Safety experts said the investigation will focus on why the fan blades snapped — whether mistakes were made in manufacturing or maintenance or if problems were missed during inspections — and whether blade inspections need to be done differently or more often. They will compare Saturday's incident with similar ones in December in Japan and in 2018 on another United flight to Hawaii.
Investigators will also look at why the cowling, which covers the front of the engine, broke off along with other parts. Photos showed a large gash in the fairing, a piece of composite material that makes planes more aerodynamic by smoothing out joints where the body meets the wings.
"That was a substantial hit," said John Goglia, a former member of the National Transportation Safety Board, which is investigating Saturday's incident. "If that had hit the wing, things might have been different because the wing is full of fuel" and the broken engine was still on fire.
Sumwalt said, however, that "there was no structural damage" to the plane.
Another concern: The engine remained on fire even after pilots presumably shut off its fuel supply. That could indicate a fuel leak, said Todd Curtis, a former Boeing engineer and now a safety consultant.