Rising fuel prices have forced two small but well-known US regional operators out of business permanently. Big Sky Airlines shut down in January after a disastrous eastwards expansion, while Air Midwest faces liquidation later this year. Both operate on thin margins, relying on federal subsidies for rural service to smaller communities. Their demise is also a sign higher operating costs may force more domestic capacity cuts. The demise of Big Sky forces its parent MAIR Holdings into liquidation. MAIR's major holding had been Mesaba Aviation, which Northwest Airlines bought last year, leaving it with only small turboprop operator Big Sky...