Two employees at Boeing had exchanged troubling messages on the now-grounded passenger jet. The company knew about it but waited for several months before passing them on to the authorities, US officials have said.
Boeing should "immediately" explain employees' messages concerning its 737 MAX passenger jet and its decision to delay informing the authorities about them, the US Federal Aviation Administration (FAA) said on Friday.
"Last night, I reviewed a concerning document that Boeing provided late yesterday to the Department of Transportation," FAA Administrator Steve Dickson said in a brief letter to Boeing CEO Dennis Muilenburg.
"I understand that Boeing discovered the document in its files months ago," he added. "I expect your explanation immediately regarding the content of this document and Boeing's delay in disclosing the document to its safety regulator."
According to the FAA, the item shows instant messages exchanged by two Boeing employees in 2016. The messages were sent while the FAA was certifying the 737 MAX aircraft.
Investigators suspect the MCAS system automatically pushed the noses of the aircraft downward based on readings from a single malfunctioning sensor, which incorrectly told the MCAS that the aircraft were entering a stall. This presumably played a key role in two crashes which killed a total of 346 people in Indonesia and Ethiopia. Regulators worldwide responded by grounding the latest 737 model.
Boeing said the document, which was finally disclosed on Thursday, contained communication of a former employee. It did not identify the author or provide an explanation for the apparent delay.
On Friday, the FAA said it was "disappointed" that Boeing failed to immediately inform it about the item. It also said there was no "prescribed timeline" for returning the 737 MAX to service.
"The agency will lift the grounding order only after we have determined the aircraft is safe," the FAA said.