WASHINGTON, Feb 24 (Reuters) - Two senior U.S. lawmakers on Wednesday said the Federal Aviation Administration (FAA) has failed to turn over a report to Congress on airline engine safety required under a 2018 law.

Sam Graves, the top Republican on the House of Representatives Transportation and Infrastructure Committee, and Garret Graves, the senior Republican on the aviation subcommittee, cited Saturday's engine failure on a United Boeing 777-200 plane in urging the FAA to quickly submit the required report.

Without the report, they wrote FAA Administrator Steve Dickson, it was impossible for anyone "to know whether the best practices and recommendations to improve airline engine safety could have helped to prevent the engine mishaps that have taken place since the October 2019 safety review."

The letter cited other recent engine failures, including a December failure in Japan and the 2018 Southwest Airlines Flight 1380 failure of a CFM56-7B turbofan engine that led to the death of passenger after shrapnel shattered a window.

The FAA said this week that after the Japanese incident it had been considering stepping up required inspections.

The lawmakers added they were concerned any "recommendations to improve airline engine safety have been languishing for well over a year. Even more concerning is the potential missed opportunity to address similar airline engine safety issues before they occurred again."

The FAA did not immediately comment.

The engine that failed on the 26-year-old United Boeing 777 and shed parts over a Denver suburb was a Pratt & Whitney PW4000 used on 128 planes, or less than 10% of the global fleet of more than 1,600 delivered 777 widebody jets and only a handful of airlines in the United States, South Korea and Japan were operating them recently.

Late Tuesday, the FAA ordered immediate inspections on all 777 airplanes with PW4000 engines after the National Transportation Safety Board said a broken fan blade was consistent with metal fatigue.

Operators must conduct a thermal acoustic image inspection of the large titanium fan blades on each engine, the FAA said. (Reporting by David Shepardson, Editing by Rosalba O'Brien and Lincoln Feast.)