(ABC 6 News) — The U.S. Department of Transportation says it is prepared to take airlines to court over flight delays. The agency has filed a lawsuit against Southwest, accusing the airline of illegally operating chronically delayed flights and disrupting passengers’ travel plans.

Southwest is disappointed that DOT chose to file a lawsuit over two flights that occurred more than two years ago," a spokesperson for the airline said.

American Eagle flight 5342 collided with a Black Hawk helicopter over the Potomac River, home to some of the country's most restricted airspace.

A Southwest ... to the U.S. Department of Treasury, but the other half can be waived if the airline doesn't operate any chronically delayed flights for the next three years. “Airlines have ...

The Department of Transportation defines a flight to be chronically delayed if it is “flown at least 10 times a month and arrives more than 30 minutes late more than 50 percent of the time. Cancellations are included as delays within this calculation.”

A massive search operation is underway after a passenger aircraft carrying 64 people collided midair with a US Army Black Hawk helicopter near Reagan National Airport outside of Washington, DC, according to aviation and defense officials.

Secretary of Transportation Sean Duffy said Americans should be assured that "you're safe" when flying in the wake of a deadly crash between a commercial airliner and an Army helicopter.

A downbeat outlook tarnished an otherwise strong quarter for Southwest, which topped profit estimates on robust holiday travel.

Search efforts continue after an American Airlines plane from Wichita, with 64 people on board, collided with an Army helicopter near Washington, D.C., and crashed into the Potomac River.

Air traffic controllers asked Flight 5342 to switch runways when coming in to land. Black box recordings from the jet and the military helicopter it collided with will help reveal if this contributed to the accident.

The crash of an American Airlines passenger jet and a US Army Black Hawk helicopter near Reagan Washington National Airport has highlighted issues around the congested airspace shared by civilian and military aircraft over the US capital.

Military spokesman Ron McLendon II said the Army is joining an investigation into the crash headed by the National Transportation Security Board.