All of the major airlines in the US have gone bankrupt since 1989, with the exception of Southwest Airlines as of 2013. Southwest Airlines is thought to have escaped bankruptcy due to having a lower cost operation model than other airlines. The amount of bankruptcies by major US airlines increased after 1979, once they were no longer regulated by the Civil Aeronautics Board. These bankruptcies tend to be Chapter 11, in which companies can restructure by negotiating with creditors without necessarily surrendering their assets, rather than Chapter 7 in which all assets are liquidated and given to creditors.
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