No one was injured in the test, which took place on Friday. That meant bending the wings of the plane while it was parked on the ground. Boeing said the test bent its wings "to a level far beyond anything expected in commercial service," and that the problem arose 99% of the way through the test.
But during the test, the rear of the fuselage became depressed, according to the Boeing statement. A person familiar with the test said one of the doors came out of the plane. The company said it is now investigating the test results to determine the cause of the problem.
777X is a key product for Boeing. The length, the longest on any commercial jet, will allow it to carry between 400 and 425 passengers in two classes, and 7,600 nautical-mile flight routes.
Boeing has already taken orders for 344 of the 777X jets.
The problems with the 777X come on top of more serious problems for Boeing's 737 Max, the company's best-selling aircraft, which has completed orders for more than 4,500 of the jets.
Boeing also released sales and delivery data for August Tuesday. It showed only six new orders, compared to 99 during the same month last year. Part of that decline is due to the fact that 2018 was a strong year for flight orders and many customers have already placed orders to meet their long-term needs.
But the orders have been very low since Max was grounded. Boeing has only reported 73 orders since the ground attack, compared to 497 in the same period one year ago.
Two of the orders last month were for an undisclosed buyer who ordered 787-9 Dreamliners. A source familiar with Boe's order book confirmed that the two orders represented a conversion of a previous order for three of the 737 Max.
Boeing did not disclose the prices the buyer agreed on for the three Max or the two Dreamliners replacing the previous order. The buyer is likely to pay more for the two Dreamliners, which have a list price of $ 292.5 million each. List prices for the 737 Max range from $ 99.7 million to $ 134.9 million, depending on model. But transactions are rarely made at the listed list price.