At least 56 instances of cracked bulkheads in the tail assemblies of DC9 jetliners have been reported by U.S. airlines the last five years, according to reports on file at the Federal Aviation Administration's records center here.

The cracks, which authorities say could lead to sudden losses of cabin pressure such as experienced when the tail cone on an Air Canada DC-9 fell off Monday, were found and repaired during periodic inspections by airline mechanics, the report shows.

Twenty-three of the 56 instances have occurred since January, which leads some experts here to believe that bulkheads are more subject to cracking as aircraft get older.

The DC9 is a smaller and older aircraft of McDonnell Douglas Corp., which also manufactures the wide-bodied DC10 aircraft that were grounded five weeks last summer after an American Airlines DC10 cracked May 25 in Chicago, killing 273 persons.

The history of the DC9 bulkhead flaws, which were detailed in records here, supports action yesterday by the FAA in ordering stepped-up inspections of the tail assemblies.

The bulkhead separates the pressurized passenger cabin from the tail cone, which is designed to come off for emergency evacuation of passengers.

A sudden cracking or pressure leak in the bulkhead can cause a blowout of the tail section, leaving a gaping hole in the rear of the plane and creating a vacuum effect, authorities said.

"Fortunately, we've never had this happen during a flight on any domestic airline," Ernest Ouellette, acting chief of the safety data branch here said. "The Air Canada incident was the first."