(New York) The investigation continues Tuesday into the Boeing MAX 9 planes the day after the announcement by the American airlines United Airlines and Alaska Airlines of the discovery of loose elements during checks after the loss of a door in mid-flight last week by a device of this type.

The American Civil Aviation Agency (FAA) has requested inspections on 171 Boeing 737 MAX 9s, grounded pending this review.

United, which operates the world’s largest fleet of 737 MAX 9s with 79 aircraft, discovered “bolts that needed to be tightened” during checks on the boarded doors of its 737 MAX 9s, the same one torn off Friday during ‘an Alaska Airlines flight in the United States.

“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 explained in a statement sent to AFP.

Alaska Airlines also announced that it had detected “loose equipment” on some of its aircraft of this type after preliminary inspections.

The US National Transportation Safety Board (NTSB) announced Monday evening that it had not found any bolts among the elements that came loose from the Alaskan plane on Friday.

The left door of the Alaska aircraft, which was sealed, had detached from the cabin in mid-flight, causing the aircraft to depressurize. She was found on Sunday in a teacher’s garden in Portland, Oregon, a city in the northwest of the United States from where the plane took off for Ontario, California.

Further research will “determine whether the bolts were there,” NTSB Chairman Jennifer Homendy told reporters.

Hundreds of flights had to be canceled after part of the Boeing MAX fleet was grounded.

“While operators conduct required inspections, we remain in close contact with them and assist them in processing each finding,” Boeing said Monday evening. “We are committed to ensuring that every Boeing aircraft meets the highest safety and quality specifications and criteria.”

Locking certain doors is offered by Boeing 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.

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 a loose nut in the same location on an aircraft not yet delivered.

On Wall Street, the planemaker’s stock fell 8.03% on Monday and that of its main subcontractor, Spirit AeroSystems, fell 11.13%.

On Sunday, Boeing CEO Dave Calhoun replaced a conference of the group’s top executives 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 and FAA are seeking to establish the exact circumstances of the incident which only caused a few minor injuries, but could have ended “more tragically”, according to Jennifer Homendy.

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

For his part, the head of the Irish airline Ryanair Michael O’Leary estimated on Tuesday that Boeing, but also its European counterpart Airbus, must “considerably improve” their quality controls.

However, “the 737 is very good. But it doesn’t need these kinds of reputational problems in the short term,”  Mr O’Leary said in comments to the Financial Times (FT) on Tuesday, confirmed to AFP by the company.

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 the FlightAware website, mainly for the Alaska and United companies which operate 144 of the 218 MAX-9s in circulation.

According to the European Aviation Safety Agency (EASA), no operator in Europe uses the 737 MAX 9 with the affected options.

Boeing has experienced numerous technical problems, but especially the crashes of two 737 MAXs, in October 2018 in Indonesia and in March 2019 in Ethiopia, which caused the deaths of a total of 346 people.

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.