Boeing’s new widebody jet, the 777X, suffered a setback Thursday afternoon during a high-pressure stress test on the ground when one of the airplane’s cargo doors exploded outward.

One 777X employee working in a nearby bay at Boeing’s Everett plant said he heard “a loud boom and the ground shook.”

The accident happened to what’s called the “static test airplane,” one of the two airplanes in any new jet program that are built for ground testing only and will never fly. It was during the final test that must be passed as part of the airplane’s certification by the Federal Aviation Administration (FAA).

The failure of the door will require careful analysis to find out why it happened, and it may mean Boeing will have to replace the door and repeat the test.

The 777X program is already delayed due to a problem with development of the GE-9X engine that will power it. In July, Boeing CEO Dennis Muilenburg revealed on a quarterly earnings call that the first 777X intended to fly, which rolled out of the Everett factory in March, will not make it into the air until next year.

This ground test failure is another blow.

The static test plane is the one that is deliberately stressed well beyond the limits of normal service. The airplane is surrounded by a metal framework while weights passing through pulleys are fixed to the wings and other parts of the airframe.

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During the ultimate load test, the wings are then pulled upward. To pass the test and be certified, the wings must bend without breaking until the load on them reaches at least 150 percent of the normally expected load.

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In addition, the skin panels that cover both the wings and the fuselage are pressurized to the maximum stress that would be expected at the edge of any extreme maneuver anticipated in service. The pressure is ratcheted up by pumping air into the cabin.

Sometimes this final test is continued beyond the 150 percent load target until a wing actually breaks. But not always. The carbon-composite wings on the 787 Dreamliner are so flexible that when Boeing tested those in 2010 they bent upward by about 25 feet and, having comfortably surpassed the target load, Boeing halted the test without breaking them.

The massively larger wings of the 777X are also carbon composite, with a folding tip, and during Thursday’s test those must have flexed in a similarly impressive way to those of the 787. This time, however, though the wings did not give way; it was one of the doors that failed — an outcome that is definitely not supposed to happen.

The entire area around the static airplane is typically cleared during this test, with all the measurements taken by monitoring equipment and with engineers watching anxiously on a video link as the load slowly inches up toward the target and the pressure increases.

No one was injured in Thursday’s door explosion, which happened shortly after 1:30 p.m., and everyone was able to exit the building.

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On Friday, according to Boeing employees, caution tape was attached to all the entry doors and no one was allowed into the building.

After the incident was first reported Friday by KOMO News, Boeing confirmed that a serious incident had occurred but offered few details.

“During final load testing on the 777X static test airplane, the team encountered an issue that required suspension of the test,” it said in a statement. “The event is under review and the team is working to understand root cause.”

Boeing went on to emphasize that “the testing conditions were well beyond any load expected in commercial service” and that the plane used in the test “will never fly or be used in passenger service.”

Because the GE engine issue has already pushed the jet’s flight tests into next year, it’s possible Boeing may have time to analyze and redo the ultimate load ground test without a further hit to the schedule.