ATLANTA — How many chimes should pilots ring to signal the plane is about to land — two or four? Should flight attendants first pour drinks into a cup or just hand over the can?

Airline mergers are complex and tough to pull off — witness the troubled marriage of People Express and Continental Airlines in the 1980s or the continuing problems in integrating America West and US Airways six years after their merger. So when Delta Air Lines acquired Northwest three years ago, executives knew they would have to resolve major labor, technology and financial issues.

What they had not fully anticipated were the thousands of tiny details that go mostly unnoticed by passengers but can make the difference between a successful merger and a failed one.

All airlines have their own way of doing things, developed over time and through labor negotiations. All have specific working rules, flying procedures, maintenance schedules and computer programs. And all have their own cultures. Delta always thought of itself as the gracious host. Hence its flight attendants poured the requested drinks. Northwest was the practical carrier; its attendants just handed over the can.