An annual airline quality report card released this week shows that the quality of airline service hit a record high in 2017.

The Airline Quality Rating (AQR) calculates the performance of carriers based on four factors measured by the U.S. Department of Transportation (DOT): percentage of bags mishandled, on-time arrivals, complaints to the DOT and denied boardings. Three of the four metrics improved in 2017.

One year after a United Airlines passenger was dragged off a plane for refusing to be bumped, United and most of the other 12 airlines in the study dramatically reduced their rates of bumping passengers from overbooked flights. Overall, involuntary denied boardings improved to 0.34 per 10,000 passengers in 2017 from 0.62 per 10,000 passengers in 2016.

The 2017 consumer complaint metric for the airlines declined to 1.35 per 100,000 passengers from 1.52 per 100,000 passengers in 2016. 

The industry mishandled baggage rate decreased from 2.70 per 1,000 passengers in 2016 to 2.46 per 1,000 passengers in 2017. (One reason may be that fewer people are checking bags due to baggage fees.)

One metric that did not show improvement: As an industry, the 2017 on-time arrival percentage slipped to 80.2 percent from 81.4 percent in 2016.

The overall AQR score for all airlines improved slightly last year from 2016, and the 2017 score was the best in the 27-year history of the ratings.

In the four categories that make up the annual AQR, top performers in 2017 were: Hawaiian Airlines, for having the best on-time arrival rate; Delta Air Lines, for bumping the lowest percentage of passengers; Spirit Airlines, for doing the best job handling bags; and Southwest Airlines, for registering the fewest customer complaints with the DOT.