Indonesian investigators on Wednesday said a malfunctioning flight-control system fed widely divergent readings to the pilot and co-pilot during the second-to-last flight of the Lion Air jet that plummeted into the sea last week.
If cockpit crews fail to respond appropriately to such conflicting data, automated safety protections on a Boeing Co. 737 Max 8 model can put the plane into a steep dive, according to safety experts and an alert issued by the plane maker.
In extreme cases, according to safety experts, such automated commands can overwhelm pilots’ manual inputs that would maintain safe flight.
Investigators are trying to determine whether that sequence of events contributed to the disaster.
In response to the crash,
late Tuesday issued a new bulletin about how pilots should react when they suspect the plane’s safety systems are receiving faulty information from certain sensors.
The U.S. Federal Aviation Administration on Wednesday issued a mandatory, emergency directive flagging the potential safety hazard.
It noted that the flight-control system could result in cockpit crews experiencing “difficulty controlling the airplane” and in extreme cases, even lead to a crash,
It requires airlines to incorporate more-specific operational instructions in flight manuals for 737 MAX 8 and Max 9 variants within three days. That would help pilots counteract such dangers when they encounter confusing or inaccurate information about airspeed and trajectory.
Boeing’s notice amounts to an official warning for pilots, highlighting potential hazards from the interaction of certain software with other cockpit alerts.
The directive generally tracks information contained in Boeing’s voluntary safety bulletin, but appears somewhat more detailed.
The Boeing bulletin and the Indonesian investigators’ revelation together were the first public indications that investigators are looking at a suspected software issue—possibly combined with misinterpretation by pilots—related to an essential system that measures a plane’s “angle of attack,” or how high or low a plane’s nose is pointed.
The company alert doesn’t call for replacement or inspection of any specific onboard system. But it does serve to remind pilots about the threats that can arise when they suspect certain sensors are malfunctioning.
In such instances, the Boeing bulletin stresses that proper procedures include turning off certain automated protections. Those safeguards can have the unintended consequence of pushing the plane’s nose down, even if pilots repeatedly act to raise it with manual controls. Those automated commands can be so extreme that they result in the plane reaching “its nose-down limit,” according to Boeing’s message.
The Indonesian probe hasn’t determined that such a scenario led to the crash of Lion Air Flight 610, which plunged into the Java Sea at a steep angle and high speed last week, killing all 189 aboard. There will be months of further data analysis and flight-simulator recreations of the event before definitive conclusions are released.
But the investigators’ latest statements, combined with Boeing’s bulletin, offer the best glimpse yet into the challenges confronting the pilots as they struggled to control the aircraft. It’s not clear whether regulators or Boeing intend to take further safety actions in response to the crash.
The wide-ranging probe also is looking into the aircraft’s software design, Lion Air’s pilot-training requirements and the carrier’s maintenance practices, according to people familiar with the matter.
The crash is the first major accident involving a Boeing 737 Max 8, the latest variant of its popular 737, raising scrutiny over the new model. The plane that crashed was delivered in August to Lion Air, one of Asia’s largest low-cost carriers. Boeing is participating in the investigation of the crash, along with U.S. safety experts.
Indonesia investigators have said that the Lion Air jet’s last four flights all experienced airspeed-indicator problems. On Wednesday, they said that after the first two of those flights, technicians chose to change an angle-of-attack sensor. The two systems are related, though the jet’s cockpit doesn’t have a devoted angle-of-attack display.
But such measurements factor into calculating all-important airspeed. The speed values are then relayed to both the pilot and co-pilot in separate indicators in the cockpit, with disagreements in the readings likely resulting in additional cockpit alerts and cautions.
After the replacement of the sensor, the jet was cleared for its second-to-last flight. Investigators, citing information gleaned from the jet’s recovered flight-data recorder, said the angle-of-attack inputs for airspeed for the pilot and the co-pilot differed by 20 degrees during that flight.
The problem of divergent airspeed indications was identified by the crew, which made in-flight adjustments and was able to continue to Jakarta and requested a priority landing. The pilots flew manually for about an hour and 45 minutes, according to a person familiar with the matter, which is highly unusual for a modern passenger jet.
Airline and FAA safety experts haven’t identified a previous pattern of similar software issues in the Boeing 737 Max 8 fleet, according to U.S. government and outside safety experts familiar with the details.
During the plane’s last trip, the crew reverted to manual flight after experiencing unreliable airspeed indications shortly after takeoff from Jakarta in good weather, according to preliminary information gathered by the investigation. Minutes after the crew communicated the situation to air-traffic controllers and gradually gained altitude as part of an apparent effort to troubleshoot the problem, the twin-engine plane plunged into the water at a steep angle and high speed.
—Robert Wall, I Made Senata and Andrew Tangel contributed to this article.
Appeared in the November 8, 2018, print edition as ‘Indonesia Crash Probe Turns to Flight System Indonesia Crash Probe Looks at Flight Gear.’