Some mergers take years to complete. Just ask a flight attendant.

Travelers may not have noticed but in the eight years since United and Continental merged to form United Continental Holdings, the parent company of United Airlines, flight attendants from both carriers had been working separately.

After a new contract, and scheduling system changes, the some 24,000 flight attendants can work on the same planes, starting Monday. American Airlines, which merged with US Airways in December 2013, is taking a similar step today, too, with its more than 25,000 flight attendants. American's pilots combined two years ago while United's merged about five years ago.

Having a single flight attendant pool at each airline will help the airlines staff flights more easily, particularly when they need backup crews, the carriers say. Since they receive the same training, most passengers won't see different service on board.

But the changes have not all gone over smoothly. United's flight attendant union, the Association of Flight Attendants, complained about glitches last month that prevented flight attendants from making some schedule changes. But the labor union praised United CEO Oscar Munoz for getting a deal done with the flight attendants.

American's flight attendant's union complained about what it called a punitive new attendance policy that takes effect Monday.

"It is much more punitive than what we have worked under in the past and we believe it will result in flight attendants working sick, fast tracking them to termination," said Lori Bassani, president of American Airlines' flight attendants union, the Association of Professional Flight Attendants. "It is a new policy for both [US Airways and American crews] with some similar components to the former AA policy, such as a point system."

For his part, American's CEO, Doug Parker, told reporters at an airline industry conference in Boston last week: "There's just so much change being placed upon our flight attendants and change is difficult."

American Airlines spokesman Ross Feinstein said the carrier will work with the union "to address their concerns and get through this integration as quickly as possible."

The two airlines are not the only U.S. carriers left to complete this milestone after a decade of consolidation in the industry.

Alaska Airlines, which merged with Virgin America in December 2016, is still in the process of combining its crews. Its 5,400 flight attendants reached an agreement with the company in April but they will start bidding on trips in January and flying together the following month.

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