The Air Force has revamped its next five-year budget plan so it can buy at least 126 Northrop F20 Tigershark fighters, which Defense Secretary Caspar W. Weinberger has said are no match for Soviet planes.

The burgeoning controversy over the F20 has embroiled Congress, split the Air Force, unleashed an intensive lobbying campaign by the competing contractors and raised questions about who in the administration is pushing the plane on the Pentagon.

According to documents obtained by The Washington Post, not only would the Air Force buy 126 F20s in fiscal years 1987 through 1991, it also would open its competitions for the next generations of ground-support and air-defense aircraft to the Northrop plane.

In one Air Force paper, the term "directed buy" is used in connection with the 126 F20s to be procured, apparently without a competition. The paper does not say who did the directing.

In addition to its inclusion in the Air Force Program Objective Memorandum, the new five-year budget plan, the F20 won a legislative scrap in the House Armed Services Committee during a recent closed-door markup of the Pentagon's fiscal 1986 procurement bill. The House panel directed the Air Force to conduct a competition between the F16 and the F20 in fiscal 1986 before proceeding with its planned purchases of fighters for that year.

General Dynamics Corp., the giant aerospace contractor under fire for billing the Pentagon for nonreimbursable expenses of its executives, stands to lose millions of dollars in F16 sales if the Air Force buys the F20. General Dynamics has launched a counterattack against the plane, with glossy brochures designed to convince lawmakers and others that the F16 offers the nation much more for the money.

"We are fully prepared for a competition," Alvin A. Spivak, a General Dynamics spokesman, said yesterday in response to the House panel's action. "In the long run, we are sure that the demonstrated performance superiorities and cost advantages of the F16 will continue to be recognized."

Pentagon executives expressed surprise over the Air Force's sudden turnaround on the F20 as evidenced by its decision to build the purchase of 126 of the planes into its new five-year plan. "It's strange," said one official involved in the internal Air Force debate.

Thomas V. Jones, Northrop's chief executive officer, is an acquaintance of President Reagan and for the past several years has pressed administration officials to buy the F20. Northrop has built three of the planes, one of which crashed, but has been unable to go into production for lack of sales here and abroad. The first F20 flew in August 1982.

Weinberger, in attacking an Atlantic magazine article on the F20, wrote in a letter printed in the magazine's November 1984 issue that "the F20 was not designed for or intended for U.S. forces. We simply cannot put our pilots in the position of flying planes we know cannot meet scenarios our pilots may have to face . . . . We have not bought it because we must have planes better than aircraft used by the Soviets in a difficult scenario such as the skies over central Europe . . . . We also must weigh the costs associated with introducing a new aircraft into our inventory . . . . "

Jones of Northrop, in a letter published with Weinberger's, said "the F20 was not intended for the Air Force fighter-force structure. The Air Force is fully aware of the capabilities of the F20."

Jones stepped away from his November statement last month by urging the Air Force to buy a mix of F20 and F16 fighter planes, and he offered to deliver 396 F20s for $15 million each. "The F20 and the F16 have complementary combat capabilities," he said. "In some missions the F20 is better; in some the F16C is better."

Pentagon officials said Air Force Secretary Verne Orr has been pressing military leaders to put more emphasis on developing a new ground-support aircraft and has been reluctant to go ahead with an advanced tactical fighter for the 1990s until cost questions are resolved. Orr also has been under congressional pressure to find room in his budget for the F20.

The Air Force's new five-year plan calls for buying 30 F20s in fiscal 1987 and 24 in each of next four years.

Under the plan, F16 purchases would drop from 180 in fiscal 1986 to 150 in 1987 and 96 for the four subsequent years.

The plan also calls for buying almost 200 air-defense fighters through an annual competition and 120 ground-support planes under a multiyear contract starting in fiscal 1988. The F20 could compete for those contracts along with the F16, assuring a continuation of the current battle between Northrop and General Dynamics and their champions in Congress and the executive branch. A Pentagon spokesman refused to comment on the internal Air Force budget plan.