THE Civil Aviation Department is preparing for more night flights at Hong Kong's congested Kai Tak airport this winter as demand from airlines threatens to outstrip the airport's capacity. The possibility of more flights, perhaps breaking the midnight to 6.30 am curfew, has raised concerns about increased noise nuisance. But the International Air Transport Association (IATA), which has asked for night-flight restrictions to be relaxed, and the tourism industry have warned that Hong Kong will suffer economically if more flights are not allowed. The airport is at full capacity for all but a few hours between 6.30 am and 9 pm, with planes taking off and landing every two minutes. Flights are cut by about a third between 9 pm and midnight, and none are scheduled during the curfew, although aircraft are allowed to land and take-off in emergencies. The restrictions are designed to protect Kowloon City and Eastern District residents who live below the airport's two approaches. A department spokesman said IATA had made an ''informal'' request last month for more night flights, but said the department would not make a move until IATA provided figures to support the request. The South China Morning Post has learnt that the Environmental Protection Department has already begun work on drafting guidelines that would allow more night flights without imposing further disruption to residents. ''One of the possibilities is to allow a larger number of quiet, new aircraft to use Kai Tak [after 9 pm] and restricting all the noisier type of aircraft,'' said environmental protection officer Andrew Cheung Sau-cheong. Speaking from Denmark, IATA aviation consultant Hans Fugl-Svendsen said the final figures on winter demand would be supplied to the department from IATA's Bangkok office. He said Kai Tak was at full capacity now and warned that without more night flights Hong Kong risked losing valuable business, perhaps even jeopardising the potential of the new Chek Lap Kok airport. Mr Fugl-Svendsen said that with Macau's new airport opening in 1995 (two years ahead of Chek Lap Kok) and international flights into Shenzhen airport, airlines could be forced to bypass Hong Kong. ''They may find that by the time the new airport opens they are doing okay at Shenzhen and Macau,'' he said. With more direct flights opening up between Asia and Europe through Chinese and Russian air space, airlines wanted more late departures from Hong Kong to put their passengers into European ports at ''sociable'' times. Economic Services secretary Gordon Siu Kwing-chue denied the airport was at full capacity - flights were still available between 6.30 am and 8 am - but agreed the problem needed addressing. ''Some of the flights that leave for Europe arrive at 4.30 am, so you can imagine the problem,'' he said. ''As a general rule I would prefer as large a number of flights in and out as we can provide [so long as safety and noise issues were addressed].'' The department said that if IATA ''justified'' its request, it would consult district boards to see what relaxation of restrictions would be acceptable, but one option would be to increase flights between 9 pm and midnight. Flights between 9 pm and midnight were restricted to 18 flights per hour, compared with 28 per hour during the day. He said noise pollution went ''right to the heart of the people'' and ensured no changes would be made without first consulting residents through district boards. The chairman of the Legislative Council's airport financing sub-committee, Samuel Wong Ping-wai, said he would rather Kai Tak lost business than increase noise pollution. ''There is no such thing as a quiet aircraft,'' Mr Wong said. ''I think it [more night flights] should be discouraged, so I think 'so what?' if we lose some flights to nearby airports.'' The Hong Kong Tourist Association said visitors to Hong Kong were worth $50 billion annually to the territory's economy and that it backed more night flights. ''We believe that the point that must be stressed to anybody who is affected by noise pollution is that this is only a temporary measure [until Chek Lap Kok opens],'' association chairman Martin Barrow said.