In the wake of several recent airplane crashes, the question has begun to pop up more frequently: Can airplanes and their engines last indefinitely? As the nation's commercial aviation fleet rounds out its fifth decade of operation, a number of experts insist it is only aircraft parts that wear out. If components are replaced at the right time, they say, planes and their engines can operate safely for an unlimited period of time.
As proof, they point to the 50-year-old DC-3, once predicted to last only five years but now widely regarded as one of aviation's most effective workhorses. By some estimates there are twice as many DC-3s flying today as in the early 1940s.
``It's sort of like the boy who had a `new' knife,'' explains Homer Mouden, technical vice-president of the Virginia-based Flight Safety Foundation. ``It was the same knife, but it had two new blades and three new handles.''
``An engine goes on and on and on,'' agrees David Lord, a spokesman for the Pratt & Whitney Group, one of the major producers of jet aircraft engines. ``The core of it may be out there for perhaps 20 years, but it can be essentially new because of replacement activity.''
``As long as a plane is maintained and its parts are replaced properly, it has an infinite lifespan,'' says Federal Aviation Administration (FAA) spokesman Dennis Feldman.
Proponents of this never-worn-out theory say that cracks, cited as a factor in a number of recent aviation accidents, are not necessarily cause for alarm.
``Don't think a crack means that an airplane is about to fall apart,'' says Captain Mouden, who was a commercial pilot for more than 30 years. ``Any metal, including your car, is going to develop cracks from stress risers such as a hole, a rivet place, or a scratch. It's designed . . . to accommodate them.''
Mouden, who also recently served as head of Eastern Airlines flight-safety division, says parts-maintenance rules often vary with the size and location of cracks or other signs of wear. For instance, he says he knows of a private plane with a 10-inch crack in the engine cowling. Inspectors, however, say the crack hasn't yet progressed to the point where anything needs to be done about it, he says.
``I've flown many an airplane in which a fuselage crack had developed because a high loader [vehicle] had run up against the side and a little crack was started,'' he says.
Often a tiny hole is drilled at the end of such a crack to temporarily keep it from growing, he says. And most aircraft, he adds, are equipped with ``crack stoppers,'' aluminum ribs that add structural support to the fuselage and limit the reach of any cracks that develop.
He recalls watching the wing tip of a Boeing 707 jet being slowly bent by a series of jacks into a position 13 feet up, and then to a position 7 feet down without failing. At one point the wing was actually broken to see what kind of pressure it would take. ``That's the kind of testing done all the time that the public isn't aware of,'' he says.
Experts and government regulators admit it is a constant challenge to keep aviation technology at a state-of-the-art level. But they say mechanical problems have often proved less of a barrier to safe flying than human error.
``The introduction of jet aircraft with the highly reliable jet engine has resulted in a decline over the last 25 years in mechanical problems. We had more of them with piston-driven engines,'' says National Transportation Safety Board (NTSB) spokesman Robert Buckhorn.
``Now mechanical problems have dropped off, but human performance problems as a factor in accidents have remained about the same,'' he says.
Frequently cited as a case in point is the near crash of an Eastern L-1011 jet in May 1983 which temporarily lost power in all engines. The NTSB traced the cause to a failure by mechanics to follow established maintenance procedures in attaching certain seals to prevent engine oil leaks. The FAA says it has now remedied the problem by requiring a better system of mechanics checking on one another.
In a similar case, a May 1979 crash of an American Airlines DC-10 in Chicago was attributed in part to a crack in a wing pylon. Instead of separating the pylon from the engine as recommended, airline maintenance workers removed the two as one unit, causing the crack. The engine ripped loose from the wing on takeoff.
And most recently, the Boeing Company has admitted that a team of its workers did not properly repair the rear pressure bulkhead of a Japan Air Lines 747 damaged in a 1978 landing. It is the same plane that crashed in Japan last month, killing 520, but Boeing says more research is needed to establish a definite link.
Many accidents are due more to human mistakes than lack of rigorous enough rules and timetables, says independent safety consultant Charles Miller, former chief of the NTSB's Aviation Safety Bureau. ``I'm more inclined to look to the quality of inspection than some basic procedural problem,'' he says. He cites the recent engine explosion aboard a 737 in Manchester, England, as an example where better inspection was needed.
In any case, says Mr. Miller, there is no substitute for a watchful eye. ``Whether you're talking about tired engines or tired tails, it's not an absolute science. Nobody can write a guarantee slip for a part that's going to go 1,500 hours but not 1,501. You've got to keep an eye on it and adjust your thinking, particularly in terms of inspections, as you go along.''
FAA spokesman Dennis Feldman says aviation safety rests in large part on the ``competance and integrity'' of the people involved. But in his view the system is ``very tight'' and is working well.
``If you look at the history of aviation accidents, it's unusual to find that maintenance was the causal factor,'' says Mr. Feldman. ``Most are caused by pilot error or weather conditions or a combination.''