Sept 11 (Reuters) - Frontier Airlines (ULCC.O) expects its third-quarter margins to improve compared to prior forecast as capacity cuts offset the impact of moderating domestic travel demand, the low-cost U.S. carrier said on Wednesday. The airline raised its adjusted pre-tax margin to a range of flat to down 2%, compared with a prior view of down 4% to 6%. It lowered its capacity growth forecast to between 4% and 5% from the earlier forecast of 4% to 6%. Frontier added it is expecting to benefit from the changes to its flight network. CEO Barry Biffle had said in April the company would add flights to “high-fare” markets, where it faces less competition from other carriers and can charge more.