A similar series of events involving faulty sensor data and an automated system suspected in bringing down a Boeing plane in Indonesia may have also caused the crash of the same type of jet in Ethiopia in March, according to people who have been briefed on the contents of the black box in Ethiopia.
Data from a vane-like device, called an angle-of-attack sensor, incorrectly activated the automatic, computer-controlled system, called MCAS, which pushed the nose of the plane down, eventually leading to a crash that killed all 157 people aboard.
The black box, also called the flight data recorder, contains information on dozens of systems aboard the plane. The black boxes on both planes, Boeing’s latest generation of the 737, survived the crashes, allowing investigators to begin piecing together what caused the disasters. Both investigations are ongoing and no final determinations have been made.
There are two angle-of-attack sensors on the Boeing 737 Max, one attached to the fuselage on the pilot’s side and another on the co-pilot’s side. Investigators in Indonesia, who have produced a preliminary report and released some of the information from the box, saw that one sensor produced a reading that was at least 20 degrees different from the other as the plane took off and began its ascent.
The system was programmed to use data from only one of the sensors, which on that flight was malfunctioning. With the bad data, MCAS was activated, erroneously pushing the nose of the plane down. The pilots on the Indonesian flight tried repeatedly to override the system, but after about 12 minutes lost their battle and the plane crashed.
None of the people briefed could say whether the black box data indicated how or whether the Ethiopian pilots tried to counteract the system. But the same bouncing, bobbing trajectory of the plane seen in the Indonesian flight as the pilots tried to save the plane is apparent in publicly available flight data for the Ethiopian plane.
Air traffic controllers in Ethiopia also said they saw the oscillating trajectory before the plane crashed. The pilot radioed back that he was having trouble controlling the aircraft, but did not give details on what systems were causing problems.