DALLAS - In a travel season rife with late arrivals and canceled flights, several airlines have begun flying at lower altitudes, sacrificing fuel efficiency to get there on time.
More than a year ago, the Federal Aviation Administration gave airlines approval to operate some short flights - up to 500 miles - at altitudes between 8,000 feet and 23,000 feet. Initially, airlines resisted because flying through denser air at lower altitudes burns more fuel.
But with flight delays angering travelers and drawing the attention of federal regulators, more airlines are turning to this quick fix.
Low-altitude routes are not as congested as the high-altitude ones, allowing planes to travel faster. The less-busy flight paths also enable planes to get off the ground more quickly instead of having to wait in line to be cleared for takeoff, said FAA spokesman Paul Turk.
In addition, planes flying low-altitude routes spend less time climbing to cruising altitude and descending for a landing.
Standard cruising altitude for jetliners is between 33,000 and 39,000 feet.
Northwest Airlines, TWA, Delta, Continental and US Airways tested the lower-altitude routes in some cities this spring. At Chicago's busy O'Hare Airport, United began rerouting some planes to lower-altitude flight paths in June. And Fort Worth-based American Airlines could begin low flights this week, pending FAA approval.
At United, 30 to 40 such low-altitude flights take off from O'Hare daily, saving an average of two minutes on the ground and about 10 in the air, spokesman Joe Hopkins said. And the airline is considering adding up to 30 more.
''If you multiply that out over 365 days, that could potentially be a very significant savings,'' Hopkins said.
Northwest, which started flying low-altitude flights out of Detroit and Minneapolis in March, found that on-time arrivals were up 33 percent in July, compared with June, in part because of the procedure, said Lorne Cass, director of flight dispatch.
Lower-altitude flights raise concerns of increased turbulence because of air currents interacting with the ground, said Michael Barr, director of aviation safety at the University of Southern California and a former Air Force fighter pilot.
The risk of ice tends to be worse at lower altitudes, and a pilot trying to navigate a thunderstorm may be under it instead of above it, he said. Some pilots also worry about bringing big jets down into skies usually reserved for small planes that don't have transponders to provide critical information about them on radar screens.
But low flights by airliners are generally kept above 18,000 feet, not ''down in the weeds with the two-seaters,'' Turk said. Furthermore, commercial planes have collision-avoidance computers on board to monitor the airspace around them.
Some 670 million Americans will fly this year, up 20 million from a year ago. The FAA reported more than 44,000 flight delays in July alone, and that was an improvement from June. Delays have been attributed to increased numbers of people traveling, bad weather and labor unrest.