One day before the Lion Air plane crash on October 29, the sensor of the angle of attack (AOA) of the nearly new Boeing 737 Max 8 jet had appeared erroneous, according to Indonesian National Transportation Safety Committee (KNKT) on Wednesday.
Boeing, the maker of the two-month old 737 Max plane, issued a safety bulletin directing pilots on what to do whenever there is an erroneous AOA sensor.
The FAA said that if the condition is not addressed, it could cause the flight crew to have difficulty controlling the plane, possibly with significant loss of altitude.
Experts say the angle of attack is a crucial parameter that helps the aircraft's systems understand whether its nose is too high relative to the current of air - a phenomenon that can throw the plane into an aerodynamic stall and make it fall. Flight crews are taught to handle "uncommanded nose-down stabilizer trim" by memorizing a procedure to disengage the angle-of-attack inputs to the plane's computer system.
The replaced AOA sensor has been sent to Jakarta for investigation by the KNKT and later it will be sent to its manufacturer in the United States for a further probe, said Tjahjano.
Some of the families of the victims aboard Flight 610 were taken by boat to the crash site location on Tuesday to pray for their loved ones and view the recovery process in-person, Muhammad Syaugi, head of Indonesia's Search and Rescue Agency said.
Soerjanto Tjahjono, chairman of the transport safety committee, said on Wednesday that airspeed indicator malfunctions on the jet's last four flights, which were revealed by analysis of the flight data recorder, were intertwined with the sensor issue.
A Lion Air passenger plane reportedly hit a lamp post, fewer than two weeks after Indonesia's discount carrier crashed a jet off the coast of Java, killing all 189 people on board. It stressed that pilots should follow procedures in the flight manual when encountering erroneous data.
That angle-of-attack sensor is meant to measure the direction of air flow over wings so that they maintain lift. The malfunction can cause the computers to erroneously detect a mid-flight stall in airflow, triggering a dive to regain speed to keep flying.
Shortly after Boeing's bulletin, the FAA, also issued a follow-up directive, mandating that the Boeing recommendations be used by pilots and carriers of the 737 Max.
Pilots raise and lower the nose of Boeing jetliners by pushing and pulling on a yoke in the cockpit, which controls panels at the tail known as elevators. That case didn't involve the angle-of-attack system.