An Ethiopian Airlines Boeing 737 MAX 8 flight to Nairobi that crashed on Sunday with 151 passengers and eight crew members aboard had unstable vertical speed, according to flight tracking site Flightradar24. It was the second crash in five months involving a 737 Max 8. Last October, a 737 MAX 8 operated by Indonesian budget carrier Lion Air crashed 13 minutes after take-off from Jakarta on a domestic flight, killing all 189 passengers and crew on board. Pilots say they were not trained in new features of an anti-stall system in the aircraft that differ from previous models of the popular 737. The automated system is designed to help pilots avoid raising the plane’s nose too high, which can cause the plane to stall, or lose the aerodynamic lift needed to keep flying. The system automatically pushes the nose of the plane down. But if that nose-down command is triggered by faulty sensor readings – as suspected in the Indonesian Lion Air crash – pilots can struggle to control the plane, which can go into a dive and perhaps crash, according to a Boeing safety bulletin and safety regulators.