American Airlines filed a notice of appearance in Spirit Aviation's bankruptcy proceedings, according to a court filing obtained by Reuters on Sunday (December 7).

American Airlines requested to receive all notices, which includes operating reports, plans of reorganization and liquidation, in connection with Spirit's ongoing second Chapter 11 bankruptcy proceedings, in a filing in the Southern District of New York last Friday (December 5). Spirit announced it was filing for Chapter 11 bankruptcy for a second time in a year in August and would consider all options for its future.

"The value maximizing outcome may be a merger or sale of the company; Spirit is actively working to explore all potential opportunities. The company is actively engaged in discussions with a number of interested counterparties," Spirit said in an October SEC filing via Reuters.

Union workers previously warned that Spirit's second bankruptcy would be "more difficult" than its first.

“This bankruptcy will be much more difficult than the last one and we must be prepared to act to protect our interests,” the Association of Flight Attendants said Wednesday (September 17) in a memo obtained by Reuters.

Spirit, which is the United States' largest budget carrier, has already dropped 11 cities from its route map following its decision to file for bankruptcy again in August. The airline announced plans to cease operations in airports located in Albuquerque, New Mexico; Birmingham, Alabama; Boise, Idaho; Chattanooga, Tennessee; Columbia, South Carolina; Oakland, California; Portland, Oregon; Sacramento, California; Salt Lake City; San Diego; and San Jose, California, a spokesperson confirmed to airlinegeeks.com.

Additionally, a previously planned route to Macon, Georgia, that was set to begin in October was also suspended. Spirit listed its assets and liabilities within the range of $1 billion and $10 billion.

Billy the Kidd