WASHINGTON, Sept 17 (Reuters) - The U.S. Transportation

Department said on Tuesday it had agreed to allow Alaska

Airlines to close on its $1.9 billion acquisition of

Hawaiian Airlines, after the carriers agreed to maintain

key Hawaiian routes and adopt consumer protections.

The Justice Department in August opted not to block the deal

that was announced in December by Alaska, the fifth-largest

domestic U.S. airline, to Hawaiian, the 10th-largest carrier.

DOT said Alaska and Hawaiian agreed to protect the value of

frequent flyer rewards, maintain existing service on key

Hawaiian routes to the continental United States and

inter-island, ensure competitive access at the Honolulu airport

and provide travel credits or frequent flyer miles for

disruptions that are the fault of the airline.