The Air Force has been breaking apart brand-new airplane engines bought for emergency use to get the parts it needs to keep its F15 and F16 fighters flying, according to internal documents obtained by The Washington Post.

This cannibalization is occurring even though Congress has appropriated billions of dollars in recent years to buy spare parts for engines already in those planes, officials said. A major reason for this, they said, is that contractors are not delivering the spare parts on time.

Internal Air Force messages shuttling between various commands document that the service is anxious to come up with "a recovery game plan" before the situation balloons into another controversy about Pentagon procurement.

"The increased level of interest" by Congress and the Air Force top brass in the spare-parts shortage makes it "imperative that we move ahead rapidly with corrective action," one message says.

The Pentagon's inspector general, Joseph H. Sherick, recently began an investigation of why spare-parts deliveries for the Pratt & Whitney F100 engine, which powers both the F15 and F16, have been as much as a year late.

Over the last five years, Congress has appropriated about $2 billion to buy spare parts for the F100 engine in response to stories about first-line fighters becoming "hangar queens" for want of engine parts. When parts are not available for engines already in airplanes, mechanics often cannibalize -- and thus disable -- new engines that are supposed to be kept at the ready for emergencies, such as war.

In the three months from July 1 through Sept. 30, 1984, Air Force figures show that mechanics at the 1st Tactical Fighter Wing at Langley, Va., took parts off new F100 engines 265 times. Cannibalizing was three times higher at Hill Air Force Base at Ogden, Utah, where mechanics for the 388th Tactical Fighter Wing took parts off new engines 864 times.

In an information paper, the Air Force attributed the situation to "technical problems and parts shortages related to relatively poor fiscal 1981 and 1982 spare-parts funding. As a consequence of technical fixes being incorporated and better fiscal 1983-84 funding, we expect cannibalization actions to decrease in fiscal 1985 and continue to decrease in subsequent years."

Air Force figures show that as of last month the goal was to have 108 spare engines for the F15 ready to go, out of 210 on hand. But only 59 were serviceable, according to the figures. The picture was brighter for the F100 version that is used in the single-engine F16 fighter. The goal was to have 57 of the 133 spare engines ready, and the Air Force said 62 were available for duty.

But an internal Air Force paper warns that the availability rate for these spare engines is likely to drop "in the next few months" because of the "reoccurrence of lack of spare parts in the field and a backlog of work . . . . " The memo blames "the growing pains" of putting spare parts up for competitive bidding rather than rely on a relatively small group of suppliers.

"The growing pains of this breakout are evidenced by many of the new vendors failing to perform at all or producing inferior parts," the memo states.