LEGISLATORS are likely to approve today a government request for $283 million to continue Bechtel's airport project management contract for another two years, despite a 21 per cent increase in costs. The rise is because of higher salaries and allowances, even though the number of staff Bechtel and its partner, Halcrow, will employ will largely remain unchanged. Under the contract, which comes up for renewal at the end of this month, Bechtel, the US engineering giant, and British consultant Halcrow, are supporting government officials in the New Airport Co-ordination Office to ensure the 10 airport core projectsare finished on time and within budget. Bechtel's contract was last renewed in January, 1992, when $237 million was approved after a heated debate by legislators. ''In principle, we agree that Bechtel's contract should continue because construction has to go on and the project management of the airport core programme must carry on,'' said Liberal Party legislator, Steven Poon Kwok-lim. He added, though, the Liberal Party would seek assurances on four key points including the huge rise in costs. ''We also need to ask if there is any conflict between Bechtel and Dr Hank Townsend, a former Bechtel senior vice-president, who is now chief executive of the Provisional Airport Authority.'' Mr Poon said the Liberal Party also wanted to know how much was spent under Bechtel's present contract and whether the Government would get anything back. Support for the request was also promised by Frederick Fung Kin-kee, chairman of the Association for Democracy and People's Livelihood, who said his party would vote in favour, but demand why there was such a large increase in salaries and allowances. The deputy convener of the Legislative Council's airport group, Samuel Wong Ping-wai, said his group would support the Government's plan. But he added it could demand Bechtel's contract be kept at the present rates until the Government justified the increase. Defending the work done by Bechtel and Halcrow, James van Hoften, head of the Bechtel group at the airport office, said: ''It has been a very successful, tightly run programme.'' He said the extra cash was needed to pay higher salaries demanded by site supervisory staff.