During the three weeks before Lion Air Flight 610 dived into the waters off Indonesia,
it replaced two malfunctioning flight control sensors of the same type that were publicly implicated in the accident, according to a summary of the southwest maintenance records reviewed by the Wall Street Journal.
Both US maintenance issues involved a
737 MAX 8, the same model that crashed last month in Indonesia. The sensors measure whether the jetliner is tilted above or below the flight level. Those sensors, or related hardware, needed repairs in southwestern instances, according to the summary document. The document also indicates that the Southwest pilots reported not being able to activate automatic engine settings, similar to cruise speed control on a car.
A Southwest spokeswoman said the sensors did not fail and were removed as a precautionary measure as part of a problem-solving process. He said that at least one was repaired.
Investigators confirmed the same type of sensor failed on the Lion Air flight, but did not accurately determine what happened between that failure and the accident.
From the accident, which killed 189 people, Boeing warned airlines of the potential for incorrect data from so-called "angle of attack" sensors. "We did not find a sensor failure or a flight problem as described in Boeing's bulletin," the Southwest spokesperson said.
The accidents in the south-west did not cause emergencies and nobody was hurt. They pushed what appear to be routine reports from mechanics who experience sensor problems. One was written on October 9 in Baltimore and the other on October 21 in Houston, the documents show, and indicate that both sensors have been repaired.
Turn the switch
Pulling back the lever will not stop a barn prevention system in Boeing's new 737 MAX floors, if the nose comes down, if you are getting incorrect sensor data. But turning off the system will do it.
In the 737 older models of Boeing,
a common practice of pulling back the control column (yoke) will prevent the cab systems from automatically pushing down the nose of the aircraft.
In the new 737 Max models by Boeing,
pulling back the yoke will not work if its stall prevention system is receiving faulty data from the sensors.
But an existing procedure works for both:
the pilots rotate the switches to prevent the aircraft from pushing the nose down.
Just eight days later, shortly after the Lion Air jet took off from Jakarta on October 29, an attack angle sensor on that flight sent erroneous data to flight control computers, according to Indonesian and US investigators. Boeing and the Federal Aviation Administration subsequently issued high priority security notices, highlighting that the problem occurred. The resulting sequence of events is at the center of the international crash probe.
Boeing and FAA did not have immediate comments on the southwest maintenance documents. The Southwest spokeswoman said the carrier replaced the two sensors in October on the same aircraft and determined that they were not the source of the automatic throttle problem. Two other Southwest 737 MAX 8s have replaced an attack angle sensor as part of routine maintenance due to external damage, such as a bird's shot in flight.
Security experts point out that it is too early for the definitive answers on the LIon Air tragedy because a number of other factors were at stake. But until now, investigators suspect that the angle of attack problems may have touched a burst of interactions between the various flight control computers and the pilot actions that led to the crash.
Within the next few weeks, Boeing and the FAA should agree on a mandatory safety standard that modifies some MAX 8 flight control software, according to industry and government officials involved in process monitoring. The most likely change, they said, will deactivate the automatic nose-down controls in the event that there is a large disparity between the angle of attack data flowing to the co-pilot's captain and cockpit displays. Some officials anticipate action as soon as two weeks.
In the context of the American-Indonesian cooperative effort to unveil the complex interaction between the automated systems and the crew of the cockpit of Lion Air, the investigators are now deepening the lessons learned over the years on the potentially disastrous consequences. defective or unreliable angle of attack indicators, according to the US government and industry safety experts.
Aircraft manufacturers and aviation security regulators have long realized and addressed the flight control risks arising from such malfunctions on a number of other models, including long-range Airbus SE A330 and A340 aircraft, as well as body jets. narrow A320 and A321 widely used by the European manufacturer.
An important aspect of the Lion Air probe is to examine whether Boeing's engineers and technical managers, supervised by the FAA, have fully incorporated those previous lessons on the design of the cockpit automation for the 737 MAX 8 and MAX 9 fleet. There are currently over 200 Max variants that have been delivered worldwide, out of thousands of 737 in total.
The certification processes for the new models "include consideration for lessons learned from accidents and in-service events," an FAA spokesperson said in a written statement on Thursday, but "care must be taken when making comparisons such as aircraft are not all the same and respond differently from sensor inputs. "
The spokesperson confirmed that a look back on security events, including those caused by attack angle problems and how the agency responded to them, is "part of the ongoing investigation".
Since the October incident, Boeing officials have refused to discuss probe specifications or flight control system now under scrutiny, saying they are helping and cooperating with investigators and regulators.
Without elaborating, the Chicago aircraft manufacturer said the latest 737 models are safe and pilots have previously been provided with appropriate checklists to counter the dangers of repeated and automatic nose commands. Boeing also stated that it is "taking all measures to fully understand all aspects" of the crash.
But at this point, much of the investigative work, led by the Indonesian authorities but assisted by US incident investigators, by the FAA regulators and Boeing itself, seems to be focused on potential dangers arising from problematic attack angle values.
The Lion Air pilots indicated that before the accident they had not been informed of the new flight control system that had been implicated by the investigators.
"Not just me, but others as well," said Yusni Maryan, a senior airline pilot in Indonesia flying MAX jets. "[Boeing] I did not specifically mention it. "
Boeing has not commented on communications with specific airlines or their pilots.
-Ben Otto contributed to this article.
Write to Andy Pasztor on firstname.lastname@example.org and Andrew Tangel on Andrew.Tangel@wsj.com