Boeing has made changes to a controversial control systems related to two fatal crashes of its 737 Max aircraft, which has been released in the past five weeks.
But it is still not certain when the planes that were grounded worldwide this month are allowed to fly.
Investigators have not yet identified the cause of the accident.
As part of the upgrade, Boeing will install a warning system by default, which was previously an optional security feature.
None of the planes operated by Lion Air in Indonesia and Ethiopian Airlines, which were involved in the fatal crashes, bore the warning systems to warn pilots when sensors produce conflicting readings.
Boeing said that airlines would no longer be charged extra for installing the security system.
The planemaker has also issued an upgrade to the software associated with the crashes.
The maneuvering characteristics Augmentation System (MCAS), designed to keep the aircraft from falling, responds to sensors that detect whether the beam climbs at a too steep angle.
But an investigation of the Lion Air flight last year revealed that the system failed-war and nose of the aircraft forced more than 20 times down before it crashed into the sea and killed all 189 passengers and crew.
The US Federal Aviation Administration (FAA) says there are similarities between this crash and the Ethiopian accident on October 10, 2014.
Boeing has redesigned the software to disable MCAS when it receives conflicting data from its sensors.
Ethiopian Airlines crash: six charts of what we know
The FAA itself was also examined on Wednesday.
At a Senate hearing to discuss aviation safety, senators asked the acting head of FAA, Daniel Elwell, how the regulatory authority involves the employees of an aircraft manufacturer in the inspection, testing and certification of the company’s own aircraft.
The practice was described by a senator Richard Blumenthal, who left as “the Fox guarded the chicken house”.
Mr. Elwell denied that it was a” self-certification “and argued that the FAA kept” the firm’s strict regulatory authority”. He said that the practice was used “globally”, including by the European Aviation Safety Agency.
Mr. Elwell added that if the FAA cannot delegate these tasks to the planner, should you hire 10,000 more employees, the regulatory authority an additional 1.8 billion