The so-called door plug tore off the left side of an Alaska Airlines jet following take-off from Portland en route to Ontario, California, forcing pilots to turn back and land safely with all passengers and six crew with only a few minor injuries reported.

The panel, put in place on some planes in lieu of an additional emergency exit, is likely to have landed somewhere in the western suburbs of Portland but has not yet been found.

“I imagine this was a pretty terrifying event. We don’t often talk about psychological injuries but I am sure that occurred here,” National Transportation Safety Board (NTSB) chair Jennifer Homendy told reporters on Saturday, adding that it was too early to say what caused the event.

The accident has put Boeing back under scrutiny as it awaits certification of its smaller MAX 7 as well as the larger MAX 10, which is needed to compete with a hot-selling Airbus model.

The United States Federal Aviation Administration (FAA) on Saturday grounded 171 Boeing jets installed with the same panel after the emergency landing of the plane that had been in service for just eight weeks.

The grounding is expected to disrupt flight schedules for several days for certain carriers.

The vast majority of that model of plane used in the United States are operated by United Airlines and Alaska Airlines while Turkish Airlines, Panama’s Copa Airlines and Aeromexico also grounded jets for inspections.

NTSB chair Jennifer Homendy says seats next to a part of jet fuselage that blew out were unoccupied. (AP PHOTO)
Airlines must inspect the panel and carry out any necessary repairs.

Before US airlines can start flying the planes, the FAA must additionally agree to detailed inspection criteria proposed by Boeing.

It was unclear if it would do so on Sunday.

Alaska Airlines halted the use of 18 MAX 9 planes on Saturday that it had resumed using following earlier maintenance checks on the FAA’s order.

On Sunday, it cancelled 163 flights, or 21 per cent, and said travel disruptions from the grounding are expected to last through at least mid-week.

United cancelled 230 flights on Sunday, or 8 per cent of scheduled departures.

NTSB chair Homendy said the two seats next to the portion of fuselage that blew out were unoccupied.

The independent US agency has opened an investigation.

“We are very, very fortunate here that this didn’t end up in something more tragic,” Homendy said.

Parts of the seat next to the fuselage, including the head rest, were missing.

The extra exit door is typically installed by low-cost airlines using more seats that require additional exit routes.

However, those doors are plugged on jets with fewer seats.

To passengers, the area looks like a normal window seat.

The fuselage for Boeing 737s is made by Kansas-based Spirit AeroSystems, which separated from Boeing in 2005.

Spirit manufactured and installed the plug that suffered the blowout, a source told Reuters on Saturday.

Sources familiar with the installation process said Boeing also has a potentially key role, since it typically removes the semi-fitted left-side door panel to feed in cabin equipment and speed up production before completing final installation.

Investigators are expected to examine whether any installation flaws occurred at Spirit or Boeing plants, sources said.

Spirit referred questions to Boeing, which did not respond to a request for comment on whether it had been involved in the final installation.

– with AAP