The head of the U.S. National Transportation Safety Board (NTSB) criticized U.S. aviation regulators for “downplaying the urgency” of addressing a possible rudder malfunction on some Boeing 737 aircraft, in an unusually public rebuke.  

Faulty parts that could cause the 737’s rudder control system to jam may be in use by at least 40 non-U.S. carriers, NTSB Chair Jennifer Homendy said in a letter to the Federal Aviation Administration’s (FAA) top official on September 30. Information provided by Boeing in August suggests that 271 suspect components may be in use on aircraft operated by those overseas airlines, while 16 may still be in use in the U.S., Homendy said. Dozens more haven’t yet been installed.

In the letter to FAA Administrator Michael Whitaker, Homendy said she is concerned that the FAA “did not take this issue more seriously until we issued our urgent safety recommendation report." 

The unusual public criticism comes after the NTSB last week warned that faulty parts supplied by Collins Aerospace, a unit of RTX Corp., for some 737 Max and 737 NG aircraft could lead to the rudder system jamming. The NTSB issued urgent recommendations to the FAA and Boeing on the issue.

The FAA’s oversight of Boeing has been criticized since a door-sized panel blew off a 737 Max during a January flight. The agency has also faced scrutiny in the wake of two 737 Max crashes in 2018 and 2019 that killed 346 people.  

The FAA on September 30 said that it is taking the matter seriously and moving quickly to contact aviation authorities in other countries to “ensure they have the information they need from the FAA including any recommended actions.” The regulator is also planning additional simulator testing in October, it said in a statement.

The NTSB’s recommendations stem from an investigation into a February 6 incident in which rudder pedals on a 737 Max 8 operated by United Airlines got stuck during landing at Newark Liberty International Airport in New Jersey. 

The safety board said about 350 parts were affected by the issue. Last week, the FAA said United is the only U.S. airline that had the affected parts and that the regulator’s “understanding” is the parts are no longer in service. 

United said on September 30 that it “removed this component from our nine affected aircraft earlier this year.” The NTSB didn’t clarify which other U.S. airlines might be using the parts.

Boeing couldn’t immediately be reached for comment.