Federal investigators said “metal fatigue” was likely to blame for causing the engine of a Boeing 777 jet to break apart at the weekend, raining metal on suburban Denver.

The incident, which occurred four minutes after take-off, forced the United Airlines flight bound for Hawaii to turn around and make an emergency landing back at the Colorado state capital’s airport. No one was injured, either on the flight or from the falling debris.

The situation aboard Saturday’s flight bore similarities to a Japan Airlines flight in December and a United flight three years ago. In both cases, fan blades snapped in a Pratt & Whitney PW4000-112 engine on a 777.

The US National Transportation Safety Board is investigating the incident in Colorado. The US Federal Aviation Administration has ordered inspections of 777s equipped with the PW4000-112 turbines, with the likely outcome of shortening the intervals between check-ups.

Dozens of Boeing 777 widebody aircraft with the engines have been grounded across the world following the incident. Boeing recommended airlines stop flying the jets until aviation regulators lay out “the appropriate inspection protocol”.

“We want to understand if there’s any relations between this event and the other events that have happened across the globe,” said NTSB chairman Robert Sumwalt on Monday.

Sumwalt said one of the turbine’s 22 fan blades had snapped off at its root and hit a second blade, snapping it at the midpoint.

“Regarding the fan blade that was fractured at the root, a preliminary, on-scene exam indicates damage consistent with metal fatigue,” he said, adding that investigators planned to study the jet’s maintenance records.

Records from the aviation data provider Cirium suggested that maintenance was performed by United Technical Operations, the maintenance, repair and overhaul division of the Chicago-based airline, with Pratt & Whitney working on some engine components. UTO’s website said the unit’s San Francisco facility is the only one in the US equipped to work on the PW4000-112 engine.

The 777 also sustained damage on Saturday from falling metal to the area where the wing joins the body. The cowling, which covers the front of the engine, ended up in a yard in Broomfield, Colorado.

Investigators will seek to understand why a fire in the engine continued to burn after crew members cut the fuel feeding it and why the cowling became dislodged. Engineers design planes to avoid “uncontained engine failure” because of the dangers presented by shrapnel.

Although Saturday’s incident was technically not an uncontained engine failure, Sumwalt said, “from a practical point of view of the flying public, it really doesn’t matter”.



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