The head of the National Transportation Safety Board blasted the Federal Aviation Administration, saying the agency is not taking seriously enough the potential for jammed flight controls on some Boeing 737s.

In a new letter shared with CNN, NTSB Chair Jennifer Homendy wrote FAA Administrator Mike Whitaker to say that the regulator failed to act when the pilots of a United Airlines Boeing 737 MAX reported that their rudder pedals became stuck when coming in for a landing in Newark, New Jersey on February 6.

Last Thursday, the NTSB issued an “urgent” safety warning, saying that some 737s equipped with certain rudder actuators could careen off the runway on landing – the latest black eye for Boeing’s embattled 737 MAX line after a door plug blowout in January and two fatal crashes abroad.

“I am disappointed that it does not appear that the FAA has initiated urgent actions to address the risk of jammed rudder controls in the 6 months since our preliminary report on this incident was issued,” Homendy wrote.

United Airlines says the rudder actuators in question have been removed from its 737s, but Homendy says “we are concerned of the possibility that other airlines are unaware of the presence of these actuators on their 737 airplanes.”

In a statement, the FAA says it has convened a team to address the issue, is sending notification to airlines internationally, and will do more simulator testing. The FAA said, “we thank the NTSB for the recommendations, and we are taking them seriously.”