THE air traffic system which has operated at Kai Tak airport for 20 years has been abandoned for safety reasons and replaced this morning by an arrangement that directs 113 more planes a week over the quarter of a million people living in North Kowloon. Urgent changes to early morning and late night flight paths will be gazetted today, suspending the use of the ''opposite runway'' system in which an aircraft takes off over the sea and climbs in a staggered pattern directly towards an incoming plane. When the planes passed there should have been at least 610 metres between them. The system, for flights scheduled between 6.30 am and 7 am and 9.30 pm and midnight each day, was introduced in the 1970s to cut down noise pollution, although it was often suspended because of wind direction or weather conditions. Under the new scheme, planes will overfly North Kowloon every night, whereas under the ''opposite runway'' this occurred only 40 per cent of the time when made necessary by wind conditions. A taskforce has been set up to identify ways of minimising the impact on Kowloon residents of the decision to abandon the old strategy. However, the limit of 18 flights per hour for the periods when the ''opposite runway'' had been in use, as well as the midnight curfew, will remain. ''Opposite runway'' operation was deemed ''a definite hazard to flight safety'' by British air traffic control inspectors. A team from the UK Civil Aviation Authority was called in by Hong Kong's Director of Civil Aviation, Peter Lok Kung-nam, after three ''losses of separation'' in the first six months of this year, compared to an annual rate of between none and two for each of the past 15 years. None of the near-misses this year occurred while the ''opposite runway'' was in use but the investigation team urged that the practice be scrapped immediately, even before its report was completed. The full list of recommendations on how to improve safety at Kai Tak will be presented to Mr Lok soon, and is expected to include changing the shift system of air traffic controllers to ease their workload. The alert was raised this summer after the third occasion in six months when planes came closer than the stated separation of 610 metres - about the length of four rugby pitches. The investigation into air traffic control procedures by the UK Civil Aviation Authority Air Traffic Control Inspectorate praised the competence of the air traffic controllers but found the system to be under increasing pressure. The inquiry team attributed this to a 141 per cent increase in annual aircraft movements at Kai Tak from 54,300 to 131,000 in the past decade; the large growth in air traffic to and over China, requiring more intense air traffic control co-ordination; a 130 per cent increase in overflights within the past five years; and the pressure on controllers from the ''opposite runway'' system. It is believed that the only other airport to operate an ''opposite runway'' is Los Angeles International, but there the planes can bank sharply after takeoff to follow a different flight path, whereas the terrain around Kai Tak forces pilots to take thesame route merely keeping a vertical space. A one-way system will now be used permanently, with incoming and departing planes both using runway one-three heading to Lei Yue Mun Gap in normal conditions, or runway three-one over the sea when made necessary by the wind or other weather conditions. Mr Lok said air traffic controllers ''are juggling too many balls now''. However, he said Kai Tak was still safe, and by suspending the ''opposite runway'' mode, he was confident that the chance of infringing the minimum separation between aircrafts would be lowered. ''Kai Tak is a difficult airport to get in, but because of that it is one of the safest in the world,'' he said. He revealed that the last infringement of the minimum separation during the ''opposite runway'' mode was in 1988. When asked why he did not decide to conduct an audit then but waited until this year, Mr Lok said the 1988 case was an isolated one and the Government did not feel that there was an unfavourable trend. ''You can see that four years after the 1988 case there has not been a single incident relating to the 'opposite runway' mode,'' he said. ''That has justified what we thought at that time was right.'' Mr Lok also said 83 per cent of aircraft using Kai Tak had quiet engines. ''We conducted two noise studies, in 1978 and 1990,'' he said. ''Aviation movements doubled in the 1990s, but noise levels decreased by 30 per cent. ''And there are increasingly more and more twin-engine aircraft, which climb steeply and therefore leave a much smaller noise footprint,'' he said. The Deputy Director of the Environmental Protection Department, Robert Law, said: ''We'll try to arrange all the landing flights to get in before 9 pm. If it could not be possible, we'll try to arrange the arriving flights to land as close to 9 pm as possible.'' The noise taskforce consists of representatives of the Economic Services Branch; Planning, Environment and Lands Branch; Civil Aviation Department and the Environmental Protection Department. The Deputy Secretary for Economic Services, Elizabeth Bosher, said the taskforce would try to persuade airlines to schedule as many in-coming flights as possible before 9 pm.