According to a news report by Bloomberg, the faulty sensor that led to the deadly Indonesian Lion Air 737 Max 8 crash, as well as a turbulent flight the day before on another of the airline’s 727 Max 8s, had been repaired in an American maintenance facility just before the incidents occurred. This information was revealed during the ensuing investigation.
Based on the investigation documents that are being prepared for presentation to the Indonesian Parliament, both Indonesian as well as US investigators have been assessing the work that had been done by a workshop in Florida on the planes’ angle-of-attack sensor.
The sensor had sent erroneous signals to the airplane’s computer, which triggered the repetitive nose-down movements of the plane on October 29. Pilots struggled with the controls in an effort to correct these movements, but were unsuccessful, leading the plane crashing into the Java Sea. All 189 people on board the plane were killed in the crash.
Five months later, on March 10, Ethiopian Airlines flight 302, also a Boeing 737 Max, crashed due to a similar fault in the plane’s sensor, leading to the death of all 157 people on board the plane – just 6 minutes after take-off.
Angle-of-attack sensors act like the wind vane on the sides of a jet, and are designed to show how the air around the aircraft is flowing relative to the direction in which the nose of the aircraft is pointed. If the aircraft is in a too-steep climb, then the sensor can warn the pilots of a possible aerodynamic stall which can cause a crash.
In the Lion Air 737 Max jet, the left sensor was sending an erroneous signal that the aircraft’s nose was pointed 20° higher that it actually was. This led to the computer’s anti-stall system to assume that the aircraft was about to lose lift, and the system tried to repeatedly push the plane’s nose down.
The struggle between the pilots trying to push the nose back up and the computer system trying to push it back down is what led to both crashes.
These two tragedies led to governments across the world to ground the company’s best-selling 737 Max airplanes until further notice. It also sparked off a global outcry against American aviation regulators. Investigations into both crashes have focused on the role the sensors played in causing the disasters.
The repair facility located in Miramar, Florida, called XTRA Aerospace Inc., had reportedly worked on the sensor that was later installed on the Lion Air plane just one day before the crash. This was done after pilots of the aircraft had complained that they had been facing problems with the display of speed and altitude on their instruments.
It is common practice for repair stations such as XTRA Aerospace to overhaul older parts from an aircraft so that they can be resold. Airlines can save costs by buying used, overhauled parts, however, American regulation require all these parts to meet legal standards before they can be resold.
According to Embry-Riddle Aeronautical University’s Department of Aviation Maintenance Science’s Chairman Charles Horning, even if a part has not been repaired correctly, or had been damaged in transit, the installation procedure would have identified any problem with the device.
Horning stated that there definitely would have been a return-to-service test to certify the installation. If the test were conducted correctly, it would have identified any problems with the device.
At this point, it is unclear why the test did not identify the malfunction in the part.