United Airlines grounded many of its flights for several hours Wednesday due to a technology issue. Flights at major hubs including Newark, Denver, Houston, San Francisco, and Chicago were impacted; according to FlightAware, around 28% of the airline's flights were delayed. A source tells CNBC the airline experienced an outage of its weight calculation system. As the AP explains, "The impacted system, called Unimatic, houses flight information that is fed to other systems including those that calculate weight and balance and track flight times." After the issue was resolved, the airline said in a statement, "While we expect residual delays, our team is working to restore our normal operations."