Joe Jacobsen, the third witness, worked for Boeing and later for the FAA, America's air regulator, before retiring in 2021.

He has harsh words for the follow-up after fatal crashes of Boeing planes in 2018 and 2019, which killed a total of 346 people and led 737 Max planes to be grounded for more than a year.

He says FAA leaders backed Boeing in limiting the scope of the review.

"The re-certification of the MAX has been characterised as the most comprehensive in the history of aviation," he says. "This is also a false narrative."

In his opening remarks, Salehpour said "gaps" were regularly found on Boeing's 787 planes, and that they were often found to contain debris.

“I found gaps exceeding the specification that were not properly addressed 98.7% of the time,” he told the panel.

“Inspection of the data shows that the debris ended up in the gaps 80% of the time,” he continued.

He added that workers would struggle to bridge the joints in the planes' mechanisms, and would sometimes use blunt force.

"I literally saw people jumping on pieces of the airplane to get them to align," he said.

"I call it the Tarzan effect."

Image source, Getty Images

Ed Pierson, the director of the Foundation for Aviation Safety, begins his testimony by saying that the world should be "shocked" by what he says are Boeing's lack of safety protocols.

"Every person stepping on board a Boeing airplane is at risk," he says.

The only thing more dangerous than a dangerous environment is the illusion of a safe dangerous environment, he adds.

"This is a criminal cover-up," he says, accusing Boeing's bosses of a conspiracy.

We won't hear directly from any Boeing representatives today to counter these witness claims, but the firm says it is co-operating with proceedings by offering to provide "documents, testimony and technical briefings".

Sam Salehpour says he felt compelled to escalate his fears, remembering the experience of one of his old carpool buddies, who worked on NASA's Challenger shuttle, which exploded in 1986.

That engineer had raised concerns about the engineering, which were ignored.

"Since then, I have always said that if I were in the same situation, I would do everything I could to speak up to try to avoid a tragic result," Salehpour says.

"I'm not here today because I want to be here," he says. "I have serious concerns ... and I'm willing to take on professional risk to talk about them."

Image source, Getty Images

Sam Salehpour, a quality engineer for Boeing, has begun his testimony.

He is a current Boeing employee and is a key witness before the committee.

"I'm not here today because I want to be here," he begins.

"I'm here because I felt compelled to come forward" to call out Boeing's safety shortcomings, he continues, adding: "I'm willing to take on professional risk to talk about them".

"I have raised these issues over three years," he says, adding that Boeing "ignored" him

"I was told effectively to shut up," he says, adding that his bosses had given him "physical threats".

Image source, Getty Images

There are two hearings into Boeing, both happening at the same time today.

Earlier, we were listening to aviation experts at the Subcommittee on Aviation Operations, Safety and Security.

But now we will shift our focus to the Homeland Security and Governmental Affairs subcommittee, which is hearing from Boeing whistleblower Sam Salehpour.

Salehpour starts his statement by saying: "effectively they are putting out defective airplanes."

You can follow the hearing live by pressing the play button at the top of this page.

We're just heard from Alaska Airlines about the FAA's decision today to ground most of its planes.

"This morning we experienced an issue while performing an upgrade to the system that calculates our weight and balance," says the Seattle-based company.

"We're working to resolve the issue as quickly as possible," it adds.

"We apologise for the inconvenience and encourage guests to check the status of their flights... prior to heading to the airport."

In the past hour, the US Federal Aviation Administration (FAA) has issued a notice that grounds most Alaska Airlines planes.

"All Alaska mainline and subcarrier flights ground stopped," the FAA said.

The advisory adds that flights for SkyWest, which provides regional flights for Alaska Airlines, are excluded.

The aviation agency did not give a reason for its decision, or which manufacturer or aircraft models could be impacted.

Alaska Airlines was the operator of the Boeing 787 Max flight that saw a door fall off over Portland in January.

Dr de Luis tells the senators that his investigation has found whistleblowers were too afraid to flag issues that they spotted, out of fear for their job safety.

"There was a very real fear of retribution and payback if you held your ground,” he says.

"It shouldn’t take a whistleblower report" in order for Boeing to further inspect safety protocols, he says.

"We’re talking a career ending move," he adds.

We're hearing from aviation safety specialist Dr de Luis, who is talking about a photograph of a Boeing plane door that was missing bolts.

He asks, why did a mechanic who installed the door walk away and leave it in that condition?

"Why wasn't he or she trained to know that you can't do that?"

He says there appears to be a "disconnect" between management at Boeing and the workers, and there was pressure to meet deadlines on the factory line.

He gives an example from an interview he conducted in one of Boeing's receiving rooms, where certain parts arrive.

Dr de Luis says he was told "foreign object debris" was the major issue staff would encounter. And they would report it, but not track down where it came from "so it doesn't happen again".

Dr Javier de Luis, an expert in aeronautics, is one of the first people to speak at the hearing.

He tells the hearing his sister died in the Boeing 737 Max crash in March 2019.

Graziella de Luis y Ponce was a passenger on the flight, which crashed shortly after takeoff in Addis Ababa, Ethiopia. A total of 157 people died in that crash.

Senate Commerce, Science, and Transportation Chair Maria Cantwell tells Dr De Luis: “I can’t imagine the tragedy of losing your sister in one of the Max crashes and then continuing to be involved in trying to correct and improve our safety culture."

There are multiple aviation safety specialists, as well as current and former Boeing employees, who are on the witness list for today.

Here are some of the notable names:

• Sam Salehpour - current quality engineer at Boeing who filed a whistleblower complaint
• Ed Pierson - executive director of the Foundation for Aviation Safety and former Boeing engineer
• Joe Jacobsen - aerospace engineer and technical adviser to the Foundation for Aviation Safety and former FAA engineer
• Shawn Pruchnicki - professional practice assistant professor for integrated systems engineering at Ohio State University

Hello and welcome to our live coverage of this US Senate hearing.

Boeing is expected to face criticism from a whistleblower who says he feared its planes could fall apart mid-air.

Engineer Sam Salehpour told NBC News he had raised concerns about the 787 Dreamliner, which Boeing dismissed.

The company is under scrutiny after a number of safety incidents, including a mid-air panel blow-out in January.

But it has sought to defend its practices, and says it is co-operating with the Senate inquiry.

Salehpour discussed the alleged safety issues with the 787 Dreamliner in an interview with NBC ahead of this hearing.

He said he feared the plane could simply "drop to the ground" unless manufacturing issues were addressed. He referred to gaps between components that emerged during the plane's assembly.

Asked if he would put his own family on one of 787s, he responded: "Right now, I would not."