Only three flights suffered excessive tarmac delays in July, all of them at Chicago’s O’Hare International Airport, as the threat of hefty, new fines continued to make an impact on air travel, new data shows.

But passengers don’t always benefit from the new rules, imposed at the end of April.
While the number of tarmac delays declined substantially in July from year-earlier results, the volume of canceled and diverted flights rose, according to data released Monday by the U.S. Department of Transportation.

The three flights that sat on taxi-ways for more than three hours were  operated by American Eagle at O’Hare July 23 and couldn’t take off as “a very nasty line of thunderstorms” from the southwest hovered over Chicago, said American spokeswoman Andrea Huguely.

American had difficulty getting the Eagle regional jets in question back to a gate as the three-hour limit approached and couldn’t let passengers off its planes because lightening made ramp operations hazardous.

“Our first priority is always the safety of our passengers and employees and we will not deplane passengers if we know there is a safety risk,” Huguely said.

The longest of the Eagle tarmac delays was three hours, 37-minutes. By contrast, 164 flights reported tarmac delays of more than three hours in July 2009, before the government limits. Of those flights, 29 sat for more than four hours.

The turnaround in performance is almost certainly because of rules that threaten airlines with fines of $27,500 per passenger, or about $3 million for a typical narrow-body flight, for any plane that sits on the ground for more than three hours without letting passengers  exit.

“It’s absolutely clear that people are taking this very, very seriously in part because of the outsize fines involved,” said aviation consultant Robert Mann.

Airlines had warned that the rules would increase the number of flight cancellations. That appears to be accurate, though it’s too soon to know whether weather, staffing and other factors also skewed results, Mann said.

In fact, 1,332 more flights were canceled during July 2010 than July 2009, while 318 additional flights were diverted. On a percentage basis, the increase was minor: 1.4 percent of all commercial flights were canceled versus 1.2 percent a year earlier.

When viewed against air traffic totals, the percentage of flights suffering long tarmac waits has always been tiny. The 164 excessively delayed flights during July 2009, for example, amounted to just .028 percent of the 580,134 commercial flights flown that month.

But for passengers stuck on the planes as summer storms fouled air travel, the experiences were often memorably unpleasant. The nightmare endured by those aboard a 50-seat Continental Express regional jet that sat overnight in Rochester, Minn., in August 2009 prodded regulators into action.