Air Force buyers, ordered to spend as much money as possible at the end of 1985, bought more than $1 billion worth of spare parts during the last 12 days of December, according to a congressional investigation. The purchases represented more than a tenfold increase in the normal buying pace.

The spending order apparently caused the Air Force to shelve some recently adopted changes aimed at curbing price abuses. At one buying center, for example, three-quarters of the contracts were "unpriced orders," under which the Air Force agrees to buy parts at a price to be negotiated later.

The Washington Post reported in January that the Air Force Logistics Command, in a Dec. 20 message, ordered spare parts buyers to obligate funds "to the maximum extent possible" by Dec. 31. Air Force officials said that an embarrassing backlog of unspent fiscal 1984 and 1985 spare parts funds had made the Defense Department reluctant to release fiscal 1986 monies.

Critics charged that the backlog had accumulated because the Reagan administration has provided the Air Force with more money than it can handle. While agreeing in part, Air Force officials also said that reforms in parts procurement had slowed the process.

One official said at the time that the Dec. 20 order "won't cause people to do dumb things." But Rep. John D. Dingell (D-Mich.), Energy and Commerce Committee chairman, asked the staff of his subcommittee on oversight and investigations to examine the order's impact.

Dingell said his inquiry shows that the Air Force is still making mistakes in its spare parts buying policy.

"The spare parts scandals are not over," said Dingell, a frequent critic of Defense Secretary Caspar W. Weinberger. "The promising rhetoric of Weinberger's reform initiatives appear to be withering in the face of the cold reality of business as usual in the field."

The Air Force declined to comment on the Dingell report.

The Dingell subcommittee found that during the first 20 days of December, the Air Force spent $117.5 million of 1984 and 1985 spare parts balances. But during the final 12 days, the service spent $1.06 billion.

The Dec. 20 message cautioned buyers to "continue to operate within the constraints of law, directives, prudence and bona fide need rules." But the message also said that if those directives "constrained" the buyers' ability to spend, they could "pursue one-time waivers."

The extensive use of unpriced orders -- which the Air Force now calls "undefinitized contractual actions" -- appears to typify such waivers. Because the Air Force is at a disadvantage in bargaining once it has promised to accept the goods, it has been trying to minimize use of such orders.

But at Warner Robins Air Logistics Center in Georgia, congressional investigators found, three-quarters of all actions in the last 12 days of the year were unpriced orders.

In addition, when emergency needs make unpriced orders inevitable, the Air Force normally obligates only half the maximum contract amount as a safeguard and to increase its leverage in later negotiations. But in the last 12 days of the year, the Air Force committed 100 percent of the maximum to swell the total spent, the investigators found.