On February 20, 2021, a 777-200ER being operated as United Airlines Flight 328 suffered a failure of its starboard engine. The cowling and other engine parts fell over a Denver suburb. The captain declared an emergency and returned to land at the Denver airport. An immediate examination, before any formal investigation, found that two fan blades had broken off. One blade had suffered metal fatigue and may have chipped another blade, which also broke off. Boeing recommended suspending flights of all 128 operational 777s equipped with Pratt & Whitney PW4000 engines until they had been inspected. Several countries also restricted flights of PW4000-equipped 777s in their territory. In 2018, there had been a similar issue on United Airlines Flight 1175 from San Francisco to Hawaii, involving another 777-200 equipped with the same engine type.