The United States' aviation regulator on Thursday launched an investigation into aircraft maker Boeing, in response to a mid-air blowout of a plugged door during an Alaska Airlines flight last week.

The Federal Aviation Administration (FAA) notified Boeing in a letter that it will check whether the aviation giant's completed products conformed with the approved design and were in a condition for safe operation under FAA rules.

"This incident should have never happened and it cannot happen again," the FAA said in a statement on its website. It also cited further discrepancies in the Boeing 737 Max 9 model.

"The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service," it said.

On Saturday, the regulator grounded 171 Boeing Max jets with the same panel, pending safety inspections.

Most of these planes are operated by Alaska Airlines and United Airlines, both of which reported loose hardware in the initial inspection post incident. 

"We will cooperate fully and transparently with the FAA and the NTSB on their investigations," Boeing said in a statement. The NTSB refers to the National Transportation Safety Board.  

Fliers fear for safety

Reuters news agency reported that more travelers are checking the model of an aircraft before booking flights.

Booking Holdings-owned Kayak said it has seen a spike in the use of a filter on its website that allows customers to check the plane model.

Last week's mid-air blowout is only the most recent safety incident encountered in a Boeing-made aircraft.

Two MAX 8 aircraft crashed in 2018 and 2019, killing 346 people and prompting a worldwide grounding of all MAX 8 and MAX 9 planes that lasted nearly two years.

As of now, Max 8 planes are still in service.

mk/wd (Reuters, AFP)