CORRECTION

October 30, 2019

In a previous version of this story, we misspelled Dennis Muilenburg's last name as Muilenberg in several references.

Boeing's 737 Max jet is at the center of two international crashes that left 346 people dead in just five months over the past year. Boeing CEO Dennis Muilenburg and John Hamilton, company vice president and chief engineer, are expected to testify before the House Committee on Transportation and Infrastructure on Wednesday. The aircraft has been grounded since March, and this week's hearings are focused on aviation safety concerns and the status of the aircraft.

On Tuesday, the Senate Commerce, Science and Transportation Committee explored issues regarding the design, development, certification and operation of the 737 Max. Muilenburg and Hamilton testified along with Robert Sumwalt, chairman of the National Transportation Safety Board, and Christopher Hart, chairman of the Joint Authorities Technical Review. Tuesday marked one year since an Indonesian Lion Air jet crashed into the Java Sea minutes after takeoff, killing all 189 people on board. Another 157 people were killed when an Ethiopian Airlines flight from Addis Ababa to Nairobi crashed in a remote part of Ethiopia on March 10.

"On behalf of myself and the Boeing company, we are sorry, deeply and truly sorry," Muilenburg said in his opening statement to the committee. "As a husband and father myself, I'm heartbroken by your losses. I think about you and your loved ones every day, and I know our entire Boeing team does as well. I know that probably doesn't offer much comfort and healing at this point, but I want you to know that we carry those memories with us every day. And every day that drives us to improve the safety of our airplanes and our industry. "

This is the first time Boeing executives have testified since the two crashes. Here's what brought them to Capitol Hill.

What should I know about the 737 Max?

The Boeing 737 Max family — including the Max 7, Max 8, Max 9 and Max 10 — is the fourth-generation of the Boeing 737. Competition with Airbus, which announced an updated version of its A320 model in 2010, spurred Boeing to develop a more fuel-efficient aircraft under time constraints. It decided to reengineer the existing model rather than undergo the long and expensive process of creating and certifying a new plane.

Boeing announced the launch of the 737 Max in August 2011 and delivered the first plane for commercial use in May 2017. It became the fastest-selling airplane in company history, with roughly 5,000 orders from more than 100 customers worldwide.

As of March 2019, three U.S. carriers have 737 Max jets: Southwest has 34 in its fleet, American has 24 and United has 14. China and Indonesia are two of the aircraft's biggest users.

What happened during and after the crashes?

The Boeing 737 Max 8 — and the company — fell under intense scrutiny after two deadly crashes.

Lion Air 610 from Jakarta to Pangkal Pinang, Indonesia, was in the air for just 13 minutes before it suddenly pitched into the Java Sea on Oct. 29, 2018. Less than half a year later, on March 10, Ethiopian Airlines Flight 302 crashed shortly after takeoff from Addis Ababa.

Airlines and government regulators worldwide, including the Federal Aviation Administration, grounded all 737 Max passenger planes pending further investigation.

Preliminary reviews of the Ethiopian Airlines plane's flight data showed that there were "clear similarities" between the two accidents. Both involved a new flight control system called the maneuvering characteristics augmentation system, or MCAS, which is designed to prevent aerodynamic stalls. The new 737 Max's larger engines were placed farther forward on the wing, which changed the aircraft's aerodynamic lift. The flight control system was intended to make the new model feel similar to its predecessor for pilots, even though it was designed to activate only in rare and extreme circumstances.

The MCAS system would push the nose of a plane downward if its sensors and flight control computers sensed a problem with low airspeed. But Ethiopian officials announced in March that the plane's flight data recorder showed that its pilot, like that of the Lion Air flight, struggled to keep the plane from nosing down at high speed shortly after takeoff.

What other issues might have contributed to the accidents?

In March, The Seattle Times reported that Boeing had downplayed the risks of crucial flaws in its safety analysis of the new flight control system and that FAA oversight was lacking. Multiple FAA technical experts told the Times that safety assessments were rushed to meet certification dates and that FAA engineers were pressured to increasingly delegate technical assessments to Boeing itself. The System Safety Analysis on MCAS was one such delegation.

Because the MCAS would only be activated in extreme situations, The Seattle Times reported, "Boeing decided that 737 pilots needed no extra training on the system — and indeed that they didn't even need to know about it. It was not mentioned in their flight manuals."

The Joint Authorities Technical Review — a panel assembled by the FAA and composed of U.S. and international civil aviation authorities — released a report earlier this month that criticized both Boeing and the FAA for the way the 737 Max was developed and certified to fly. It confirmed that information about MCAS was absent from pilot and training manuals and that Boeing withheld details about it from the FAA during the certification process.

And in mid-October, evidence emerged that key Boeing employees were discussing problems with the MCAS system over text messages as early as 2016, without alerting the FAA.

Senior company pilots described seeing fundamental issues with the flight control system but telling federal regulators the plane was safe, in what one pilot described as an unknowing lie. In an Oct. 18 statement, the FAA said Boeing had alerted the Department of Transportation to the existence of the messages, which the company had discovered "some months ago." The FAA said it found the substance of the messages and their handling concerning, and has since shared them with the relevant congressional committees.

Ted S. Warren/AP

Passenger flights using the 737 Max have been grounded since March.

Last week, Indonesia's National Transportation Safety Committee released its long-awaited report, criticizing Boeing for the design and certification of the 737 Max and the MCAS system in particular. Investigators found that while MCAS was the primary cause of the Lion Air crash, a faulty sensor, inadequate maintenance, poor pilot training and a failure to heed previous problems with the same aircraft were all contributing factors.

Boeing insiders told NPR's David Schaper that a shift in corporate culture over the past decade has led to the prioritization of profits and shareholder value over safety. Since the 737 Max earned a common "type rating" with the previous model, for instance, pilots only had to undergo minimal additional training, which Boeing promoted on its website as a cost-saving measure. Experts such as aviation industry analyst Bjorn Fehrm say a culture of cost-cutting has financial as well as reputational consequences that Boeing must address.

How have Boeing and the FAA responded?

Boeing officials have insisted safety is their top priority and has said its engineers are working to fix software problems and other issues. The company has since developed a software fix for the MCAS system and announced an overhaul of internal safety procedures and reporting protocols.

Since the Lion Air accident, the "737 Max and its software are undergoing an unprecedented level of global regulatory oversight, testing and analysis," Boeing said in a statement responding to the Indonesia report. It added that the company is "updating crew manuals and pilot training, designed to ensure every pilot as all [the] information they need to fly the 737 Max safely."

The FAA also issued a statement saying it welcomed the report's recommendations and would consider them as it reviewed proposed changes to the Boeing 737 Max.

"The FAA is committed to ensuring that the lessons learned from the losses of Lion Air Flight 610 and Ethiopian Airlines Flight 302 will result in an even greater level of safety globally," it read.

Sources told NPR in June that FAA test pilots had discovered an additional issue that "affected their ability to quickly and easily follow recovery procedures for runaway stabilizer trim and stabilize the aircraft." Boeing said it was working on the required software fix and was working closely with the FAA to "safely return the 737 Max to service."

Last week, Boeing reported a 50% drop in profits in the third quarter related to the grounding of the Max. Both domestic and foreign airline carriers have experienced shrinking profits and crowded planes as a result of grounding their 737 Max aircraft.

Boeing also announced last week that it would be replacing the head of its commercial airplanes unit, the first firing of a top executive since the crashes. Kevin McAllister had arrived at Boeing when the 737 Max was already in the final stages of design and certification but had faced customer criticism over his handling of the crisis.

What's happening on the Hill?

Boeing officials faced intense questioning at Tuesday's hearing, and tensions are expected to remain high on Wednesday.

Boeing CEO Muilenburg acknowledged the company had made mistakes and was working closely with the FAA and other regulators to fix them.

"Regulators around the world should approve the return of the MAX to the skies only after they have applied the most rigorous scrutiny, and are completely satisfied as to the plane's safety," Muilenburg told lawmakers. "The flying public deserves nothing less."

Senators asked Muilenburg whether the company intentionally hid information about issues with the MCAS system from regulators and pressed him on why Boeing pushed regulators to permit 737 Max planes to continue flying after the Lion Air crash.

Sen. Tom Udall, D-N.M., criticized the company for being in "too cozy a relationship"with the FAA, and Sen. Ted Cruz, R-Texas, asked why the 2016 pilot text messages were not turned over until earlier this year. Muilenburg answered that he did not see the details of the exchange until "recently" and supported "diving deep into this and understanding what he said and what he meant." He disputed the idea that the company would "lie or conceal."

"We know we made mistakes and got some things wrong," Muilenburg said in his opening statement. "We own that, and we are fixing them."

Among software improvements and organizational restructuring efforts, Muilenburg said, the company has also pledged $100 million to help meet the financial needs of the communities and families affected by the crashes.

Nearly 20 family members from several countries who lost loved ones in the Ethiopian Airlines crash are attending the hearings. At one point on Tuesday, they were asked to stand up while holding photographs of their lost relatives.

Family members, who had just returned from collecting the remains of those killed in the crash, also spoke with Muilenburg in a private meeting after his testimony.

Andrew Harnik/AP

Senators criticized Boeing President and CEO Dennis Muilenburg at a hearing on 737 Max safety.

Rachel Treisman is an intern on NPR's National Desk.

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