Air France Flight 447 Crash 'Didn't Have to Happen’
I can't understand why AF 447 crashed since the flight envelope protection was there. I thought the software would add thrust and push the nose down automatically to prevent a stall.
Also, why did FO Bonin keep pulling back on the side stick? He should know better than to keep the nose up at 35 degrees. And why didn't FO Bonin, FO Robert, or Capt Dubois respond to the stall warnings? That was the only warning they had until they got the GPWS at 2000 feet.
When the Airbus A330 encountered severe icing, the external airspeed sensors (pitot tubes) were blocked. This caused the autopilot to trip because the computers were not able to cope with conflicting information.
The failure caused the plane to change from ’normal law’ to ‘alternate law’ where the flight envelope is basically lost – hence no alpha floor protection against the stall. If only the crew knew about it, the stall could be manually recovered by pushing the sidestick down instead of up. Sad to say, things happened so quickly that the crew lost situational awareness and never recovered from this jet upset
Apparently, the pilot flying (FO Bonin) had the misconception that the plane was over speeding (loud and unfamiliar slipstream noise due to stall-related buffeting) when it fact it was the reverse. As a reaction, FO Bonin kept pulling back on the side stick to reduce the speed - it was a fatal mistake.
It is true that the stall warning came on. It was a true indication but was fooled by the ‘unreliable speed’ indication.
This explains why the cockpit crew did not respond to the stall warning and believe it was over speeding. According to the investigators (BEA) report which attempt to answer question similar to yours : "Human operators notice and react according to their mental representation of the situation, not to the real situation."
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