Not even gamma rays can get through five centimeters of lead. Just pack the computer in that and put it in the thickest part of the plane. Nothing gets in, nothing gets out. You end up with no net-weight gain because most of the cockpit can be removed. Pilots are surprisingly fragile creatures- they can get sick, make simple math errors, press the wrong button. That sort of stuff doesn't happen to computers. Computers never get bored, either. That's a big one because most of flying is letting the autopilot take you from place to place; if a pilot had to fly an airliner's course manually for eight hours straight he'd likely miss or drive himself nuts staring out across the clouds.
-Penguin