MAIR Holdings' ( MAIR) Mesaba Aviation unit filed for bankruptcy Thursday, unable to overcome a revenue shortage brought upon by Northwest Airlines' ( NWACQ) Chapter 11 filing. Mesaba, which operates as a Northwest Airlink partner, said it will continue its regular operations during the reorganization. "This was a difficult but necessary step; Northwest's actions since filing bankruptcy along with the continuing distress affecting the entire industry requires us to change and to do so quickly," said John Spanjers, Mesaba Airlines' president and chief operating officer, in a statement. "The changes imposed on us by Northwest since its filing in September have left us with insufficient revenues to support our cost structure." Mesaba warned last week that it was considering a Chapter 11 filing after Northwest failed to make certain payments it owed the company and said it would reduce its fleet size. Mesaba has a net unsecured claim of $30 million in Northwest's bankruptcy case. "The combination of the loss of $30 million of revenue and the reduction in our fleet size by at least 28 percent has left us with no choice but to take this difficult step," Spanjers said. "We view bankruptcy as a last resort, but a necessary one because no other alternatives would allow us to change as rapidly as we need to." Mesaba's parent, MAIR Holdings, offered to extend debtor-in-possession financing to the airline, Mesaba said. MAIR shares recently changed hands at $4 in after-hours trading, down 66 cents, or 14%. The shares have dropped more than 50% since Northwest's bankruptcy filing on Sept. 14.