Breaking: Faulty Sensor Data May Have Caused Boeing 737 MAX Crashes
Andrew Wheeler posted on March 12, 2019 |

Sunday's crash resembles the Boeing 737 MAX jet that crashed into the Java Sea in Indonesia and killed all 189 people aboard the ill-fated Lion Air flight.

The Ethiopian Airlines flight that crashed soon after takeoff in Addis Ababa last Sunday had some of the same takeoff behavior as the ill-fated Lion Air flight that crashed into the Java Sea off Indonesia last October. Could it have been bad data from a single angle of attack sensor? (Image courtesy of Airways Magazine.)
Story updates:
  • March 15 - added photos of AOS sensor and cutoff switches
  • March 13 - FAA grounds Boeing 737 finally. See FAA announcement

 

Boeing's 737 MAX, the model being flown by Indonesia's Lion Air that crashed last October killing all 189 on board, had been criticized for relying on a single sensor to detect the aircraft's angle of attack, according to today's Wall Street Journal. A preliminary investigation by Indonesia's National Transportation Safety Committee (NTSC) blamed the stall prevention system (known as Maneuvering Characteristics Augmentation System, or MCAS) which relied on a the single sensor that provided erroneous data, causing the MCAS to misfire, leading to a series of events that put the plane into a nosedive. 

After the October crash, Boeing began updating the plane's software so multiple sensors would provide data to the stall prevention system, the company said in a statement on Monday.

This critical software change would be done no later than April, says Boeing.

It may not have been in time. Last Sunday, a 737 MAX flown by Ethiopian Airlines, crashed soon after takeoff in Addis Ababa, killing all 157 passengers and crew. Both jets fell from the sky suddenly after takeoff, showing similar behavior before they crashed.

The 737 MAX  is a new design and two crashes with a few months of each other set off alarms worldwide. Reaction has been swift, with analysts and industry regulators all publicly expressing concern and taking action by grounding Boeing 737 MAX jets everywhere -- except for US and Canada.

While the cause of Sunday's crash in Ethiopia is still unknown and the Lion Air investigation is ongoing, the effect on the airlines using the Boeing 737 MAX is pronounced: over 40 percent of Boeing's 737 MAX fleet has been grounded. The European Union Aviation Safety Agency banned the aircraft for all flights in and out of Europe, as has Asia, Latin America and Australia for their countries. 

Update: The FAA has announced that they're grounding all Boeing 737 MAX jets based on new evidence from the crash site and "newly refined satellite data."

Though it is too early for investigators to have gathered information to guide them in unraveling Sunday's crash, here's what we know so far. 

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Angle of attack (AOA) sensor to detect stall on the 737 MAX. (Picture courtesy of www.b737.org.uk)

The 737 MAX with its larger engines mounted even more forward needed a separate sensor to sense stall, the nose up attitude of a plane that turns off aerodynamic lift over the wings. Stall sensors are common, but the MAX is the only 737 to have a blade-like sensor on the exterior of the plane that can automatically pitch the plane down when it detects stall conditions -- without input any input from the crew --according to Seattle Times veteran aerospace reporter, Dominic Gates, who is close to Renton, Washington where the 737 MAX is assembled.

According to the preliminary report from Indonesia's NTSC and in a Boeing service bulletin, Lion Air flight 610 was providing erroneous data from a angle of attack (AOA) sensor. It appears the crew may have been fighting against: bad sensor data, a multitude of warnings and an automatic flight control system that seemed to be malfunctioning by automatically pushing the nose of the plane down repeatedly during the 11-minute flight. Ground control was contacted numerous times by the crew, who were repeatedly requesting permission to return to the airport. 

Similar to the Lion Air crash, the crew of Sunday's Ethiopian Air flight radioed ground control about a flight control issue and repeatedly requested permission to return. Within six minutes, the plane disappeared from radar and crashed.

In an unusual safety warning issued after the Lion Air crash, Boeing instructed pilots on how to handle a "runaway trim" situation. Pilots are trained to handle stalls. The 737 MAX has a cut off switch in the center console between the pilots, according to the Seattle Times.

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The stabilizer trim cutoff switches can shut down electricity to the stabilizer controls. Either pilot could flip up the guard that keeps the switch in the normal (up) position and pull the switch down. Then the stabilizers are controlled manually by the wheels on either side of the console. (Picture courtesy of www.b737.org.uk)

The MCAS could have sidelined but turning it's control of the stabilizer, says Boeing. However, the Seattle Times reported when the system was relatively new, several pilot unions from US complained that Boeing had not provided them with enough information about it. United's pilot union, however, took issue and said training on the MCAS was not necessary, that it was system meant to work unattended, in the background. 

Boeing, the FAA and US airlines maintained that the 737 MAX is safe, though the FAA grounded the 737 MAX jets earlier today, reversing their initial decision.

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