On Thursday, bipartisan senators sought amendments to a significant defense bill after warnings from crash investigators and victims’ families. They expressed concerns that the legislation might reverse critical safety reforms established after a devastating collision between an airliner and an Army helicopter in Washington, D.C., resulting in 67 fatalities.

The National Transportation Safety Board’s lead investigator, along with victims’ families and senators from the Commerce Committee, criticized the bill advanced by the House on Wednesday. They argued it could compromise aviation safety in the U.S. by allowing the military to revert to pre-crash practices. This incident was deemed the most catastrophic in over 20 years.

Senators Maria Cantwell, a Democrat, and Ted Cruz, the Republican Chairman of the Committee, introduced two amendments on Thursday. These amendments aim to remove problematic helicopter safety provisions and substitute them with a proposal from last summer to enhance safety requirements. However, there is uncertainty over whether Republican leadership will permit changes to the National Defense Authorization Act at this stage, as it could delay its approval.

“It’s our duty to ensure that genuine safety improvements become law, rather than providing the Department of Defense with more loopholes,” the senators emphasized.

The bill would roll back reforms

Presently, the bill includes allowances that would enable military helicopters to navigate the congested airspace around the nation’s capital without employing the ADS-B system, which broadcasts their locations. This system became mandatory by the Federal Aviation Administration in March. NTSB Chairwoman Jennifer Homendy described the bill as a “significant safety setback” that risks a repeat of the past disaster.

“It represents an unacceptable risk to the flying public, to commercial and military aircraft, crews and to the residents in the region,” Homendy said. “It’s also an unthinkable dismissal of our investigation and of 67 families … who lost loved ones in a tragedy that was entirely preventable. This is shameful.”

The biggest unions representing pilots, flight attendants and other transportation workers joined the chorus criticizing the bill on Thursday. Sara Nelson, who is president of the Association of Flight Attendants, questioned why this was proposed. She said these provisions are “not only reckless and indefensible, but also a direct undermining of the NTSB’s safety guidance.”

Congress may turn to another bill to fix the concerns

Senate Majority Leader John Thune said he is looking into the concerns but thinks they can be addressed by quickly passing the aviation safety bill that Cruz and Cantwell proposed last summer that would require all aircraft operators to use both forms of ADS-B, or Automatic Dependent Surveillance Broadcast, the technology to broadcast aircraft location data to other planes and air traffic controllers. Most aircraft today are equipped with ADS-B Out equipment but the airlines would have to add the more comprehensive ADS-B In technology to their planes.

That legislation would also revoke an exemption on ADS-B transmission requests for Department of Defense aircraft.

“I think that would resolve the concerns that people have about that provision, and hoping — we’ll see if we can find a pathway forward to get that bill done,” said Thune, a South Dakota Republican.

Military routinely flew without ADS-B turned on

The military used national security waivers before the crash to skirt FAA safety requirements on the grounds that they worried about the security risks of disclosing their helicopters’ locations. Tim and Sheri Lilley, whose son Sam was the first officer on the American Airlines jet, said this bill only adds “a window dressing fix that would continue to allow for the setting aside of requirements with nothing more than a cursory risk assessment.”

Military helicopters like the Black Hawk involved in the crash did send some location data to controllers through a transponder, but the FAA has said that ADS-B data is more precise and the NTSB has been recommending for decades that all aircraft be equipped with such systems. The Army was concerned about using those systems because anyone — including a plane enthusiast on the ground — can use them to know precisely where a helicopter or airplane is located.

Homendy said it would be ridiculous to entrust the military with assessing the safety risks when they aren’t the experts, and neither the Army nor the FAA noticed 85 close calls around Ronald Reagan National Airport in the years before the crash. She said the military doesn’t know how to do that kind of risk assessment, adding that no one writing the bill bothered to consult the experts at the NTSB who do know.

The NTSB’s final report on the cause of the D.C. crash won’t be released until next year, but investigators have already identified a number of factors that contributed, including that the helicopter was flying too high on a route that only provided scant separation between helicopters and planes landing on Reagan’s secondary runway.

Homendy said part of the investigation focuses on the limitations of the various systems that are designed to alert other pilots and air traffic controllers about the location of an aircraft. The pilots of the American jet that was flying into D.C. from Wichita, Kansas, did get a warning about traffic nearby 20 seconds before the collision. But at the low altitude the plane was traveling as it prepared to land, the basic collision avoidance system recommended by this bill was partly inhibited to prevent false alarms and because there is little room to manuever.

The White House and military didn’t immediately respond Thursday to questions about these safety concerns in the bill. But earlier this week Trump made it clear that he wants to sign the National Defense Authorization Act because it advances a number of his priorities and provides a 3.8% pay raise for many military members.

The Senate is expected to take up the bill next week, and it appears unlikely that any final changes will be made. But Congress is leaving for a holiday break at the end of the week, and the defense bill is considered something that must pass by the end of the year.