UNITED AND ALASKA Airlines are to investigate the discovery of loose hardware on some of their planes after a panel fell out of a plane mid-flight last week.

It comes as multiple aviation experts have questioned why a Boeing jetliner that suffered an inflight blowout over Oregon was flying at all after warning lights were triggered on three previous flights.

Expert scrutiny comes in the wake of new information that the plane was intentionally not being used for flights to Hawaii.

Alaska Airlines decided to restrict the aircraft from long flights over water so the plane “could return very quickly to an airport” if the warning light reappeared, Jennifer Homendy, chairwoman of the National Transportation Safety Board (NTSB), said on Sunday.

Homendy said the pressurisation light might be unrelated to Friday’s incident in which a plug covering an unused exit door blew off the Boeing 737 Max 9 as it cruised about three miles over Oregon.

Friday’s flight was headed from Oregon to Southern California and made it back to Portland without serious injury to any of the 171 passengers and six crew members.

But the decision to allow it to fly over land in the first place struck some aviation experts as illogical.

“If you are afraid to take the airplane far from land, what is the reason for that? That has to be answered by Alaska Airlines,” said Steven Wallace, an air safety consultant and commercial pilot who once headed accident investigations for the Federal Aviation Administration (FAA).

Alan Diehl, a former crash investigator for both the NTSB and the FAA, said Alaska should have grounded the plane.

However, he and other critics said the decision to stop flying the plane to Hawaii might have averted a disaster.

If the blowout had happened halfway to Hawaii, pilots would have been forced to fly low enough so passengers could breathe without oxygen masks, which, in turn, burns more fuel. And the gaping hole in the fuselage would create drag.

The plane might have run out of fuel before reaching land, experts said.

“As far as I’m concerned, there’s an angel in Alaska,” Mr Diehl said.

“Whoever made the decision to do that probably saved a lot of lives.”

On Monday, the FAA approved guidelines for inspecting the door plugs on other Max 9 jets and repairing them, if necessary. That move could speed the return to service of the 171 planes that the FAA grounded.

Alaska has 64 other Max 9s, and United Airlines owns 79 of them. No other US airlines operate that model of the Boeing 737.

Shares in The Boeing Co fell 8% and those of Spirit AeroSystems, which builds the fuselage for Boeing’s 737 Max, tumbled 11% on Monday, the first day of trading since the incident occurred.

Shares in Alaska Airlines were nearly unchanged after slumping earlier in the session.

The auto-pressurisation system warning on the ill-fated Alaska Airlines jet lit up during three previous flights.

Homendy said she did not have details about a December 7 incident, but that it came on again during a flight on January 3 and after the plane landed on January 4 – the day before the blowout.

“We plan to look at that more, and we’ve requested documentation on all defects since delivery of the aircraft on October 31,” she said.

The NTSB said the lost door plug was found on Sunday near Portland, Oregon, in the back garden of a home.

Investigators will examine the plug for signs of how it broke free.

Investigators will not have the benefit of hearing what was going on in the cockpit during the flight. The cockpit voice recorder, one of two so-called black boxes, recorded over the flight’s sounds after two hours, Homendy said.

At a news conference on Sunday, Homendy provided new details about the chaotic scene that unfolded on the plane.

The explosive rush of air damaged several rows of seats and pulled insulation from the walls. The cockpit door flew open and banged into a toilet door.

The force ripped the headset off the co-pilot and the captain lost part of her headset. A quick reference checklist kept within easy reach of the pilots flew out of the open cockpit, Homendy said.

Two mobile phones that appeared to have belonged to passengers on Friday’s terrifying flight were found on the ground. One was discovered in a garden, the other on the side of a road.

However, the plane made it back to Portland, and none of the 171 passengers and six crew members was seriously injured.

Hours after the incident, the FAA ordered the grounding of 171 of the 218 Max 9s in operation, including all those used by Alaska Airlines and United Airlines, until they can be inspected. That led to flight cancellations at both carriers.

Early on Monday, Alaska Airlines was forced to cancel 20% of all flights, 141 in all. United cancelled 221 flights, or 8% of its total flights scheduled for Monday.

Boeing chief executive Dave Calhoun called a companywide webcast to talk about the incident with employees and senior leadership for Tuesday.

“When serious accidents like this occur, it is critical for us to work transparently with our customers and regulators to understand and address the causes of the event, and to ensure they don’t happen again,” Mr Calhoun wrote in a message to employees on Sunday.

“This is and must be the focus of our team right now.”

Alaska Airlines flight 1282 took off from Portland on Friday for a two-hour trip to Ontario, California.

About six minutes later, the chunk of fuselage blew out as the plane was climbing at about 16,000 feet.

One of the pilots declared an emergency and asked for clearance to descend to 10,000 feet, where the air would be rich enough for passengers to breathe without oxygen masks.

Videos posted online by passengers showed a gaping hole where the panelled-over door had been. They applauded when the plane landed safely about 13 minutes after the blowout.

Firefighters came down the aisle, asking passengers to remain in their seats as they treated the injured.

It was extremely lucky that the airplane had not yet reached cruising altitude, when passengers and flight attendants might be walking around the cabin, Homendy said.