Of the 19 industries the ACSI asked consumers about, only the cable and satellite TV industry, at 62, fared worse. In comparison, the IRS scored 65.

"If a company has a score close to the IRS' score, something is awfully wrong," says Claes Fornell, the study director.

According to the study, the year-to-year drop is the result of "the same problems that have pulled airline passenger satisfaction down the past few years — disenchanted employees, increasing fuel costs, bankruptcy, and now also record levels of lost, delayed and damaged luggage."

United Airlines' customers were the most dissatisfied. The airline, which emerged from bankruptcy-court protection in 2006, scored 56, the lowest among the seven airlines mentioned in the survey. It also registered the largest year-over-year drop — 7 points — and is "one of the lowest-scoring companies measured by the ACSI," the study says.

"We are committed to doing a better job at giving customers the service they expect," says United spokeswoman Robin Urbanski.