A BOYCOTT by international engineering consultants of design contracts for the Provisional Airport Authority (PAA) has ended. But Legislative Councillor Mr Albert Chan Wai-yip yesterday demanded that the PAA explain how the issue was resolved. ''It is important for us to know whether the PAA has made any concessions which will lessen the safeguard against cost overrun,'' Mr Chan said. Next Friday is the new deadline for tenders for the design of temporary and permanent utilities, storm water and waste water sewerage and irrigation systems for the new airport at Chek Lap Kok. The date for receipt of tenders in relation to the design of airfield tunnels, pavement design, and ground and apron lighting will be March 15. In an unprecedented move, the world's leading consulting engineers earlier refused to bid for any of the contracts in protest at conditions they claimed were ''onerous''. The boycott was said to be one of the reasons for the sudden departure of former PAA chief executive Mr Richard Allen. A PAA statement yesterday said: ''Firms short-listed have been assured that concerns raised by them will be amicably resolved once the tenders have been received.'' PAA's project director Mr Douglas Oakervee said the delay would not have an effect on the budget or completion date for the airport.