High above the sunbaked tarmac of a California airfield late last month a group of VIPs and business people perched on metal folding chairs peering into the sky at an aerial oddity: a ''plastic'' plane.
The experimental six-passenger craft had an airframe made almost entirely of composites - tough plastics reinforced with fibers. It was being tested by a California firm, Avtek Corporation, which sees it leading to a new era of lightweight, fuel-stingy corporate aircraft.
Another devotee of plastic planes, meanwhile, Lear Fan Ltd., has had trouble making its version airworthy. A prototype of its Lear Fan (70 percent plastic) suffered a cracked fuselage, among other things, during tests.
These aircraft encapsulate the problems and promise of what is expected to be one of tomorrow's key materials: composites. After more than 30 years under study in the laboratory, these mixtures of plastic resins and reinforcing fibers - many stronger and lighter than steel - are still struggling for commercial acceptance.
True, composites are now used in everything from tennis rackets to car grills to missile nose cones. And they are among those forms of matter expected to be a part of the ''new-materials age,'' in which exotic stuff like high-performance ceramics, metallic glasses, and specialty alloys increasingly replace such materials of the Industrial Revolution as iron and steel.
Yet the superstrong plastic compounds are emerging gradually, particularly in big industries like aerospace and automobiles.
This isn't, certainly, for any lack of admirable qualities on their part. Composites are tough (some bulletproof vests are made of them), lightweight, and noncorrosive. They can also be molded into complex shapes with little machining, often replacing several metal parts with one plastic piece. Different fibers and resins can be mixed to meet different needs. But they also have their warts:
* Costs. Raw materials in some advanced composites cost $50 a pound, says Don Slivka, a materials economist with Battelle Laboratories, vs. $2 to $3 for aluminum and less than $1 for steel.
* Retooling. There's also the problem for big companies of replacing their conventional equipment.
* Durability. Uncertainty remains about how long composites will last and how they will react to impacts. How will a plastic wing hold up after thousands of landings? Adding to the problem: Defects inside the material (delamination, for instance) may not be visible on the surface.
* Fabricating parts. Particularly with sturdier long-fiber systems, production is often slow and labor intensive. As many as 400 layers of fiber can go into an aircraft wing - each laid up by hand. This handwork makes it tough to control production and get uniform parts. Plastic also requires precise engineering. You can't slap it in on an anvil and bang it into shape.
''The major hurdle is manufacturing,'' says Dr. Byron Pipes, head of the University of Delaware's Center for Composite Materials. ''Current manufacturing of high-performance materials is not automated. It produces a wonderful product - but a handmade one.
''This is changing. New computer-aided design systems, robotics, and other machinery to work with the materials are being installed in factories. New raw materials (thermoplastic resins, for example) have also been developed; they are more heat resistant and easier to inject into molds, making them suitable for mass production.
All this has helped plastics find their way into everything from golf clubs to shower stalls. The most promising area, though, may be aerospace. The industry seems far short of earlier forecasts that the composite content of aircraft will hit 60 percent by 1990. Boeing Aircraft Company's latest commercial planes, for instance, are 3 percent plastic by weight (although 30 percent of the outside skin is composite). And, except for some military craft, plastics aren't used in any jetliner parts in which failure could cause a crash - for now.
The Federal Aviation Administration is considering certification for five different corporate planes with all-composite airframes (including the Avtek 400 and Lear Fan). The featherweight materials are expected to help the planes cut fuel consumption by up to 40 percent over aluminum counterparts. Boeing, meanwhile, recently tested composite support elements in the tail assemblies of five of its 737 jetliners. In July, a Connecticut firm put the first helicopter with an all-plastic airframe in the air - one 24 percent lighter and with 75 percent fewer parts than a comparable metal chopper. Still, a commercial version probably won't be ready until the 1990s.
In the automobile industry, last year's production of the Pontiac Fiero - the first General Motors composite-body car since the Corvette in 1953 - has boosted the prospect of more plastics in cars. Previous forecasts that the average car would carry 500 pounds of plastic by 1995 still seem vastly inflated. But the material is spreading beyond mainly decorative trim to structural areas like bumpers, fuel tanks, and springs. Other items taking on more plastic: leisure goods (fishing rods, boat hulls), construction wares (pipes, building panels), and military parts (jeep hoods, helmets).
Still, the ''composite age'' will dawn slowly. ''Steel has been around for 200 to 300 years,'' notes Dr. Joseph Lees, manager of composites research for the E.I du Pont de Nemours & Co. ''You don't get rid of something that is still doing a job. You move judiciously.'' Besides, he adds, ''the metals people are not going to sit still.''