5 things to know about the United Airlines ground stop

Here are five things to know about the stoppage.

1. United Airlines is one of the largest carriers in the country with more than 16 million passengers a day with 138 million passengers in 2014. The company serves 373 airports world-wide with 4,935 departures daily. United has 691 planes in its fleet.

2. United bought Continental Airlines in 2010. It ran until 2012 as a separate operation under United Continental Holdings.

3. United Airlines requested the ground stop from the FAA just before 8:30 a.m. ET/5:30 a.m. PT. It was for all United flights including subcarriers due to "automation issues" for all airports and facilities. Post reported that about an hour later, only "mainline flights" were still grounded. United released its subcarriers. By 10 a.m. ET, all United flights were released and said that the computer issue had been resolved. About 3,500 flights were affected, MSNBC reported.

4. There were reports that ticket agents were issuing hand-written travel documents, MSNBC reported. The FAA blamed the glitch on "automation issues." If passengers did not have a hard-copy of their boarding pass, one Connecticut airport would not let them check in.

5. Flight delays were expected due to the groundings. Experts told NBC News that delays of 90 minutes can have a snowball effect of delays at each of the late flight's destinations not only on the runways, but also at the gates. The airline offered customers who were affected a waiver to change their flights.

Some passengers complained on social media about conflicting information from the airline.