The second quarter was the airline industry's best quarter in six years, government statistics confirmed Monday, even as U.S. carriers potentially confront the new challenge of falling demand in a slowing economy.

A group of 21 airlines reported a domestic operating profit margin of 7.9%, the highest since the second quarter of 2000, the Bureau of Transportation Statistics said. The group included the seven largest carriers in each of three groups: network, low-cost and regional.

"At this point, what we would be most concerned about is a general slowdown in the economy," said Calyon Securities analyst Ray Neidl in a research report.

Though he noted that demand is "somewhat dampened," Neidl was generally optimistic, citing falling fuel prices that "have helped absorb some of the negative effects of August's somewhat reduced traffic numbers as well as reduced

revenue per available seat mile guidance from

AirTran

(AAI)

."

AirTran said recently that rising East Coast and Midwest capacity is putting the brakes on revenue growth in those regions and is having more of an impact than security concerns.

JetBlue Airways

(JBLU) - Get Report

said demand was slowing, and several other carriers reported that growth in revenue per available seat mile eased in August.

On a positive note, Neidl also said that overall fare levels "have not been hit as much as usual" during the normally slow post-Labor Day travel period, even though tightened security measures were implemented after last month's announcement that authorities foiled a terrorist plot to blow up a number of airplanes flying trans-Atlantic routes.

Low-cost carriers led the pack in the second quarter, with a domestic operating profit of 10.6%, the BTS said. Regional carriers reported a 7.9% margin, and network carriers reported a 7.2% margin. Domestic operating margin measures profit or losses as a percentage of total domestic operating revenue.

The network carriers, whose generally profitable international results aren't included in the report, had a combined domestic operating profit of $1.3 billion, combined with a domestic operating loss of about $400 million in the same quarter a year earlier, the BTS said.

The top operating profit margins were reported by low-cost

Southwest Airlines

(LUV) - Get Report

, network carrier

Northwest Airlines

(NWACQ)

, and regional carrier Atlantic Southeast Airlines.

The agency also noted that network carrier cost per available seat mile rose 1.0 cents, while regional and low-cost carrier CASM rose by 1.4 cents.