The largest US pilot union is urging US officials to address what it calls a significant aviation "safety deficiency" related to a new flight control system on Boeing's 737 Max.
In a letter to the heads of the Federal Aviation Administration and National Transportation Safety Board (NTSB), the Air Line Pilots Association (ALPA) requests help in ensuring pilots receive all relevant information about Boeing's 737 Max.
The letter references a new flight control system Boeing reportedly did not disclose.
"We are concerned that a potential, significant aviation system safety deficiency exists, and we are writing to ask for your immediate help and assistance in clarifying the issues with respect to the pitch control system of the aircraft," says the 15 November letter from ALPA, which represents some 61,000 pilots.
"Reports indicate that information regarding the normal and non-normal operation of this system was not provided to… flight crews and maintenance technicians," it adds. "The lack of critical safety information being provided to the air carriers and frontline operators is concerning."
The letter cites reports that Boeing did not tell airlines that it equipped the 737 Max with the so-called "manoeuvring characteristics augmentation system" (MCAS).
That technology pitches the aircraft's nose down at high atitudes if flight data tells the system the aircraft is close to stalling. But the system might also send the aircraft diving if it receives incorrect angle-of-attack data – a scenario implicated as a factor in the 29 October crash of Lion Air flight 610, which killed 189 people.
Boeing reportedly created the system because the 737 Max behaves slightly different in flight from earlier-generation 737s, and the technology nullifies some of those differences.
The FAA declines to comment about ALPA's letter, saying it is assisting international counterparts with the Lion Air investigation. The agency also says it is not separately investigating Boeing, countering a recent media report.
Boeing also declines to comment about the letter but says it is working to understand the Lion Air crash and remains confident in the 737 Max's safety.
The NTSB did not immediately respond to a request for comment.
Following the Lion Air crash Boeing issued a service alert calling attention to established cockpit procedures for countering uncommanded stabiliser movements.
The FAA responded with a similar airworthiness directive.
American Airlines and Southwest Airlines – both 737 Max operators – have since said Boeing never told them about the existence of the MCAS.
The Allied Pilots Association, which represents American's pilots, and the Southwest Airlines Pilots Association have said they were in the dark, too.