Simulation tests imitating a flight on a Boeing 737 MAX with a faulty nose position sensor showed that if the pilots from Lion Air and Ethiopian Airlines engaged the problems with the plane's automatic anti-stalling system (MCAS), they had only 40 seconds to prevent the crash, The New York Times reported, citing people participating in the tests.
During the simulation, the system would constantly kick in, pushing the plane's nose down for 10 seconds and then making 5 second pauses, as was intended. However, since the jet's position sensor was feeding erroneous data to the system, instead of preventing stalling, the MCAS was gradually sending the plane into an uncontrolled dive. In this way the plane would inevitably crash within 40 seconds once the system kicked in for the first time, the sources indicated.
The pilots could extend the window of opportunity to fix the problem by flipping a switch located near their thumbs — it would stop some of MCAS's work. The Lion Air pilots flipped the switch "more than two dozen times" the logs reportedly showed, but that didn't stop MCAS from engaging and the plane eventually crashed.
To avert this outcome and save the lives of the 189 people on board, pilots would have had to completely shut off the power from the anti-stalling system, but that would require knowledge about two more switches that apparently the pilots didn't have. According to the NYT, prior to the Lion Air crash both Boeing and regulators didn't even bother notifying the pilots about the system, which appeared only in the newer MAX jets, considering it unnecessary.
Soon after the second 737 MAX crash in just five months that took the lives of 157 passengers on Ethiopian Airlines Flight 302, the aviation giant announced a patch named 12.1 that would implement "additional limits and safeguards" to the MCAS. Now, instead of using just one sensor, the anti-stalling system would use two (the second one is already installed on the planes) and if the data from them would conflict — the system won't kick in at all, eliminating the failure bottleneck.
Another important addition makes the MCAS a less aggressive system. Some of the participants in the simulations told the NYT the system was "surprisingly powerful". Now MCAS will engage less persistently and only once in most cases. In addition, the system won't be able to push the nose more than a pilot can fix by pulling it back using manual controls.
Boeing, meanwhile, has failed to elaborate why such flaws were originally present in a system shipped to passenger planes as well as why it used only one sensor, while two were installed.
The questions about the 737 MAX's anti-stalling system arose after two planes of the series crashed within 5 months soon after taking off. The investigation is still ongoing, but officials have already reported finding similarities between the October 2018 Lion Air Flight 610 crash in the Java Sea and the Ethiopian Airlines Flight 302 crash on March 10. According to some media reports, the Lion Air captain was hastily looking through the plane's technical documentation trying to figure out how to switch off MCAS, which was reportedly pushing the plane's nose down.
Another report has suggested that the pilots, who flew the plane prior to them encountered similar issues, but managed to figure out the solution on their own. However, they failed to pass the info about it to the next team.