From United Air lines' most recent 10-Q, it is chapter 11, I think chapter 13 is personal bankuptcy (perhaps a small point):
Voluntary Reorganization Under Chapter 11
Bankruptcy Proceedings. On December 9, 2002 (the "Petition Date"), UAL, United and 26 direct and indirect wholly owned subsidiaries filed voluntary petitions to reorganize their businesses under Chapter 11 of the United States Bankruptcy Code in the United States Bankruptcy Court for the Northern District of Illinois, Eastern Division (the "Bankruptcy Court"). The Bankruptcy Court is jointly administering these cases as "In re: UAL Corporation, et al., Case No. 02-48191." The consolidated financial statements shown here include certain subsidiaries that did not file to reorganize under Chapter 11. The assets and liabilities of these subsidiaries are not considered material to the Consolidated Financial Statements.