More than half of the reserve engines for F15 and F16 jets, backbone of the U.S. land-based fighter force, are "unusable" for lack of key components, the Air Force said yesterday.

Shortages of components resulted from poor forecasting of spare-parts needs and delays caused by new, time-consuming procurement requirements, Maj. Jim Jannette, an Air Force spokesman, said.

While the shortages have depleted spare-engine stockpiles, they have not affected readiness because more than three-fourths of the planes have serviceable engines, he said. The Air Force has 725 of each fighter.

"There's no problem in peacetime," an Air Force official said. "The crunch comes if you have to replace parts quickly in a war."

Reserve engines for the two aircraft have quickly dwindled since January when Air Force inventories reached historic highs as 60 percent of the 110 F15 spares and 90 percent of the 57 F16 spares were serviceable, the official said.

Today, 20 percent of the F15 engines and 72 percent of those for the F16 are usable, according to the official.

Stockpiles have dwindled because of an unexpectedly high "wear-out rate" of parts in installed engines and should become even scarcer until August, when new supplies arrive, according to Jannette.

Air Force officials failed to forecast deterioration of certain parts and did not order replacements far enough in advance, he said.

Jannette also cited the Defense Department's new emphasis on competitive bidding to explain the Air Force's failure to replenish stocks at an adequate pace. He said the process of soliciting and reviewing bids from spare-parts contractors has stretched the average time for ordering new engine components from 60 to 270 days.

Pratt & Whitney produces the F100 engine that, with modifications, fits both planes. The company formerly arranged for subcontractors to supply spare parts, but the Pentagon now does so, and the process takes longer, Jannette said.

Jannette said intensified public scrutiny of the defense procurement process has contributed to the problem. He said Air Force officials responsible for buying reserve engine parts have developed an "audit mentality," making them "very conservative" in their orders for fear that Defense Department auditors will criticize them for accumulating unneeded supplies.

Because of the shortfalls, the Air Force is expected to reassign officials at the San Antonio Air Logistics Center where most of the engine parts are kept, he said.