The investigation into an engine explosion on a jetliner taking off from Denver is focusing on broken fan blades, a development reminiscent of a fatal failure on board another plane in 2018. These and other recent engine failures raise questions over long-held assumptions about how long fan blades last and whether they are being inspected often enough. A Boeing 777 operated by United Airlines had to make an emergency landing in Denver after one of its engines blew apart, spewing huge chunks of wreckage that landed in suburban neighborhoods. Passengers captured video of the crippled engine, wobbling and still on fire, as pilots made a safe return to the airport minutes after the plane bound for Hawaii took off. WHAT HAPPENED? U.S. officials said late Sunday that two fan blades in the Pratt & Whitney engine broke off. Experts said it is likely that one blade snapped first and chopped off the second. Federal Aviation Administration head Stephen Dickson said inspectors quickly concluded that inspections should be done more frequently for the type of hollow fan blades in certain Pratt & Whitney engines that are used on some Boeing 777s. As a result, 69 planes and another 59 in storage were grounded in the U.S., Japan and South Korea, the only countries with planes using this particular engine. United, the only U.S. carrier with affected planes, said it grounded 24 Boeing 777s and 28 others will remain parked. Japanese regulators ordered Japan Airlines and All Nippon Airways to ground 32 planes, and South Korea’s Korean Air and Asiana Airlines said Monday they will ground their Boeing 777s. WHAT ARE INVESTIGATORS LOOKING INTO? Safety experts said the investigation will focus on why the fan blades snapped — whether mistakes were made in manufacturing or maintenance, or 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 purported to show a large gash to the lower side of the plane. “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. 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. HOW MUCH DANGER WERE PASSENGERS IN? Saturday’s incident is called an “uncontained” failure because debris blew off the disintegrating engine, creating shrapnel that can damage key systems like hydraulic lines or hit the passenger cabin. The last accident-related death on a U.S. airline flight occurred in 2018, when a broken fan blade triggered an engine breakup on a Southwest Airlines Boeing 737. Part of the engine housing struck and broke a window. The passenger in the window seat was blown halfway outside and died of her injuries. That engine was made by a different company, CFM International, a joint […]

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