If US Airways succeeds in buying Delta Air Lines, the big acquisition will require blending two complex companies, each with their own distinct corporate cultures and key differences in everything from unions to what kinds of planes they fly.

Tougher still, US Airways already is not quite a complete mixture of two airlines - the old US Airways and the company that acquired it and took its name, America West.

''You're really talking about a three-way combination,'' said Dan Petree, dean of the College of Business at Embry-Riddle Aeronautical University. ''Combining three cultures should not be underestimated.''

Yet the rewards could outweigh the risks in an industry in which mergers are seen as all but inevitable.

''It has to happen,'' said Dean Headley, a Wichita State University marketing professor who co-authors an annual survey on airline service quality. ''There are too many players out there.''

Arguably, the toughest task in merging two airlines lies in meshing their work forces. Les Hough, a consultant and labor expert, said Delta has a history of avoiding union representation. US Airways is unionized through and through, with five associations that represent employees. At Delta, only pilots and dispatchers are organized. Delta dispatchers are represented by a different union than their counterparts at US Airways.

For unions, however, the acquisition of Delta could lead to a breakthrough. The International Association of Machinists, or IAM, which represents three departments at US Airways, has been trying to organize ramp workers at Delta by launching a bid a month ago.

Joseph Tiberi, a spokesman for the IAM, said combining companies with opposite union histories need not be an ordeal.

''If the management team is willing to work with employees and unions, it's possible for there to be a smooth transition,'' he said.

Some changes are certain to be contentious, however, especially when it comes to combining all-important seniority rankings.

At an airline, a seniority system governs when a person works, how much he or she gets paid and when a vacation can be scheduled.

''It controls virtually every aspect of your life,'' said Kit Darby, president of Air Inc., an Atlanta company that specializes in helping pilots with their careers.

In a merger, the seniority lists of both sides would be blended. Some people surely would consider themselves to be worse off. Even in the fairest system, Darby said, it will be virtually impossible to make everyone happy.

Dale Oderman, professor of aviation technology at Purdue University, said ''people problems'' are the toughest task in a merger. ''If one group feels they're getting a bum deal, that's going to be very difficult,'' he said.

Corporate culture also looms as a major barrier to making two companies work as one.

Delta is one of the nation's most venerable airlines; its history dates to 1924. America West, the dominant corporate player in the new US Airways, started flying in 1983.

The two companies are different in heritage and in philosophy. The picture is even more muddled because America West-US Airways is still in flux.

''Delta's an old-line carrier,'' Headley said. ''We don't know exactly what the US Airways-America West culture is. They're still trying to figure it out.''

The list of challenges goes on. Oderman said Delta primarily flies Boeing planes, while US Airways has a largely Airbus-built fleet. The planes have different crews, different mechanics and different inventories of spare parts.