The story has been updated November 7 to reflect the issuing of the service bulletin from Boeing and Emergency Airworthiness Directive from the Federal Aviation Administration.
Boeing has issued a service bulletin to 737 Max 8 and 9 operators worldwide warning operators that the jet’s angle of attack (AOA) sensor can produce erroneous indications causing the single-aisle jet to enter an aggressive dive, according to the company.
The bulletin comes after a newly-delivered Lion Air 737 Max crashed off the coast of Indonesia on October 29, killing all aboard. The published bulletin specifically identified Indonesian air safety officials findings that Lion Air flight 610 experienced erroneous AOA input.
The investigation into the crash is on-going, but the factual findings by Indonesian investigators and the resulting guidance from Boeing and impending regulatory directive provides a central theme as the multi-national team continues its inquiry into the brand new jet’s crash. The precise cause of the erroneous AOA data has yet to be identified, but a senior industry official familiar with the details of the investigation said a software issue is a focus of the inquiry.
According to the bulletin, Boeing warns operators that the AOA issue can occur during only manual flight. The erroneous AOA input can pitch the aircraft’s stabilizer trim down for up to 10 seconds as a time.
The plane maker cautions pilots to use the electric stabilizer trim to reverse the downward stabilizer trim, but the nose-over can begin again five seconds after those switches are released.
The repeated uncommanded nose down action can be stopped by deactivating the stabilizer trim system, according to the bulletin. Boeing warns that the stabilizer system can reach its full downward position if not counteracted by pilot trimming the aircraft and disconnecting the stabilizer trim system.
Subscribe to TAC“Whenever appropriate, Boeing, as part of its usual processes, issues bulletins or makes recommendations regarding the operation of its aircraft,” the company said in a statement, without discussing the details of the bulletin. Boeing confirmed it issued an Operations Manual Bulletin (OMB) on November 6 “directing operators to existing flight crew procedures to address circumstances where there is erroneous input from an AOA sensor.”
The service bulletin was the prelude to the formal emergency airworthiness directive from the Federal Aviation Administration issued on Wednesday. The FAA said the directive was prompted by an “analysis performed by” Boeing found that “an erroneously high angle angle of attack (AOA) sensor input” can cause “repeated nose-down trim commands of the horizontal stabilizer.”
The emergency directive calls for U.S.-based 737 Max 8 and 9 operators to revise operating procedures to flight crew handling runaway horizontal stabilizer trim motion. The FAA said 246 Max aircraft are operating worldwide, 45 in U.S. with Southwest Airlines, American Airlines and United Airlines.
Bloomberg News first reported the impending operator notification.
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