(New York) The American airline United Airlines said Monday that it had found loose bolts during checks on the locked doors of its Boeing 737 MAX 9 aircraft, the same as the one torn off during an Alaska Airlines flight on Friday.

This discovery comes during inspections ordered Saturday by the American Civil Aviation Agency (FAA) on 171 of the 218 planes of this model, which are grounded pending this review.

“Since we began inspections on Saturday, we have made discoveries that appear to be related to problems with the installation of the panel blocking the doors,” United announced in a statement sent to AFP. “For example, bolts that needed tightening. »

United has the largest fleet of 737 MAX 9s, with 79 aircraft.

Also on Monday, the Aeromexico company indicated that it was in the “final phase of a detailed inspection” and anticipated the return to service of its 19 MAX-9s “in the coming days”.

Locking certain doors is a configuration that Boeing offers to its customers when the number of existing emergency exits is already sufficient in relation to the number of seats in the aircraft.

In addition to the 737 MAX 9, this device already exists on other Boeing models, notably the 737-900er, launched in 2006 and which has not experienced any similar incidents since.

On Friday, during an Alaska Airlines flight between Portland, Oregon and Ontario, California, the obstructed left door detached from the cabin mid-flight, causing the aircraft to depressurize.

In December, Boeing recommended that companies equipped with 737 MAXs check the rudder control system, after a company noticed that a nut was missing on one of its planes.

According to the FAA, Boeing had also observed that a nut was loosely screwed in the same location on an aircraft that had not yet been delivered.

Wall Street reacted poorly to this incident. The aircraft manufacturer’s shares fell 8.03% on Monday alone. That of its main subcontractor, Spirit AeroSystems, plunged 11.13%.

The famous door of the Alaska Airlines 737 MAX 9 aircraft was found on Sunday, announced the president of the American transport safety agency (NTSB), Jennifer Homendy.

A teacher recovered the sign, which had fallen in his garden in the city of Portland (Northwest Oregon). “We are going to go get it and start analyzing it,” said the head of the National Transportation Safety Board during a press conference.

On Sunday, Boeing CEO Dave Calhoun decided to cancel a conference which was to bring together the group’s senior executives at the start of the week, to replace it with a safety meeting on Tuesday, open to all employees.

It will be held at the Renton (Washington State) factory, a suburb of Seattle.

NTSB, Boeing, Alaska Airlines and FAA are seeking to establish the exact circumstances of the incident, which only caused minor injuries, but could have ended “more tragically”, according to Jennifer Homendy.

According to the NTSB, no one was seated in the two seats next to the partition that flew away.

But according to passengers cited by American media, a teenager sitting in the row had his shirt torn off by the decompression, causing him minor injuries.

This incident “is indicative of a major cultural challenge” within Boeing, according to Richard Aboulafia, director of the consulting firm AeroDynamic Advisory.

“They have to change,” according to the analyst. “They can’t stagger from crisis to crisis. »

The grounding of a number of 737 MAX 9s has already led to the cancellation of more than 1,000 flights since Saturday, according to data from the specialized site FlightAware, mainly for the companies Alaska Airlines and United, which operate 144 of the 218 MAX-9s in traffic.

The European Aviation Safety Agency (EASA) has indicated that no operators in Europe are using the 737 MAX 9 with the affected technical options.

Among the long list of technical problems encountered by Boeing in recent years, the most serious of them were the crashes of two 737 MAXs, in October 2018 in Indonesia and March 2019 in Ethiopia, which caused the deaths of 346 people in total.

After these accidents, linked to the MCAS piloting software, all 737 MAXs were grounded for 20 months.

At the end of December, the manufacturer had delivered more than 1,370 examples of the 737 MAX and its order book exceeded 4,000 units.