CHICAGO (CBS)–Chicago-based Boeing has outlined a new safety plan for its planes, including a software fix and new training for pilots.

The changes follow two crashes that occurred in the past five months, killing hundreds of people.

The acting FAA administrator was grilled by senators, who demanded to know what caused the failures that led to to two 737 Max planes going down, in Ethiopia and Indonesia.

Between the two crashes, 350 people died.

During a Wednesday hearing of the Senate Subcommittee on Aviation and Space, senators questioned federal aviation and transportation safety officials how Boeing was able to sell two critical safety features as “optional” on the 737 Max.

“Should the FAA ban the practice of selling features ala carte to the airlines? Yes or no?” asked U.S. Sen. Ed Markey (D-MA).

“I will tell you that if there is any manufacturer that sells a safety-critical part a la carte, we will not permit it,” Acting FAA Administrator Daniel Elwell said.

The planes that crashed did not have those options.

Boeing says pilots must have additional training before they’re allowed to return to the cockpit.