WASHINGTON (Reuters) - A United Airlines Boeing 777 plane with a Pratt & Whitney engine that failed on Feb. 20 had flown fewer than half the flights allowed by U.S. regulators between fan blade inspections, the National Transportation Safety Board (NTSB) confirmed on Friday. The Boeing Co 777 plane had flown 2,979 cycles before its last inspection in 2016. At the time of the incident, checks were required every 6,500 cycles mandated after a separate United engine failure in 2018. A cycle refers to one take-off and landing. Reuters first reported the inspection data on Feb. 24, citing sources. After the United engine failure shortly after takeoff from Denver, Pratt & Whitney, a unit of Raytheon, issued a bulletin calling for inspections every 1,000 cycles. The Federal Aviation Administration subsequently issued an emergency order requiring inspections of all 777 airplanes with PW4000 engines before resuming flights. The NTSB said earlier that damage to a fan blade in the United engine failure was consistent with metal fatigue. The agency said the blade that fractured underwent inspections in 2014 and 2016. The 2016 inspection data was examined again in 2018 after the incident involving a United 777 that suffered an engine failure near Honolulu. The engines are used on 128 older versions of the plane, accounting for less than 10% of the more than 1,600 777s delivered. Only a handful of airlines in the United States, South Korea and Japan were operating 777s with that engine before the incident. United, which is the only U.S. operator of 777s with the PW4000 engine, declined to comment on Friday. It voluntarily grounded its fleet of 24 planes with that engine after the incident. Boeing did not immediately comment Friday. The engine failure showered engine parts over a nearby Denver suburb but no one was injured. The captain elected not to dump fuel for safety and time reasons, the NTSB said.