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Aviation History
1991
1991 - 1699.PDF
minimum of 5nm horizontal. Today, air craft flight management systems (FMS) real-time databanks can deliver continually updated oceanic position information via Satcoms to an ATC display at Oakland OCC, California; the position information transmitted is derived from existing on board INS or from Satnav systems. ADS data does not have to stop at position, time and FL. A controller's display could also show groundspeed, planned track with time and FL at the next two reporting points so that the controller could check ahead for traffic conflict and for incorrect entry of flightplan data into the FMS — the latter being one of the most common modern causes of oceanic naviga tional error. The problem is that, with the exception of INS — usually a triplicated self- monitoring system — all the other systems are, by aeronautical standards, new and untried in the air-transport environment. Failures could occur in many sectors of a complex, interdependent system: satellite systems (both communications and naviga tion satellites), antennae (aircraft and ground station), landline, aircraft data- processing systems (FMS), aircraft transmit ters and static (Doppler), avionics and interfaces at all points, including with the ATC computer, and finally the displays in the aircraft and at the OCC. Safety authorities at the International Civil Aviation Organisation (ICAO), the US Federal Aviation Administration (FAA), Eu- rocontrol and all interested national avia tion authorities have the duty to monitor the level of collision risk to ensure that it does not increase. An ICAO computerised collision-risk model (CRM), worked out for current circumstances, predicts that an oce anic area collision might occur about once in 150 years. The risk is influenced by two basic factors: firstly, sky "occupancy" — conges tion — and secondly, the simple number of navigation errors which occur each year. Since it is inevitable that the occupancy rate will increase, the number of navigation errors must be made to decrease or the collision risk will climb. Navigation errors are caused by human mistakes and equipment inaccuracy or fail ure. Therefore, if the number of errors is to be reduced, it is essential not only to provide technology which enables pinpoint navigation, but to prove its extreme reliabil ity before putting it into operation. The effects of pilot error can be reduced by a number of methods, but real-time ATC surveillance is clearly the most effective. The authorities have to debate whether TCAS should be allowed to be a factor in collision-risk calculations: ie, to affect the CRM. The issue is that, although it may reduce collision chances, it does not reduce "airmiss" or "near-miss" chances. One argu ment is that TCAS should be left as a safety-net system and not be included in the CRM. The other argument says that TCAS, even if it does not prevent airmisses, may reduce collision risk, which is the objective. Finally, before TCAS could be accepted as a factor in the CRM, its own reliability and effectiveness must have been proved beyond doubt in the radar environment. Today, it is still only just being introduced into service in the USA. Operationally also, its collision-avoidance "resolution advisory" instructions to pilots involve aircraft depart ing instantly and without notice from the ATC-cleared flight level until the risk has passed. The TCAS system ensures theoreti cally that a new collision risk will not be created, but controllers, pilots and authori ties will want to see this claim proved in action. TCAS SAFETY-NET Paul Wood, the senior UK Civil Aviation Authority representative on the North At lantic Systems Planning Group (NATSPG), does not believe that TCAS should be a factor in the CRM in the foreseeable future, but he believes that it should be a part of the plan. Wood and others maintain that TCAS range must be improved to make it useful in the high-Mach-number oceanic environment, but point out that longer range would increase the risk of interfer ence with DME bands. TCAS range and display graphics currently show less than 20nm, limiting its hoped-for extended usage in an environment where the anticipated halved lateral separation would be 30nm. At the other end of the professional- opinion scale is Capt Bill Cotton, United Airlines' manager air traffic systems. He is one of many TCAS-experienced pilots who believe that TCAS provides the ultimate redundancy to allow ADS-environment separation to reduce to domestic criteria in due course. Wood and Cotton disagree also on the rate at which ADS and subsequent separa tion reductions will be introduced, Wood again being the conservative. Both share the same enthusiasm for ADS, however, and have identical views on its ultimate pur poses. Wood sees its development as "evo lutionary", with full ADS oceanic operation allowing reduced separation by the year 2000. "Integrity standards have to be estab lished before separations come down," says Wood. "We haven't even yet decided on acceptable redundancy," he adds, referring to satellites, groundstations, landlines, air craft equipment and interface with the ATC computer. Wood says he sees the "evolving" ADS FLIGHT INTERNATIONAL 19 - 25 June, 1391 45
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