About half the nation's first-line warplanes cannot fly because over the years the Pentagon has concentrated on buying new ones rather than fixing up the old ones.

This policy has forced mechanics to take the parts off one plane and put them on another, a constant process of cannibalization that the Navy figures takes up the equivalent of 610 men doing nothing else for one year.

Besides not being ready to go to war, the high percentage of broken planes means that Air Force and Navy pilots must fly fewer hours, prompting many of them to quit in disgust.

These are the findings of Rep. JackEdwards (R-Ala.), ranking minority member of the House Defense Appropriations subcommittee, who has personally investigated the problems of aging equipment and inoperative planes.

"This is unacceptable," Edwards said. "People are lulled into the feeling that we've got an Air Force hot to go. But what we've really got are hangar queens all over the country."

There are, Edwards found, scores of warplanes in the shop waiting to be repaired.

He cited two examples he said are typical. Only 53 percent of the Air Force's hottest fighter, the F15 Eagle, were ready for combat at any one time last year, and only 53 percent of the Navy's F14 Tomcat fighter. The percentages for forward-deployed aircraft, such as F15s based in Europe, were not much better, he added.

At least 70 percent of the planes should be ready to go at any given time, the congressman contends.

Defense Secretary Harold Brown, under questioning by the House Defense Appropriations subcommittee this year, acknowledged that the problem is serious, but neither Brown nor his predecessors has taken the problem seriously enough to spend more money on spare parts and less on new planes, Edwards complained.

Edwards displays a chart of Pentagon figures to make his point. The Air Force plans to spend twice as much money to buy new tactical aircraft in fiscal 1983 than in fiscal 1975, from $4.2 billion to $8.4 billion, in comparable fiscal 1980 dollars. But no extra money is budgeted to keep the planes flying. The operating account for repairs remains at $7.4 billion in 1983, as it was in 1975. The budget plan for Navy fighter planes shows a similar trend.

"The combat readiness of our first-line fighters like the F14 and F15 is poor," Edwards wrote. "The underlying cause for this problem is quite simple: inadequate provisions of spare parts."

Half the time, Edwards continuted, mechanics are forced to rip a part of one plane to fix another or else must use parts supposedly reserved for wartime. To keep one F14 flying 100 hours, he said, the maintenance crews have to take parts off another F14 from 40 to 50 times.

"It makes no sense whatsoever," Edwards wrote, "to buy all these very costly aircraft without also buying all the other necessary parts and weapons that make these aircraft an effective combat system."

Operation and maintenance funds, which include money for spare parts, have traditionally lost out to new airplanes. Defense contractors lobby the Pentagon and Congress to buy new planes, not to fix up the ones already purchased, Edwards noted.

Air Force Gen. David C. Jones, chairman of the Joint Chiefs of Staff, has accused Congress of cutting the funds the military has requested in the past for operating and maintaining planes and other weaponry. That is a "bad rap," Edwards said.

"What I found out for the first time this year," he said, "is that the careful cuts our committee had directed the Pentagon to make in those accounts were not made."

Instead, he continued, Pentagon leaders notified field commanders, including the head of the Strategic Air Command, that they were getting less money for operation and maintenance. It would be up to the commanders to make the cuts where they saw fit.

Edwards has written letters to the chairmen of the House and Senate Armed Services committees urging them to join the Appropriations subcommittee in forcing the Pentagon to shape up.

"It's easier for a member of Congress to go back home and say I got you another 10 planes than say I spent money to make them ready," Edwards added.

It will not be easy to reorder the priorities, Edwards conceded, "but I would rather build half as many planes if I could turn the money saved into readiness. The situation we find ourselves in now is totally unacceptable."