Airbus planes have computerized protection systems designed to avoid an aerodynamic stall, a condition in which an aircraft’s wings lose lift because forward speed is too low or the wings are pointing upward too steeply. But under extreme circumstances, such as multiple failures or disconnection of redundant flight-control computers, the stall-protection system becomes inoperative. That is apparently what happened to Flight 8501, though questions remain about the primary and contributing factors.