TRAVELLERS leaving from Kai Tak are going to have to be punctual or their planes will take off without them. The Hong Kong Board of Airline Representatives (BAR), which covers more than 55 airlines using Kai Tak, has agreed to close all check-in desks 40 minutes and boarding gates 10 minutes before departure. The clampdown, which comes into effect on January 1, is being targeted at the growing number of locals who arrive at the last minute and expect to be allowed on board. The bags of passengers who have checked in, but who fail to get out of the airport lounge and duty free shops 10 minutes before scheduled take-off, will be removed and their plane will leave without them. For security reasons, it is against local and international aviation rules for a plane to take off with a person's luggage unless the owner is on board. Cathay Pacific Airways was the first to adopt the get-tough policy in October and has since reported a significant improvement in on-time departures. This has been coupled with a marked decline in customer complaints about punctuality. Cathay Pacific's on-time departure rate slipped from 72 per cent to 69 per cent between 1990 and 1992. Since introducing the new system, it has surpassed the 80 per cent rate and work is continuing to improve this. Financial costs of delays to the airline - calculated by examining the costs of staff overtime, aircraft parking fees and the effect of dissatisfied customers - have been estimated at about $3,000 to $3,500 a minute. Linus Cheung Wing-lam, a Cathay Pacific director and present Hong Kong BAR chairman, said: ''Our aim is to provide the best service for everybody.'' Bill Hutchings, Kai Tak's general manager, welcomed the move while hitting out at inconsiderate local late-comers. ''Kai Tak is supposed to be a quiet airport. The only public announcements we allow are the change of gate number and to call the last few passengers. ''But it has got to a stage where the public address system is running non-stop. People don't listen to it.''