Early on in the movie, there are two sequences (one a nightmare and one a character imagining) of a plane flying through downtown New York City and crashing into a building. While this doesn't actually take place, as it's a big "what if" scenario, the scene is still unsettling - especially when considering how reminiscent it is of what happened to the Trade Towers on 9/11. Might be especially triggering for people who remember that day or lived through the events in some form.
The flashback scene where the plane is actually flying low near some of the buildings in New York City, as professionals gaze out of their office windows in shock, is also unsettling because of its reminiscence of 9/11.
Based on true events, "Sully" is a drama about an airline pilot, Captain Chesley Sullenberger, who safely landed U.S. Airways flight 1549 on the frozen Hudson River in 2009. The film follows the investigations that took place in the wake of the incident.