By David Shepardson

WASHINGTON, Sept 18 (Reuters) - The Federal Aviation Administration said on Friday a proposed triumphal arch President Donald Trump wants to build near Arlington National Cemetery ‌in Washington would not pose a hazard to aircraft but would be required to ‌have an "eternal flame" on top.

The FAA determination requires it to be marked with a light on top of the structure and ​floodlight projectors pointed at the arch to make it noticeable for pilots. Some aviation experts have raised safety concerns given its proximity to Reagan Washington National Airport.

The determination clears an aviation hurdle for the arch, which Trump proposed to mark the nation's 250th anniversary. The project is part of the ‌president's broader drive to reshape ⁠Washington's monumental landscape, including a new White House ballroom, renovation of the Lincoln Memorial Reflecting Pool and redevelopment of a golf course in East Potomac ⁠Park.

Veterans have sued to block the arch, arguing it would destroy the historic sightline between the Lincoln Memorial and Arlington National Cemetery.

The FAA previously said the arch would need red obstruction lights but said on Friday ​the ​structure no longer is required to have them.

The FAA ​determination is subject to review if ‌anyone petitions by October 18.

In June, US Senator Tammy Duckworth, the top Democrat on the Senate aviation subcommittee, urged the FAA to reject White House pressure to approve the proposed 250-foot (76-meter) arch, saying it could pose risks to commercial flights.

The proposed arch would be 3,000 feet from Washington National Airport and within the airport's main approach and departure corridor.

The FAA generally requires blinking red warning ‌lights on buildings above 200 feet near airports to ​warn pilots at night, like on the nearby 555-foot-tall Washington ​Monument.

Duckworth cited the National Park Service ​as saying construction of Trump's arch would require cranes reaching 300 to 320 ‌feet in height and could last 20 hours ​per day for two ​to three years.

The FAA said pilots must fly at least 780 feet above the area around the arch, at least 488 feet above the top of the proposed structure.

On ​September 4, a federal judge ordered ‌the Trump administration to give 48 hours' notice before engaging in any activity on ​the arch site, except for work to identify underground cultural artifacts.

(Reporting by David ​Shepardson; Editing by Chizu Nomiyama and Sanjeev Miglani)