Air France has agreed to purchase 16 Boeing 747-400 long-haul airplanes with the option to buy a dozen more. The contract, worth $2 billion, completed a deal agreed on last January. If the dozen other planes are eventually bought, the contract would be worth $3.5 billion to Seattle-based Boeing. The planes, capable of flying 8,000 miles without refueling, are to be delivered between 1991 and 1996, Air France said. Unlike the current 747, which is designed to be piloted by a three-person crew, the 747-400 is designed to be piloted by two people.