The Office of Technology Assessment questioned yesterday the suitability of some of the equipment and design for the Federal Aviation Administration's proposed multibillion-dollar modernization of the air traffic control system. It said the plan may be based on inflated projections of traffic growth.

In addition, proposed fuel and ticket taxes that the FAA hopes would raise most of more than $6 billion in capital funds needed for its program between fiscal 1983 and 1987 would hit disproportionately hard at airlines, OTA chief John H. Gibbons told the House subcommittee on transportation.

Gibbons was commenting on a 20-year project unveiled by FAA chief J. Lynn Helms in January. It would replace the system's aging computers and communications gear, and would rank among the federal government's largest ever nonmilitary procurements, with an eventual cost of $10 billion or more. The modernization, which requires congressional approval and funding, is needed to accommodate growing traffic and maintain safety standards, according to the FAA.

Helms, testifying after Gibbons, defended the FAA's plan and rejected many of Gibbons' criticisms, ranging from the air traffic growth estimates to the design approach.

In a statement prepared for the subcommittee, Gibbons expressed support for modernization of the system and termed the FAA's plan "a significant and even bold step." But, he said, it "lacks a clear statement of priorities and contingencies."

Studies by OTA, a congressional agency, show that low capacity at airports, not air traffic control, will be the main barrier to growth in the years ahead, Gibbons said. The FAA might do better to settle airport expansion programs before drafting an air traffic plan, he said.

Gibbons, noting that "FAA's traffic and workload forecasts have tended to be too high in the past . . . ," cautioned that inflated estimates in this case could mean the FAA has selected the wrong equipment.

He also said FAA proposals to replace equipment step by step could mean that "hardware choices made in the first stages will limit the options available for the final system design."

The FAA proposes to replace aging computers by 1986 at the continental United States' 20 enroute centers, which control planes after they leave airports, but to continue using old programming until the late 1980s, Gibbons said. Improving existing computers at some centers for the time being while completely redesigning the system might be wiser, he said.

Gibbons noted some analysts believe that the FAA's provisions for automation and turning controllers into air traffic managers backed by computers could jeopardize safety and called for "close and continued examination" of this concept.

The Defense Department feels that parts of the FAA plans "will either be irrelevant to the military mission or impose more costs than benefits on the military user," Gibbons said. The Defense Department said that equipping military planes with the required communications gear would cost it more than $1 billion, he said.

FAA chief Helms, in his testimony, argued that FAA growth estimates have normally been low, not high. Improving existing computers was not feasibile, he said.

Distribution of costs to users is equitable, he said, though he acknowledged that many feel it could have been done better. Helms said the FAA would make every effort to accommodate the Defense Department's objections.