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A report last week from the U.S. Transportation Department’s statistical bureau showed that complaints from U.S. travelers rose 20.3% in the first six months of 2015, compared with the same period last year. And complaints continue to rise.

In the first six months of 2015, consumers filed 9,542 complaints, up from the total of 7,935 received during the first six months of 2014. In June 2015, the department received 2,052 complaints from travelers, up 47.3% from the total of 1,393 filed in June 2014 and up 37.5% from the 1,492 received in May 2015.

Of the 2,052 complaints received in June of this year, the largest proportion (35%) were related to flight delays. Lost baggage complaints were the second-most common with nearly 14% (283) of all complaints, followed by 250 complaints related to reservations/ticketing/boarding, 235 related to customer service and 220 related to fares.

The U.S. airlines’ on-time performance rose from 71.8% in June of 2014 to 74.8% in June of this year. That is, however, down from 80.5% in May of 2015.

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The causes of flight delays were aviation system delays (6.3%), late-arriving aircraft (8.8%), factors under a carrier’s control (7.0%), extreme weather (0.9%) and security reasons (less than 0.1%).

The mishandled baggage rate in June was 3.63 reports per 1,000 passengers. For the first six months of 2015, the rate is 3.52 per 1,000 passengers, compared with the year-ago rate of 3.84.

The three airlines with the best on-time arrivals performance were Hawaiian Holdings Inc. (NASDAQ: HA) at 90.5%, Alaska Air Group Inc. (NYSE: ALK) at 87.4% and Delta Air Lines Co. (NYSE: DAL) at 82.2%.

The worst on-time performance was posted by Spirit Airlines Inc. (NASDAQ: SAVE), where on-time arrivals occurred 49.9% of the time. United Continental Holdings Inc. (NYSE: UAL) was next worst with 66.3% on-time arrivals and Frontier Airlines with 67.6%.

The airline receiving the most complaints in June was United with 297, followed by Spirit and American Airlines Co. (NASDAQ: AAL) with 284 each.

The airline receiving the fewest complaints is Southwest Airlines Co. (NYSE: LUV) with a rate of less than one complaint per 200,000 passengers. Among the larger carriers, Delta gets about 1.5 complaints per 200,000 passengers, American’s complaint rate is near five per 200,000, United’s rate was around 6.8 per 200,000 and Spirit came in last with a rate of more than 38 complaints per 200,000 passengers.

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