August 4, 2010, 2:12 PM ET.Canceled Flights Up Slightly in July.Airlines threatened to cancel scads of flights if the Department of Transportation put through its three-hour tarmac-delay limit. The DOT did, and airlines seem to be adjusting without throwing their operations and summer air travel into cancellation chaos.

A July 15 Middle Seat looked at May and June. Now we have some preliminary July data from, which tracks airline flights, showing much the same.

The weather seems to have been worse for airlines in July. On-time performance for the U.S. industry dropped to 76.1% from 77.6% in July 2009, and there were fewer flights operating because of airline schedule cuts. (Less congestion generally translates into improved on-time arrivals.) The percentage of U.S. flights arriving 45 minutes or more past their scheduled arrival time also increased, to 9.8% of all flights tracked by FlightStats, compared to 9.1% last year.

And cancellations showed a similar small increase, to 1.4% of all flights from 1.2% last year. In terms of total numbers, there were 9,759 cancellations counted by FlightStats compared to 9,336 in July 2009.

With May and June, airlines said in the July Middle Seat that the tarmac-delay rule, which threatens massive fines if airlines leave flights stranded, had little impact. Even airlines with big increases in cancellations and plenty of incentive to blame the tarmac rule said other factors were to blame–their own maintenance issues, for example, and volcanic ash in Iceland and Central America.

To be sure, there are flights that get canceled because they are approaching the three-hour limit, or airlines simply fear launching them into a potential long delay. But volcanoes and other factors had far more impact.

A study by two aviation consultants, by the way, looked at an increase in May cancellations and concluded the tarmac-delay rule was to blame. The DOT refuted that, and airlines themselves did as much in the July Middle Seat story.

Instead of massive cancellations, airlines and airports have worked hard to put systems in place to comply with the new rule. In the end, travelers may get more reliable transportation. While some flights will get canceled because of the rule, many probably enjoyed shorter waits to takeoff and better service upon landing because of the increased monitoring of flights and changes in operations to avoid long delays.

The FlightStats numbers are raw – DOT publishes more refined data, but with a much longer time lag. Next week, DOT will release June operations numbers. FlightStats, which is used by airlines and airports as well as frequent travelers and gets data directly from airline systems and the Federal Aviation Administration, already has July available. There are other advantages: FlightStats looks at a broader universe – more U.S. airlines than DOT reports. It’s a good resource, but by no means the definitive accounting of how the system worked.

Still, by most accounts, the system is working this summer, even better than airlines themselves predicted.