The Federal Aviation Administration, concerned about the growing number of metal cracks and fatigue problems turning up on aging airliners, has launched a major study that is intended to produce much tougher inspections for planes as they get older.

Nicknamed the "geriatric aircraft program" by senior FAA officials, the study has been under way for some months but is being stepped up in light of the problems discovered this week with the McDonnell Douglas DC9 jet, FAA officials said.

Monday afternoon an Air Canada DC9 just out of Boston was forced to return after the wall at the rear of the passenger cabin, called the aft bulkhead, blew out. Air pressure in the cabin, sufficient to keep passengers comfortable at 25,000 feet, was strong enough to cause the "explosive decompression" because a major crack -- undiscovered in previous inspections -- had developed in the bulkhead.

Flight controls, including throttles for the engines and cables for the tail and rudder, run through the aft bulkhead and were damaged, but the plane landed safely.

That incident, in which passengers could turn around in the cabin and see out the back of the plane, resulted in an FAA order to U.S. airlines for detailed inspections of similar older DC9s to find other cracks. Fewer than 125 of the 377 DC9s in the fleet were subject to those inspections, but industry sources said yesterday 141 planes had been inspected.

The FAA reported 41 planes with small cracks, though only one had a crack of serious length.

At least two follow-up orders from the FAA for more frequent DC9 inspections are expected this weekend or early next week as the data is evaluated, officials said.

Charles Foster, the FAA's associate administrator for aviation standards, said yesterday that "with airline deregulation, a lot of people are getting older planes out of storage" as smaller airlines seek to become larger airlines.

As a result of the "geriatric" study, Foster said, "inspections will probably have to become more frequent" after a plane reaches a certain age.

FAA Administrator Langhorne Bond, Foster and other aides recently visited Wright-Patterson Air Force Base in Ohio to study what the Air Force has been doing on the problem of aging airplanes.

"Frankly, the Air Force is ahead of us right now," a senior FAA official said."They know more about the problem and they're helping us with this." New regulations stressing inspections are being drafted based on work from that study, officials said.

Cracks in load-bearing metal structures are a regular, expected occurrence in aviation. The trick is to be sure that inspection procedures are frequent enough to catch the cracks and repair them before they reach dangerous lengths.

The older a plane gets, the theory goes, the more fatigued its metal supports become and the more subject they are to cracking.

The DC9 incident has received considerable publicity, but a similar case occurred in May 1977 when a Dan-Air Boeing 707 crashed near Lusaka, Zambia, and killed all six persons aboard.

The British Department of Trade reported the cause of that accident was "metal fatigue and inadequate design" in the horizontal stabilizer rear spar, which means that the plane essentially became uncontrollable. Investigators found that the airline operator had followed the approved inspection schedule.

Boeing 707 operators around the world subsequently reduced the time between inspections.

The problem with the DC aft bulkhead first was reported in 1976, and McDonnell Douglas recommended to operators that they either modify the bulkhead to strengthen it or increase their inspection schedules.

Air Canada had inspected the airplane that was involved in the Monday incident in the spring of this year, using X-rays, but found no problems, according to FAA officials.

The new FAA orders for DC9s, expected this weekend, will beef up inspection schedules and techniques for the type of DC9 Air Canada was flying, and will extend a more rigorous inspection schedule to similar DC9s -- those that have stairways in the tail.

An FAA official said this last order was in the drafting stage, because of reports of cracks, when the Air Canada incident occurred.