THE former chief executive of the Provisional Airport Authority, Mr Richard Allen, has apparently left the territory for Denver in the United States. Plain white cards were mailed to friends, marked ''Change of Address'', followed by his name and a Denver, Colorado, address, then telephone and fax numbers. A new Denver international airport is nearing completion, to replace the present Stapleton International Airport. The former PAA chief, replaced in a surprise move last week, could be seeking involvement in the airport project, described as similar in its controversial nature to the Chek Lap Kok project. Denver is a major hub airport for the western United States, and the decision whether to improve Stapleton Airport or build the new one had been under discussion for a number of years. Greiner International, a leading US firm of consulting engineers, is one of the many companies working on the Denver project. Greiner worked closely with Mr Allen during development of the Chek Lap Kok airport master plan.