Major North American airline United was forced to ground all flights on Sunday night because of a technical outage. Flights were stopped for at least two hours while the issue was investigated. United Airlines didn't release many details but called it "an IT issue." The Federal Aviation Administration mandates strict procedures regarding air travel and system stability so even simple issues can ground flights.
A ground stop is in place for domestic flights due to an IT issue. We're working on a resolution. We apologize for the inconvenience.--- United (@united) January 23, 2017
What we currently know is that the issue was related to the Aircraft Communications Addressing and Reporting System.
ACARS is an air-to-ground data link for communicating small messages like flight plans, weather information, equipment health, and the status of connecting flights. It is primarily used to help pilots calculate safe takeoff speeds. Planes already in the air at the time of the outage were not in danger as the system is only used around the beginning of flight.
Standard communication between pilots and air traffic controllers was not affected. US officials said ACARS was having low bandwidth issues. There are backups and redundancies for the system, but they involve manual calculation of weights and flight plans which can be tedious. Sources said the ground stop was a "way to be incredibly cautious."
While major airline outages are uncommon, they are not unheard of. Air Canada had computer issues last week affecting multiple airports. The end of 2016 also saw outages with multiple international airlines. Southwest and Delta both cancelled thousands of flights in August due to a system failure. In September, British Airways had technical problems that caused long delays, and last October, United flights were also delayed due to computer problems.