It appears that Boeing 777 -200 and -300 airliners with the PW 4000 series engines have been grounded world wide.
Given that two of these aircraft have had an uncontained fan failure in the past there months, this seems to be a sensible precaution:
Boeing has recommended grounding more than 120 of its 777 jets worldwide following a catastrophic engine failure on a United Airlines plane in Denver.
The company said on Sunday night that airlines using the same type of engine that scattered debris across Denver before making an emergency landing should suspend operations until inspections could be carried out.
Flight 328 was flying from Denver International Airport to Honolulu with 231 passengers and 10 crew on board on Saturday when one engine failed shortly after take-off. Police in Broomfield, Colorado posted photos of pieces of debris from the plane near houses and other buildings. There were no reports of any injuries on the ground or among the passengers.
United Airlines said it was temporarily grounding all 24 of its Boeing 777s on active duty, and Japan’s aviation regulator swiftly followed suit, ordering Japan Airlines (JAL) and All Nippon Airways (ANA) to cease flying 777s that use the Pratt & Whitney PW4000 engines while it considered whether to take additional measures. Japan said ANA operated 19 of that kind and JAL operated 13.
“We reviewed all available safety data following yesterday’s incident,” the FAA said in a statement from its administrator, Steve Dickson . “Based on the initial information, we concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes.
The US National Transportation Safety Board (NTSB) said on Sunday an initial examination of the Denver engine showed two fan blades had become fractured. The voice cockpit and flight data recorders have been taken to a lab in Washington for analysis, it said.
Japan’s transport ministry said early on Monday a JAL flight from Naha to Tokyo had to return to the airport on 4 December last year due to a malfunction in the left engine. That plane is the same age as the 26-year-old United Airlines plane involved in Saturday’s incident.
My guess is some sort of fatigue or stress corrosion cracking.