US regulators are seeking to fine Boeing $5.4m (£4.14m) for “knowingly” installing faulty parts on 737 Max planes.
The move comes after the release of internal messages that raised more questions about the jet’s safety.
In one of the communications, an employee said the plane was “designed by clowns”.
Boeing has been under scrutiny since the fatal crashes of two 737 Max planes, which killed 346 people.
The fine announced by the Federal Aviation Administration (FAA) on Friday is not connected to the software system that investigators have implicated in those crashes.
It concerns “slat tracks” that are located on the wings.
The FAA said the company submitted the jets for FAA approval despite determining that the wing parts had failed a strength test. It also accused Boeing of failing to oversee its suppliers properly.
The planemaker has the right to contest the penalty, which follows a $3.9m fine the FAA proposed against the US aerospace giant for similar reasons last month.
Boeing did not immediately respond to a request for comment.
The announcement heightens the pressure on the company, which is now facing multiple investigations following the 737 Max crashes in Indonesia and Ethiopia.
Last month, the company fired chief executive Dennis Muilenburg. The firm said Friday it had denied him severance and that he had forfeited stock awards worth about $14.6m.
As part of the investigations, Boeing has provided hundreds of messages to the FAA and Congress. It said it had released redacted versions this week as part of its commitment to transparency.
“These communications do not reflect the company we are and need to be, and they are completely unacceptable,” Boeing said. (BBC)