Cathay Pacific Airways Ltd. will begin flying domestic routes in the Philippines today to fill a vacuum left by the closure of flag carrier Philippine Airlines. Cathay Chief Executive David Turnbull also said the Hong Kong-based company might be interested in buying a stake in PAL if it is put up for sale. In talks Sunday at the home of Philippine President Joseph Estrada, Turnbull agreed that two Cathay planes and crews will be leased to PNB Holdings, a unit of Philippine National Bank. The flights will nominally be operated by PNB since Philippine law prohibits foreign carriers from servicing domestic routes. PAL shut down Wednesday after a union representing ground personnel refused to accept a management plan in which workers would receive 20% of the debt-burdened airline's stock in exchange for a 10-year suspension of their collective bargaining agreement.