AVIATION DISASTERS, like the recent unfortunate loss of Germanwings 4U9525, sadden me and cause me to rewind back through human factors I investigated while a Flight Safety Officer with the US Air Force. Today, I'm thinking a lot about the way the Airbus cockpit is protected from entry because locked cockpit doors might be cited as the single cause of failure in some aircraft disasters. 
During my career as an instructor pilot in the US Air Force, I walked through wreckage of fatal aircraft accidents while investigating their causes and I walked through reconstructed crash scenes during recurrency training as an accident investigator. My role in these investigations was to dig into the human factors behind aircraft accidents. That meant trying to burrow into the flight scenario and to see the incident from the perspective of the pilots behind the controls. I would look into the personal lives of aircrew members, including their financial transactions, home lives, crew rest, duty periods and medication. I was an instructor pilot in two different types of military aircraft during my time in uniform, including the Lockheed C-141 Starlifter. I've seen highly qualified pilots make critical errors while airborne. In fact, I would induce some of those errors during local training flights or in the flight simulator. It was part of my job.