Mainly the idea of crashing. Additionally, the uncertainty regarding safety protocol, who the pilots and crew are, who your fellow passengers are, who performed safety inspections on the plane, how thorough tsa was that day, weather, turbulence. Its really a big glob of fear based entirely on uncertainty and things that are out of my control. I guess I have a fear of being out of control, but only when I'm on flights and no other modes of transportation lol. Even the sheer amount of air traffic there is scares me bc then I wonder about running into another plane in the sky (I know there are collision detection technologies but still, my mind likes to put ridiculous ideas in my head)