A locked cockpit door, a commonsense post-9/11 security measure, gave the co-pilot of Germanwings Flight 9525 all the time he needed to deliberately crash the aircraft in the French Alps, killing all 150 people on board.

As the plane descended, the pilot, who had apparently taken a bathroom break, can be heard on the cockpit voice recorder frantically pounding on the door and begging co-pilot Andreas Lubitz to let him back in. His pleas were accompanied by chilling screams in the final moments before the plane, en route from Barcelona to Düsseldorf, slammed into the remote mountains.

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The stunning crash, already being called a mass murder, raises urgent questions about how cockpit doors can be secured to keep terrorists out, while allowing intentionally locked-out crew members to get back in. The Federal Aviation Administration requires an electronic keypad in the cabin that can briefly unlock the door. But a person in the cockpit can override that system.

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One obvious measure to consider is wider international adoption of the U.S. standard that requires at least two people in the cockpit at all times. Another is seeking to develop systems that somehow allow a flight crew to open the door after keypad access is blocked -- difficult to do without compromising security if there are terrorists on board.

Live-streaming video from the cockpit could help alert air controllers when there is a problem. And it would be prudent to consider requiring pilots to routinely undergo a psychological assessment as part of medical exams required annually for those younger than 40 and twice a year for those 40 and older. Currently, pilots answer questions about their mental health on medical forms, and additional testing can be required if examiners think it's warranted.

Flying is a remarkably safe way to travel. That's in part because every tragedy is intensively investigated to determine what went wrong and how to avoid a repeat.

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But nothing can make flying risk free. And the cost-benefit of new equipment or procedures is always a factor. Here, a measure taken to thwart terrorists may have cost lives because of the actions of a suicidal co-pilot.