WASHINGTON (AP) _ 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 today 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 today that a bomb caused the breakup of an 18-year-old Pan American World Airways jumbo jet last week.

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 an airplane too old to fly safely?

Since l979, the average age of the aircraft fleet belonging to the major airlines has increased from 10.28 years to 12.53 years, according to Avmark Inc., an aircraft consulting and appraisal firm. It is estimated that there are about 2,300 jetliners in service that were built before l968.

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, willing to accept the need for added maintenance costs.

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 the Boeing 747 that crashed last Wednesday, although 18 years old and the 15th off the Boeing assembly line, had undergone extensive modifications and strengthening as recently as mid- l987. As a result, he said ''the airplane comes out almost brand new.''

Likewise, 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 at a location where there had been repairs.

Concern about aging aircraft is not new. In l983 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.

As a plane ages ''you should start to expect to see some more damage occur ... through fatigue and corrosion,'' says Ben Cosgrove, a vice president for engineering at Boeing. Boeing is about to conclude an extensive examination of 69 older jets belonging to 43 airlines in 24 countries.

Preliminary results show that most of the 69 aircraft are ''in excellent condition,'' although in some cases they are ''below expectation,'' the aircraft manufacturer observed recently.

The aircraft 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 Cosgrove, acknowledging that this view represents a change in industry thinking.

''Nobody wants anymore Alohas,'' Cosgrove said. As a result of its recent survey of older aircraft, Boeing has suggested that a modification or replacement of parts be made part of the routine maintenance program after an aircraft has reached a certain age even if there is no specific problem noticeable.

In perhaps the first official reflection of this new attitude, the FAA in October directed the replacement of 7,200 rivets on every older Boeing 737 jetliner to guard against a possible repeat of the Aloha accident.

The directive came three weeks after workers at Continental Airlines discovered a foot-long crack on an aging Continental Boeing 737 during a routine repainting job. The crack apparently had not been noticed before, even though that aircraft had been among those subject to special scrutiny because of its age.