The breakdown showed how much American air travel depends on the computer system that generates alerts called NOTAMs — or Notice to Air Missions. “So something unusual happened.”According to FAA advisories, the NOTAM system failed at 8:28 p.m. Tuesday, preventing new or amended notices from being distributed to pilots. The FAA resorted to a telephone hotline to keep departures flying overnight, but as air traffic picked up in the morning, the phone system was overwhelmed. An Air Canada jet whose pilots had overlooked a NOTAM about a closed runway nearly landed by mistake on a parallel taxiway. As the extent of Wednesday’s breakdown became clear and airlines delayed more flights, passengers scrambled to rearrange trips.