at NASA Langley and at other centres, supports the role of ICAO's secondary-surveillance radar (SSR) improvements and collision-avoidance systems (SICAS) panel, which is a focal point for ASAS developments. This panel foresees the CDTI receiving the necessary data on aircraft position and status not by interrogation, but by broadcast. This relies on the Automatic Dependent Surveillance - Broadcast (ADS-B) system, in which an aircraft's or vehicle's position, altitude, vector and other information are broadcast via datalink for use by other aircraft and vehicles and by ground sites. Separation assurance, whether provided by ground ATC or based on the ASAS, could draw on the high-accuracy navigation data thus made available.
There remains a need for collision avoidance to provide protection which is independent of ADS-B and the ASAS function. Collision avoidance is a last-resort safety protection. Its purpose is to prevent collision when the primary means of separation assurance has failed. Logically, it must be independent of the ASAS' primary means of separation assurance, since a risk of collision implies a failure in that area.
If the collision-avoidance function is to be based in any way on ADS-B data, it must be proved that the overall design provides sufficient integrity, reliability and availability of the common elements. It may require time to obtain sufficient operational experience, taking account also of any risk of deliberate or accidental jamming. So the simplest and most cost-effective way to the integrity required will probably be by the provision of independently measured data as the basis of collision avoidance.
Aircraft not equipped with SSR altitude reporting transponders, but fitted with satellite-navigation equipment, might be equipped to function with a base level of ADS-B. Traffic awareness would be enhanced if either or both aircraft were so fitted. Greater protection would be offered too by an ASAS which included a conflict-detection function.
The generation of conflict-avoidance manoeuvres with such a basic ASAS is more problematic because they would need to be co-ordinated and to be compatible with the ACAS. There will be the potential need to co-ordinate its recommendations with those of ATC.
Full collision avoidance, as a last-resort safety system, cannot be achieved without an independent means of validating the ADS-B data supporting the conflict detection - eg, active interrogation of transponders. The last-resort collision-avoidance protection would simply be missing, but the SICAS panel considers that this may well not be required by general aviation.
The ACAS cannot generate RAs against aircraft which do not have altitude-reporting SSR transponders operating. Any unplanned and uncoordinated reduction in the level of transponder equipment carried as a result of installing the ADS-B would increase the risk to aircraft which have the ACAS in operation.
Parallel runways
An example of an ASAS application which could provide significant operational benefits in airspace where there is radar coverage is the paired-approach concept.
This proposes surveillance, based on received ADS-B aircraft-position data, of dual-runway approaches to airports with closely spaced parallel runways - especially runways which, during low-ceiling and poor-visibility weather conditions, would otherwise have to revert to a single-runway approach capability because of ATC separation requirements.
In the final event, considering the range of potential applications, it is probable that the proposed ASAS will require surveillance performance levels in excess of those now provided by the ACAS. The requirements will include high-integrity data on aircraft position, intent and further operational data to be exchanged via an appropriate air-to-air datalink.
These data could be displayed either in new formats for surveillance-traffic presentation, suited to receive ADS-B aircraft-position data or, when showing extended Mode S data, by traffic displays shared with the existing ACAS.
The ASAS, says the SICAS panel, should not be considered merely as an "improved ACAS". The ADS-B system could, however, eventually achieve sufficient availability and integrity that ADS-B-equipped users may no longer have to carry transponders.
In regions where benefits can be shown for a co-operative ASAS, changes would be needed to realise its potential. These would have to be made progressively, within the controlled transition to a future air-navigation system. Where the ASAS is to be operated, all aircraft would have to be suitably equipped, procedures set in place to allow for equipment failure, and the effectiveness of failure and mode indications assessed. ATM providers will need to deal with users with mixed-equipment levels in these airspaces for the transition period.
The SICAS panel stresses that the ASAS, in its proposed pilot-operated surveillance role, will be a safety-critical system. Its performance and integrity, as a full-time system, will have to be even greater than those demanded of the ACAS. Identifying the type of airspace in which such a system can be applied, and setting the appropriate separation minima, are the next challenges for this promising concept.
Source: Flight International
