The Bush administration this week ended 29 years of continuous flights by Air Force doomsday planes equipped to direct a nuclear war after any Soviet attack on the United States, the Pentagon said yesterday, a concession to the budget squeeze and the superpower thaw.

The continuous flights will be replaced by occasional flights, the Pentagon said. The decision was approved by President Bush late last month after a discussion with his senior advisers, a White House official said. It was conveyed June 29 to the Strategic Air Command (SAC) at Offutt Air Force Base in Omaha by Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff.

The last continuous flight over the central United States landed at 2:28 p.m. (CDT) Tuesday at Offutt and was commanded by Gen. John T. Chain Jr., who heads SAC. The operation, code named "Looking Glass," was a staple of the Cold War.

The first flight, under command of Lt. Gen. John P. McConnell, went aloft on Feb. 3, 1961, the year the Berlin Wall was erected in a Cold War challenge to President John F. Kennedy by Soviet leader Nikita Khrushchev. The Air Force has kept at least one plane in the air ever since.

Defense Secretary Richard B. Cheney made a similar recommendation last December, after the Air Force sought to ground the fleet of 12 "Looking Glass" planes for economy. But senior administration officials rejected the request, saying it was the wrong signal to send as the world waited to see whether the Soviet leadership would allow democratic revolution in Eastern Europe to proceed unchecked. "That was December and this is now," said Pentagon spokesman Pete Williams, "and we thought we could back off a little bit."

In the argot of doomsday planning, the planes are known as "Post Attack Command and Control Systems." They are Boeing EC-135s, modified versions of the venerable Boeing 707 jetliner. The airborne command posts have flown a lazy concourse over Nebraska and nearby states for 29 years and five months, commanded by a SAC general designated as the "Airborne Emergency Action Officer."

A SAC description of the planes says that if SAC's underground command post is destroyed by a surprise Soviet missile attack, the airborne command post "gives SAC a further guarantee that its global forces can be controlled and launched successfully in retaliation to attack."

Air Force officials said the airplanes could launch all nuclear-armed Minuteman and MX intercontinental ballistic missiles in ground silos across the north central and western United States.

A SAC general must be aboard each flight to ensure continuity of control over U.S. nuclear forces should the president, vice president, defense secretary and SAC command bunker die or become isolated by a Soviet surprise attack.

Administration officials emphasized yesterday that curtailment of flights by the four-engine, swept-wing planes, which will save $18 million in 1991 and $23 million the following year, does not represent a change in U.S. readiness.

"Instead of flying all the time," Williams said, "they will continue to fly random sorties each week and for the rest of the time we will keep them on quick-reaction ground alert so that if we need to, we can have them back in the air in a matter of minutes and go right back to the old way of doing things."

Still, the grounding of the planes, grim symbols of a nuclear-armed world and its potential for Armageddon, is a noteworthy milestone eight months after the fall of the Berlin Wall and collapse of the Soviet-led Warsaw Pact.

A SAC spokesman in Omaha said the decision reflected greater confidence that the United States would have ample notice through improved intelligence satellites and other detection systems of Soviet mobilization for a nuclear or conventional attack.

"The chances of a bolt-out-of-the-blue kind of attack these days is virtually gone," one SAC spokesman said. A SAC statement said, "Better intelligence and warning times, plus world events and the always present need for economy, were factors" in the decision.

According to Pentagon documents, the Air Force also has recommended bringing home similar airborne command posts used by the regional commanders-in-chief of U.S. forces stationed outside the continental United States.

The "Looking Glass" designator for the airplanes derives from the planes' technical design as the mirror image of the underground SAC command bunker. During the Reagan administration, this bunker was expanded and shielded to make it immune from the "electromagnetic pulse" from a nuclear detonation aimed to knock out electronic command and control communications equipment and thereby "decapitate," or cut off, silo crews waiting for orders to launch missiles. Staff writer Ann Devroy contributed to this report.