Because that door—a sorry excuse for a divider anyway—was often open, flapping flimsily with the aircraft's movements, held in place by a pinned-down strap.

In 1991, it was through that open door, on a Scandinavian MD-80, that a dead-heading captain, sitting in my favorite seat, watched in horror as both engines failed shortly after take-off from Stockholm. The cockpit lit up, warnings erupted. Quickly, he unbuckled his seatbelt, dashed forward through the open cockpit door, crouched down on his knees between the two pilots and did what he could to help them nurse the stricken jet to a glided, engine-less landing in a snowy field near Gottrora, Sweden.

The captain who was flying the plane told me much later that he always flew with the door open.

And then 9-11 happened. And the world changed.

By the time I myself had entered the cockpit, proudly in uniform, that door had become a hard barrier. It certainly was good bladder training. The elaborate procedures to leave the cockpit, even for a short personal pit stop, were enough to limit them to one or none per flight. Codes, passwords, phone calls, decompression panels, locks, lights, deadbolts, security cameras on some airplanes — overwrought post 9-11 anxiety written into the very design behind my left shoulder.

Every time the other pilot came back in and slammed that door shut, something small, tiny in me, broke. Just a little bit. The slam would signal to the passengers, "we don't trust you". And perhaps we don't trust ourselves enough to let you into our world. Not even the eager kid in the middle seat stretching his neck to catch a glimpse of a world he might one day want to make his own.

And now, that defense has turned against ourselves. The night after I learned about the captain locked out of the Germanwings cockpit, I slept badly.

Of course, theories can carry the explanation of a defense turning deadly very nicely. The law of unintended consequences. Emergence in complexity science. Chaos theory. The heavily armored French knights at Agincourt in 1415, dragged or fallen off their horses in heavy mud, quickly found that it wasn't so much the longbow that became responsible for Henry V's victory. It was, like the cockpit door, their heavy defense of steel and armor. One day, I might perhaps be able to muster excitement for the application of such theories.

But this night, my blood seemed to have slowed and cooled and curdled in horror. This, as Chancellor Angela Merkel said, gave the whole thing a new, simply incomprehensible dimension.

There are ways to die, and then there are … Every captain I flew with took the responsibility for her or his passengers deadly seriously. The duty ethic, the notion that they alone were ultimately responsible for where their hundreds of charges ended up, and in what condition, was grafted into their every assessment action. I admired that.

This night, I tried to put myself into the shoes of the Germanwings captain. We actually all know the nightmare, at least in some version. You try to get from one place to another, running, clawing, crawling — and nothing really happens as your efferent neurons are suppressed during REM sleep and muscle tone is at its lowest. The vexation, the frustration can be overwhelming, even in sleep. You shout, you open your mouth and force it out, and nothing happens, until the shout that awakens you in shock, its echo still bouncing off the quiet bedroom walls.

There he stood. I know exactly where he stood. I once called into the cockpit to be let back in, and nobody answered. It took multiple calls. The other pilot had been on the radio to air traffic control, an innocent and operational explanation.

I can imagine, or try to imagine, the unfolding, accelerating nightmare inside and around the captain. The plane is in a steady descent. With his 6000 hours on type, I am convinced the captain even knew what kind of descent, which automation modes the co-pilot had selected to make the plane go down: with a bit of experience you can hear this from the hum and whine of the engines, feel it in your feet from their soft vibration, and in your inner-ear from the pitch of the nose.

And then, a swelling crescendo of concern, the cold, prickly anesthethising leach of adrenaline into your gut and lower limbs, a crescendo of galloping heart rate, of shouting, of pounding and disbelief and alarm and panic—with your 150 charges behind you, their eyes piercing your back, increasingly uneasy, distressed. 150 charges all with real skin in the game. And then, glimpses perhaps of mountainsides swishing through the small portholes in the doors to your left and right. And, although you are evidently near the ground, you get nothing of the usual: no slowing down, no flare, no closing of the throttles, no kiss and smoke and screetch of tires on concrete.

Instead, suddenly—nothing.

Sidney Dekker is an aviation expert at Griffith University and a pilot.