Delta Air Lines yesterday unveiled a new computerized airline reservations system for travel agents that, unlike its competitors', is designed to favor no airline's flights, not even Delta's.

Delta's new offering--which it is billing as "the world's first totally unbiased" automated reservations system--is to be operational this fall. It comes at a time when increasing questions are being raised about the biased nature of the "res systems" used by most travel agents. The Justice Department's Antitrust Division began a preliminary investigation a month ago to determine if airlines with computerized scheduling systems have used them to harm competitors.

Its industrywide investigation was opened after the disclosure that a federal grand jury in Texas was looking into allegations that American Airlines may have used illegal tactics against Braniff International to help push it into bankruptcy. The charges, denied by American, included manipulation of Braniff reservations through American's Sabre system.

The major reservations systems other than Delta's have a built-in bias that gives preferential listing to the flights of the sponsoring airline and secondary preference to the flights of other airlines that pay extra to become "co-hosts" of the systems.

Delta's new system--called DATAS II--is different; Delta officials said it will show every direct flight available between two cities strictly by departure time, giving Delta no advantage. If two or more flights leave at the same time and have the same enroute time, the listing will be in alphabetical order. The same will be true of the listings for connecting flights and for worldwide flights, Delta said.

"This system is something the travel agents tell us they wanted, and we're willing to compete in an unbiased environment," said Whit Hawkins, Delta's vice president-marketing.

Hawkins said Delta is hoping the system will appeal especially to the small- and medium-sized travel agencies that haven't committed to a system yet, and others that also want an unbiased look at available flights. Travel industry sources say that about 80 percent of the travel agencies that are automated use either American's Sabre system or United Airlines' Apollo system; smaller systems are run by Trans World Airlines and Eastern Airlines. Hawkins estimated that about half the nation's agents are automated but others think the figure may be higher.

Delta said its system has flight information for 179 domestic and foreign airlines and the capacity to store schedules for 600.