Federal Aviation Administration (FAA) chief Dan Elwell faced questions Wednesday on Capitol Hill from lawmakers about when he expects airplane manufacturer Boeing to release a software fix for its controversial 737 MAX airplane after recent crashes.

Democratic lawmakers are particularly skeptical of how the FAA under Elwell and Boeing have handled the incident and concerns have been raised towards how the FAA certifies aircraft to be ready to fly. Boeing's design choices and communication protocols were also subject to scrutiny.

"The world is watching and the FAA and Boeing must get it right," Democrat Rep. Peter Defazio of Oregon said.

"The FAA needs to fix its credibility problem," Democrat Rep. Rick Larsen of Washington added.

Rep. Thomas Massie, a Republican from Kentucky, tweeted that how the FAA handled the crash "shouldn't be a partisan issue."

Elwell said he expects to receive the software upgrade in the "next week or so" and that the FAA will grant federal permission to the 737 MAX to resume flying when it is "absolutely safe to do so… it's important we get this right," Reuters reported.

The 737 Max encountered a software anomaly that led to the airplane crashing after takeoff in Ethiopia in March and in Indonesia in October. The two crashes led to 346 fatalities and the U.S. planemaker grounded the model worldwide.

One revelation made during the hearing was that both pilots in the Ethiopia and the Indonesia crash received stick shaker alerts, a warning that urges the pilot to return to the airport of origin due to an imminent engine stall in the plane.