The Air Force may waste half or more of the $1.4 billion it plans to spend replacing the wings of its fleet of 81 trouble-ridden C5A cargo transports, a congressional hearing was told yesterday.

The Air Force and Lockheed Corp. "may be exaggerating the seriousness of the wing problem" to justify replacement rather than far less costly repair, Sen. William Promire (D-Wis.) charged.

Under a recent contract, the defense contractor's Lockheed-Georgia subsidiary, which built the huge planes, will get $1.16 billion of the total outlay. This is "the biggest cost in history to correct a mistake," just as the original $2 bilion cost overrun, discovered in 1968, was the largest in the history of defense procurement, Proxmire said.

As he described it, the mistake was Lockheed's 1965 decision to reduce each wing's weight by five tons. This helped Lockheed underbid the rival Boeing Co. for the job of supplying the Air Force with aircraft capable of carrying outsized loads.

Lockheed-Georgia president Robert B. Ormsby Jr. acknowledged to Proxmire's subcommittee on priorities and economy in government that the 10,000-pound weight reduction caused the problems that now are said to require replacement of the wings.

Under questioning, however, Ormsby denied emphatically that Lockheed had known when it decided to reduce the weight of the wings that replacement would be necessary. The company's sole objective was to reduce wing weight "to the absolute minimum within the design requirements," he testified.

When Ormsby estimated that the company will make a $140 million profit on the contract, Proxmire remarked, "I think a lot of businessmen wish they could make mistakes" like Lockheed's.

The point was developed by David Keating, legislative director of the 150,000-member National Taxpayers Union (NTU).

A $140 million profit for replacing a "clearly deficient" part sets up "a terrible incentive system," he testified. Its message for other defense contractors, he said, will be: "The more inefficient you are -- the more profit you make. Build failures into the system and you'll be rewarded."

By contrast, Ormsby said that "in terms of the critical need" for the C5As to the nation's defense, the $1.4 billion program is a bargain for the taxpayers.

The hearing's principal focus was on the assertion by the Air Force and Lockheed that a 30,000-hour wing-service life -- the goal of the $1.4 billion program -- is warranted.

The 30,000-hour service life -- for which the C5A was supposed to have been designed -- would, at projected use rates, keep the aircraft in service until the year 2019, although by that time, Keating said, it probably would be obsolete.

The Air Force estimates the current wing-service life of the C5A at an inadequate 7,100 hours. But this figure is sharply quetioned by hitherto secret portions of a three-volume report that were declassified and released under a Freedom of Information Act request by the NTU.

The report was done for the Air Force by Rand Corp., and its contents were described at the hearing by Keating and by a coauthor, Paul C. Paris, an expert on wing cracks at Washington University in St. Louis.

The report said the 30,000-hour goal "needs to be reexamined," because if it is excessive "there is no overwhelming technical reason to foreclose . . . the possibility that the service life might easily be extended to 12,000 to 15,000 hours, providing the opportunity for service to the end of the century without significantly impairing the aircraft performance capabilities."

Service to the year 2000, involving "more modest structural modifications and extending present constraints on operational use," could be had at significantly less cost than wing replacement, the report emphasized.

Coauthor Paris said the report raised the possibility that the C5As, under the operational restrainst, could reach the year 2000 without any significant modifications.

But if the Air Force hadn't imposed severe restrictions on the C5A to prevent restrictins on the C5A to prevent it from being flown in accord with its design specifications Paris said, it would have lasted fewer than 3,000 flight hours. For a major aircraft producer to miss the fatigue life of a wing by a factor of more than 10 is "absolutely incredible," he testified.

After getting the report, the Air Force, for reasons that Paris said "seem self-evident," set up what it called a Structural Information Enhancement Program. He was the only member of SIEP's steering committee who was not a regular employe of the Air Force or Lockheed.

From the start, Paris testififed, "the tacit assumption was made that since the [wing replacement] was going to be done, other less expensive options were not to be considered . . . The fact, so clearly pointed out in the Rand study, that the 30,000 hour life had no rational basis and that invoking it precluded other options, was simply ignored."

In a practice Paris termed "dangerous" and Proxmire said had "the appearance of a conflict of interest -- at least," SIEP relied for data solely on Lockheed.

Paris said that wing cracks -- the vast majority of them "inconsequential" -- developed in the C5A, but that wing cracks develop in "all" aircraft.

The report recommended that a panel of independent specialists be appointed to assess the situation. Proxmire said he wants the assessment done by the congressional Office of Technology Assessment. So do the NTU and Paris.