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Aviation History
1987
1987 - 0097.PDF
The sardine syndrome Do airliners already fly close enough for comfort at high altitude, or is there room to reduce vertical separation above flight level 290? Harry Hopkins reports. I naccurate altimetry or poor altitude holding has never caused a collision between two airliners flying at adja cent flight levels, although collisions have resulted from blunders in altitudes flown and through mistaken climbs and descents. The number of civil aircraft has increased markedly, however, since the present system of altitude intervals was introduced in the early 1960s, with con comitant pressures for greater numbers of economic cruising levels. In turn, this has raised questions about separation at high altitude, and the safety implications of any reductions, because, in the rarefied atmosphere at high levels, any barometric inaccuracies translate into larger altitude errors than lower down. At present, aircraft are separated by 1,000ft between opposite-direction traffic up to 29,000ft, and by 2,000ft at all alti tudes above that. Above a safe height over terrain, all aircraft altimeters are refer enced to the standard average sea-level atmospheric pressure of 1,013-2mb, and altitudes in hundreds of feet are called "flight levels" (29,000ft = FL290). Between mid-July and the end of September, 1963, lata undertook a ten- airline study of the correlation between barometric and radar altimeter data at each 10° of longitude on Atlantic cross ings. Comparisons were made between aircraft within an hour, half a degree of latitude, and 2,000ft of each other—all above flight level 290. The airlines thought this showed the existing vertical- separation standards to be unnecessarily large, but the matter rested there. It is 33 years since the Icao vertical separation panel was set up, but since 1974 another Icao panel, known as the "review of the general concept of separa tion" (RGCS), has taken over the exami nation of possible change. It was vital to determine the accuracy of altitude holding so that any proposals could be based on sound data. The RGCS is now near to completing its examination of the data. Having met this January, its final report is due to be published in spring 1988, for adoption by Icao that autumn. Various proposals have been made over the years: that the 1,000ft interval should be extended to higher flight levels; that an intermediate value (say 1,500ft) should be used above FL290; that pressure intervals be used instead of altitude figures (using a dedicated level indicator for the cruise, with separate optimally positioned static ports). In contrast, some parties have said that the 2,000ft interval should be extended downwards. The first alternative is seen as the only immediately practicable answer, but a pressure-interval solution has not been completely discarded for the long term. The study of altitude-keeping accuracy had its precedent in a survey of lateral navigation accuracy, made before the reduction of lateral separation standards over the North Atlantic, and the develop ment of minimum navigational perform ance specifications for aircraft flying in the area. Near-miss reports have been criticised as data sources because they are subjective (which in turn highlights the paucity of near-miss processing and analysis on a worldwide basis), while photographic and radar monitoring of civil-aircraft altitude has been limited. In 1964 the FAA published a study made on 15 airliners, using triangulation and a "pacer" aircraft fitted with a trailing-cone static-pressure source, but the mass traffic surveys made recently used precision radar. In Europe this was conducted by Eurocontrol at Aberporth, Bretigny, and Hanover, with more than 10,000 samples taken. In Japan, the Civil Aeronautics Board took 9,000 samples at two locations, while the US FAA conducted long-term surveys from six different locations with portable radar. Earlier data had been provided by Canada and the USSR. Precision tracking radars monitored geographical cruise altitudes over a number of routes. To determine the equiv alent barometric flight level under exist ing pressure conditions, sounding aircraft and balloon sondes were used to supple ment weather forecasts. The radars were sited near to ATC routes, so that individual aircraft could be tracked for long periods, typically 5min, although it was sometimes difficult to ensure a good location because of random direct routeings. The Japanese used mobile maritime radars with their scan ning axes laid horizontally along the airway. (As precision radar is too expensive for full-time monitoring or control, a marginally less accurate but cheaper device for later long-term monitoring could be important.) Data collection has finished, and final data bases are now being prepared. The initial assessment looks promising. Results obtained from computer analy sis of data has been set against a numeri cal model of the risk which incorporates measures of traffic frequency, proximity, and aircraft size. Cases considered have FLIGHT INTERNATIONAL, 11 April 1987
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