(KFYI News) – A new report from a consumer watchdog group says the number of complaints against US airlines continues to rise, as planes fly fuller and air fares head north as a result of consolidation in the industry.

The report, released locally by the Arizona Public Interest Research Group, shows Spirit Airlines had, by far, the highest number of complaints per 100,000 passengers.  In fact, Spirit's complaints were three times the rate of complaints for any other airline.

Frontier, United, and American Airlines were also near the top of the list.

US Airways, which was based in Tempe before the recent merger of US Airways with American Airlines, had also been one of the most complained-about airlines for several years, but dropped out of the top five in 2013.

The airlines that generated the fewest complaints per 100,000 passengers were Southwest, Alaska, AirTran, and JetBlue.  Southwest has consistently been the least complained-about airline, according to US Department of Transportation statistics.

The category with the largest number of complaints by passengers to the US DOT over the last five years, the report says, had to do with delayed or canceled flights.  Lost or damaged baggage, customer service, and the reservation/ticketing/boarding process also generated a large percentage of the complaints.

Will the merger of US Airways and American Airlines lead to an increase in complaints?  Based on previous mergers, AZPIRG's Diane Brown says it's a toss-up.  "When Delta and Northwest merged, the complaints actually decreased," she said, thanks to a concerted effort by management to make sure the new merged airline was customer-friendly.  But when United absorbed Continental, she says, complaints spiked.

You can access the full report at www.arizonapirgedfund.org .