Engineers and technicians responsible for the building of Boeing aircraft are reluctant to speak up about safety concerns, and when they do, they are not being heard, a panel of witnesses told Senate lawmakers Wednesday.

Three members of an FAA-appointed safety panel created to review Boeing’s safety culture following 737 Max crashes appeared before the U.S. Senate Commerce Committee in Washington, D.C., to review its findings in a February report that were critical of the aerospace giant.

Also testifying Wednesday in a separate hearing on Capitol Hill was Sam Salehpour, a Boeing engineer and whistleblower, who told the Senate Homeland Security and Governmental Affairs Committee’s investigations subcommittee that more than 1,000 Boeing 787s should be grounded due to safety risks.

“Good engineering wins the day, but you have to listen to them,” said Senator Maria Cantwell (D-Wash.),  who also chairs the Commerce Committee, said during the hearing to determine if there are more steps the federal government can take to ensure Boeing aircraft are safe to fly.

In 2018, Lion Air Flight 610 crashed, killing all 189 on board. The next year, 157 died when Ethiopian Airlines Flight 302 crashed shortly after takeoff. Following the incidents, investigators determined that both crashes were attributed to the Maneuvering Characteristics Augmentation System, commonly referred to as MCAS, acting on false data from a single angle of attack sensor that put the aircraft into unrecoverable dives shortly after takeoff.

In response, the FAA grounded the jets worldwide for several months while the cause of the crashes were investigated. The investigation pointed to a series of faulty technical assumptions by Boeing’s engineers, a lack of transparency on the part of the company’s management, and what was described as “grossly insufficient oversight” by the FAA.

The FAA panel reviewing Boeing consisted of 24 members, all considered experts in their field. Among them, Javier de Luis, an aeronautics lecturer at the Massachusetts Institute of Technology, whose sister was killed in the 2019 Ethiopian Airlines crash. De Luis noted the panel spent a year reviewing 4,000 pages of documents provided by Boeing and interviewed 250 company employees at all levels of the organization, across six Boeing locations. The effort resulted in “27 findings and 53 recommendations” for the improvement of safety at the company, he told lawmakers Wednesday.

According to the panel’s findings, Boeing has made changes since the 737 Max crashes, but there is still room for improvement. Although management tells the employees to speak up if they have a safety concern, they are reluctant to do so, fearing retaliation. Others interviewed by the panel noted that their concerns—even when raised— are ignored.

An engineer, for example, warned Boeing about the potential for lithium-ion batteries aboard the Boeing 787 Dreamliners to overheat due to thermal runaway. The Dreamliner entered service in 2011 and was grounded by FAA emergency order in 2013 due to fires from overheated batteries.

“There exists a disconnect between the words that are being said by Boeing management and what is being seen and experienced by the technicians and engineers,” de Luis said.

Following the hearing, Boeing released a statement, saying it took the FAA review panel’s critiques “to heart and will act on their findings and feedback. Since 2020, Boeing has taken important steps to foster a safety culture that empowers and encourages all employees to raise their voice. We know we have more work to do, and we are taking action across our company.”

According to the company, employee reports through its “Speak Up” portal increased 500 percent since January, which it said indicated “progress toward a robust reporting culture that is not fearful of retaliation.”

The FAA panel released its findings in February just a few days after a Boeing 737 Max 9 experienced explosive decompression when it lost a door plug in its fuselage midflight while en route to California from Portland, Oregon.

The accident resulted in a mass grounding of the aircraft and reopened questions about the manufacturer’s process and attention to safety, including the documentation of repairs made during the production phase. It was determined the bolts that hold the door plug in place had not been reinstalled after corrective maintenance on a line of rivets in the fuselage.

Boeing was given 90 days to issue a plan to address the results and recommendations. That deadline for release of the plan is May 28.

During Wednesday’s hearing, Senator Eric Schmitt (R-Mo.) noted there were no representatives from Boeing in attendance: “We should be hearing directly from Boeing.”

The 737 is not the only aircraft under scrutiny. The 787 Dreamliner is under examination as well.

Salehpour, who has worked for Boeing for 10 years as a quality engineer, called for a global grounding of the 787 Dreamliners, saying the shortcuts he allegedly witnessed on the factory floor during the building of 787s and 777s that may have led to the misalignment of parts in the jet fuselage. In an interview with NBC, he suggested that employees took shortcuts that may have resulted in parts of the jet being misaligned, which could lead to metal fatigue and weakened fuselage.

Boeing pushed back on those claims.

“Extensive and rigorous testing of the fuselage and heavy maintenance checks of nearly 700 in-service airplanes to date have found zero evidence of airframe fatigue,” Boeing said in a statement Wednesday. “Under FAA oversight, we have painstakingly inspected and reworked airplanes and improved production quality to meet exacting standards that are measured in the one hundredths of an inch. We are fully confident in the safety and durability of the 787 Dreamliner.”

WATCH: Whistleblower Testifies at Senate Hearing on Boeing Safety Culture

Editor’s Note: This story first appeared on FlyingMag.com.