While this is an unusual procedure, an autopilot can be easily overridden by a pilot at any moment if the crew wants to take control of the aircraft. Ten seconds later, both radar returns and data from the FDR reveal that the engines appeared to throttle down and there was an overall reduction in the amount of thrust. Flight 990's passengers experience complete weightlessness as the dive becomes steeper. The airliner accelerates to around 0.86 the speed of sound. The aircraft's master warning system activates and the engine monitors warn of low oil pressure.
Records appear to show that the crew ignored the warnings.
It takes the aircraft less than 20 seconds to dive 12,000ft. Seconds later, as the aircraft descends below 20,000ft, the start levers on both engines switch from run to cut-off, shutting down the engines. Boeing has said that the engines could only have been intentionally shut down. Radar traces show that the aircraft then began to stabilise and gain height, reaching some 24,000ft over the next minute. However, considering that the aircraft had entered the steep dive at extremely high speed, most of the height could have been gained by the aircraft's forward momentum.