NEW YORK - How old is too old for an airplane?

Most travelers don't think twice about it - although there's something unsettling about easing into your seat and finding the armrest still has an ashtray built in.

But fliers may be more worried than usual after a 1.5-meter hole opened in the roof of a 15-year-old Southwest jet earlier this month. Southwest quickly grounded 79 of its older Boeing 737s for inspections.

A well-maintained plane can fly for decades. Older planes do need more repairs, but experts say an aircraft's age has never been the cause of a passenger death. Pilot training and fatigue, as well as frequency of aircraft maintenance, are larger safety issues.

The average age of jets flown by US airlines is 11 years, slightly above the world average of 10 but far shy of the 28 for Venezuela's fleet - the oldest of any country with more than a handful of jets.

Theoretically, a jet could continue flying indefinitely as long as an airline maintained it, said Bill Voss, president and CEO of the Flight Safety Foundation. The costs would eventually be prohibitive, though. Deciding when to mothball an airplane is usually a matter of the economics of the individual airline.

Older planes need more frequent inspections and bigger and costlier repairs.

That means less time in the sky carrying paying passengers.

Besides being cheaper to maintain, newer planes offer substantial fuel savings, and passengers enjoy features like personal TVs.

"Aircraft become impractical a long time before they become unsafe," Voss said.

Nearly one out of every four planes flown today by US airlines is more than 15 years old. That's about the same share as a decade ago, according to aviation consulting firm Ascend.

The government requires more frequent inspections as an airplane or certain parts get older. The Federal Aviation Administration doesn't set a mandatory retirement age for planes.

Age isn't the only factor when it comes to safety. Each takeoff and landing cycle - and the pressurization and depressurization associated with it - adds stress to the skin of the plane. Aircraft that fly short, frequent routes go through more of these cycles than planes flying long distances.

In 1988, a 19-year-old Aloha Airlines Boeing 737-200 that had made frequent, short hops among the Hawaiian islands lost a large part of its roof. Corrosion and metal fatigue were to blame.

On April 1, a Southwest Boeing 737-300 with 118 people on board rapidly lost cabin pressure just after takeoff from Phoenix after the plane's fuselage ruptured, causing a 1.5-meter tear.

Passengers reached for oxygen masks as the pilots quickly brought the plane down to an altitude with more oxygen in the air before making an emergency landing at an Arizona military base.

No one was seriously injured. While the incident is still being investigated, the jet had been pressurized and depressurized 39,000 times in its 15 years and metal fatigue is suspected.

Cracks were subsequently found on five other Southwest jets with more than 30,000 cycles.

That came as a shock to the industry. Boeing engineers had forecast that the planes wouldn't need to be inspected for metal fatigue until at least 60,000 cycles.

"It reminds us that as much we know about metal structures, we haven't figured it all out yet," Voss said.

Flying is the safest form of public transportation. John M. Cox, an aviation safety consultant and former commercial pilot, says regulators do a good job of making sure airlines perform proper maintenance.

Associated Press