Ice

Most passengers are likely to be concerned about turbulence, but according to Tanya Gatlin, a pilot and associate professor at Metropolitan State University of Denver, it's not as bad as most think. "It's not something that's going to cause an accident or is even a factor to safety," she told Business Insider.

Instead, Gatlin said she is worried about ice. When necessary, ice and snow are removed from a plane before it takes off, and the plane is coated in materials that will prevent ice from building up while it's in the air — for a limited amount of time.

The difficulty can come when the plane turns down its engine while preparing to land.

"We're coming down in a very short amount of distance and there's no way we can get down that fast without the power being at idle," Gatlin said.

This means the engines don't generate as much heat as when they're taking off, which increases the chance that ice will build up on the plane and make a smooth landing difficult.