The failure of a United Airlines engine likely started inside of it Saturday before pushing outward, aviation experts said, blowing the engine’s outer cover to pieces and shedding debris over a northern Denver suburb.

Three experts could only speculate Sunday on what caused the engine failure when the Boeing 777-200 plane took off from Denver headed for Honolulu, Hawaii, and said it’s too soon to know whether poor maintenance, old age, an external force — like a bird — or various other root causes created the uncontained engine failure.

A likely culprit is one of the hundreds of spinning blades inside the massive engine.

“I’m 6’3’’ and I can stand in the inlet of that engine,” said Mike Robertson, a retired Federal Aviation Administration inspector and Army pilot who reviewed photos and videos of the incident.

“With an initial look at it, it looked like one of the impeller blades, the first set of blades you see inside an engine, it looks like one of those probably broke or came loose,” he said. “Something happened way up front in the engine. What, exactly? Until you can get your hands on it, you don’t know.”

The National Transportation Safety Board is investigating what happened, and it’ll take months of looking at the engine itself and the shards collected from Broomfield neighborhoods and a park after they crashed into a truck, some yards and CBS4 reported at least one home.

No injuries were reported as of Saturday afternoon — neither among the 231 passengers or 10 crew members aboard Flight 328, nor anyone in Broomfield, about 25 miles northwest of the airport.

The Boeing 777-200 plane has been flying since 1995 and was certified by the FAA through 2022, according to federal records. That model is a “very reliable older airplane,” aviation safety expert John Cox told The Post on Sunday, and uncontained engine failures like what happened over Broomfield are rare.

The root cause of such a failure could have happened between the rigorous, frequent inspections that planes undergo — like if a rock got into the engine between flights. Robertson said in that case, the failure would not be considered a maintenance issue. Or a budding crack could have been missed during an inspection, which would be a maintenance issue, though Robertson cautioned he didn’t know what happened in this case.

Even a tiny flaw in the engine can cause massive problems, said Ross Aimer, a retired United Airline pilot and CEO of Aero Consulting Experts.

“When it turns at that speed, with the slightest imbalance it basically tears itself apart,” he said.

He added that pilots and flight crews train for this type of situation — even if passengers were terrified — and the plane’s safe landing shows that safety measures did work.

“The airplane is tolerant of even a major failure like this,” he said.

Well-maintained planes can fly safely for more than 40 years, Cox said.

“They’re going to look very carefully at what caused the failure,” Cox said of the NTSB, “and then backtrack that to the entries on what inspections were done and what they found — all those kind of questions — to make sure it wasn’t somehow something that happened and wasn’t caught in maintenance.”

The Boeing 777 can fly on one engine, Cox said, although an uncontained engine failure will make for a bumpy ride, which a few passengers Saturday noted happened as they were returning to DIA.

“I know everybody was scared on it,” Cox said. “And what would have been most unnerving is that when you have an engine with this kind of problem, it will vibrate, and the plane will feel like it is shaking. That is to be expected, but it would be very unnerving for passengers.”

The Boeing 777 model is being phased out among a few large airlines throughout the world, including Qatar Airways, Nippon Airways, Japan Airlines and Emirates. And in May, Delta announced it was retiring all 18 of its Boeing 777 planes in favor of newer, more fuel-efficient aircraft.

United Airlines has not followed suit, Cox said, likely because they have more Boeing 777s in their fleet.

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