The Federal Aviation Administration will order special inspections of older Boeing 727s to ensure that the rear wall of the passenger cabin remains structurally sound and capable of sustaining the huge air pressures that build as a plane climbs to high altitudes, aviation sources said yesterday.

The special inspection order for the world's most widely used jetliner will be published shortly, the sources said. It follows the discovery of cracks in support beams and the rear wall during routine inspections of older 727s. The wall, called the aft pressure bulkhead, must remain intact to guarantee the safety of flight.

A spot check of U.S. airlines using 727s indicated that they do not anticipate service interruptions because of the inspections.

The 727 represents about one-third of the U.S. fleet of 3,000 jetliners. About 1,775 of the three-engine jets are in service worldwide, and Boeing estimated yesterday that about 780 of those were old enough to require special inspection. Boeing delivered its first 727 in 1963.

A faulty repair in the aft pressure bulkhead on another type of Boeing plane, the jumbo 747, is the leading suspect in the Japan Air Lines crash Aug. 12 that killed 520 people. It is theorized that the bulkhead collapsed under pressure, and that the explosive rush of air knocked off a portion of the tail and mortally wounded the airplane.

Many 747s have been inspected since the JAL crash, and although no problems have been found, sources said the FAA is beginning a special study on the "design philosophy and damage tolerance" of the domed or curved aft pressure bulkhead common to the 747, McDonnell Douglas DC10 and Lockheed L1011, among other aircraft.

That type of bulkhead has a thin aluminum membrane riveted to radial beams on the inside and circumferential beams on the outside. If there is a tear in the membrane, it is supposed to stop at the rivets on the beams, thus not spread and cause a hole large enough for an explosive decompression.

"We know that curved is better than flat as far as strength is concerned," a source said. "We're just looking at whether the detail within the concept is the best."

The Boeing 727 aft bulkhead is a flat wall with an exit door in the middle leading to the rear stairs. It, too, has a membrane sandwiched between beams.Cracks have been found in the beams as well as the membranes and were reported to Boeing and the FAA by the airlines.

A spokesman for the Boeing Commercial Airplane Co. said yesterday that in July, it sent an "Alert Service Bulletin" to all 727 operators urging special inspections. The pending FAA order will make the bulletin a mandatory, legally enforceable requirement.

One airline executive said, "We looked at all our high-time 727s, and every one of them had some cracking in the bulkhead."

According to sources, the FAA order will require that 727s be inspected within 400 flights if they have 25,000 flight hours and within 200 flights if they have 40,000 flight hours or more. A typical airline will fly a 727 about 3,000 hours a year, Boeing estimated.

Aviation safety experts have been concerned for some time about what they call "geriatric airplanes." Jetliners built in the 1950s are still in service many years after the end of their predicted life.

Normal FAA practice is to extend the interval between inspections of critical structural components as experience with a type of plane is gained. However, an FAA source said, "After a certain number of years, there has to be a potential for deterioration as a matter of age, so we have to sure and start shortening the intervals between inspections."