Hmm, you're right. I would draw a distinction between the software running on the air traffic controller's desktop (which has often been Unix for decades now) and the microcontroller which controls the air intake to the engines. The former has fairly relaxed real-time requirements (the display has to update, but once a second might be acceptable) and I would call it 'soft safety-critical'. The software running on your doctor's desktop PC could kill you if it displays the wrong notes and causes the doctor to prescribe the wrong medicine - yet this is not normally an application considered safety-critical where special software methods must be used to guarantee correctness. By contrast, a failure of the jet engine microcontroller (assuming it has one, I am speculating) can cause instant disaster.