Consumer complaints against Frontier Airlines came in at a lower rate in June than any other month of 2017 so far, the U.S. Department of Transportation reported today.

The Denver-based carrier posted a rate of consumer complaints to the USDOT of 1.86 per 100,000 "enplanements," or boarding passengers, according the agency’s latest monthly Air Travel Consumer Report.

That’s less than a quarter the complaint rate against Frontier in January — 7.87 per 100,000. Since then, the airline’s complaint rates were 2.21 per 100,000 in February, 3.39 in March, 2.42 in April and 3.01 in May.

Frontier — the third-busiest carrier at Denver International Airport by passenger counts — has been plagued by high complaint rates and frequently tardy flights in recent years as it has transitioned into a "ultra low cost" airline and has been struggling to improve performance.

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