Henry A. Duffy won't seek re-election as president of the Air Line Pilots Association.

ALPA's 40,000 members staff the flight decks of most major American-flag carriers except those at American Airlines, Continental Airlines, Federal Express Corp. and United Parcel Service.Mr. Duffy's second four-year term expires at year end. To date, no candidates have said they will seek the job, up for grabs in an October election, a spokesman said.

Mr. Duffy, 55, and a Delta Air Lines captain, disclosed his intentions in a letter he recently sent to ALPA's executive committee. He couldn't be reached for comment Thursday.

The next ALPA president will lead a union at the crossroads, said Peter Cappelli, a professor of management at the University of Pennsylvania's Wharton School in Philadelphia. He specializes in airline industry labor relations.

According to Mr. Cappelli, ALPA's new leader must determine whether the union will act like a traditional labor organization, concentrating on winning favorable contracts, or a shadow management group.

ALPA, for example, has expended tremendous effort trying retain representation of Continental Airlines' flight crews.

At the same time, the union has negotiated with several bidders for United Airlines, willing to throw its weight to the management group that best addresses members' concerns.

ALPA, based in Washington, has feet in both camps because of its unique membership.

ALPA members predominately are college graduates who, on average, earn $78,000 annually. Many, moreover, got flight training in the military. Members pay almost $2,000 a year in union dues, which has allowed ALPA to build a large war chest.

The job of ALPA's senior officer is complicated by the fact that each union local, or executive council, acts autonomously and bargains independently.

That parallels the airline industry's singular seniority system that marries an employee to a particular airline. Flight crewmembers lose all accrued seniority should they transfer to another carrier.

Seniority is key to ALPA members because it determines the schedule and routes pilots get to fly, as well as salary.

During his tenure, Mr. Duffy took pains to reign-in the somewhat freewheeling union.

He won a major victory when members agreed to change a bylaw to require that union headquarters approve contract settlements for each airline, Mr. Cappelli said.