When one-third of the roof peeled off an Aloha Airlines jet flying at 24,000 feet eight months ago, it stunned the aviation community. It could not happen, aircraft engineers and safety investigators said.

But it did - and the Aloha incident and others like it are changing long-held views on how well jetliners age.In October, a large crack was found on a Continental Airlines jet. And on Monday a 14-inch hole opened up at 31,000 feet on an aging Eastern Airlines Boeing 727.

A flight attendant was swept to her death in the Aloha incident, but everyone else survived as the pilots of the stricken Boeing 737 were able to land it safely. The crack on the Continental jet was found before it posed any danger, and the Eastern jet landed safely after an emergency descent and landing at Charleston, W.Va., without any significant injuries.

Eastern Airlines said Wednesday that a crack was found in the fuselage of another one of its 727s in "roughly the same area" where the hole tore open in the first plane Monday. Eastern officials in Boston said the Boeing 727 was grounded for repairs at Logan International Airport after company inspectors discovered the 3-inch crack in the fuselage Monday night.

But in Scotland, British investigators said that a bomb caused the breakup of an 18-year-old Pan American World Airways jumbo jet last week. (See story on A2.)

It is an axiom in the aviation industry that an old jetliner is not necessarily an unsafe jetliner. But the Eastern incident, perhaps the Pan Am tragedy and the Aloha and Continental incidents earlier have prompted federal regulators and the industry - not to mention the flying public - to ponder the question: When is a plane too old to fly safely?

Since 1979, the average age of the aircraft fleet belonging to the major airlines has increased from 10.28 to 12.53 years. It is estimated that about 2,300 jetliners built before 1968 are still in service.

The commercial jetliner fleet has grown older as airline executives - seeing fuel prices drop - have chosen to keep their fuel-guzzling geriatric jets a little longer.

A new jet can cost from $30 million to $100 million or more, depending on the model. While an aircraft might be old, some have undergone extensive overhauls.

Pan Am spokesman Jeffrey Kriendler said as a result of extensive modifications and strengthening, "an airplane comes out almost brand new."

Eastern, not long ago, decided to overhaul its older Boeing 727s, instead of replacing them. There were indications, in fact, that the crack that became a hole Monday on Eastern Flight 257 may have occurred where there had been repairs.

Concern about aging aircraft is not new. In 1983 the government began to require that older aircraft be subject to special inspections and increased maintenance. The program covers more than 1,000 jetliners, including Boeing 727s, 737s and 747s, as well as McDonnell Douglas DC-9s and DC-8s.

The industry and the Federal Aviation Administration believe that as long as old aircraft are closely monitored and subject to increased maintenance, they will perform safely.

But critics have suggested more is needed. And since the Aloha incident last April, many industry experts have begun to agree.

"In some cases modification (of the aircraft) is needed, not just (added) inspection," said Ben Cosgrove, a vice president for engineering at Boeing. "Nobody wants anymore Alohas."