For months, astronaut David Walker practiced not landing a space shuttle.

In ground simulators and on a specially modified business jet, Walker and his pilot, Bob Cabana, rehearsed keeping their hands off the controls as their craft glided in for a landing.

The real thing was to have oc-curred at the end of the next flight of the space shuttle Discovery, which is scheduled to blast off this week. With Walker as commander, Discovery - now poised for a Wednesday morning liftoff from Kennedy Space Center - was to have become the first U.S. space shuttle to land itself.

The planned "autoland" was canceled, however, after an ex-Marine pilot was appointed chief of space flight for the National Aeronautics and Space Administration.

During this shuttle flight at least, the decades-old struggle between space technology and aviation custom will remain at a standoff.

NASA space shuttles always have been capable of landing themselves, in theory at least. But a human has been at the controls since the very first test flight. (The former Soviet Union's shuttle prototype, known as Buran, landed without a crew during its first and only test flight in November 1988.)

U.S. astronauts - particularly those culled from the military specifically to command and pilot spacecraft - always have resisted surrendering the relatively few human-controlled aspects of space flight to computers, which of necessity handle nearly all of the complex functions aboard a shuttle.

That resistance dates from the earliest days of space flight, when the original Mercury Seven astronauts insisted that windows and control sticks be added to their space capsules so they could pilot their craft manually.

Now, with the prospect of a U.S. space station in orbit by the end of the decade, some in NASA want to ensure that shuttles can land safely automatically. After a long stay in space, a shuttle's commander and pilot could became incapacitated by the return to Earth's gravity, they argue, and it would be up to the space plane's computer to land what amounts to a 100-ton glider.

Walker, a Navy fighter pilot from Eustis, has mixed feelings about the decision to cancel the Discovery test.

"We worked hard on it," he said recently. "On the other hand, no pilot that I know of prefers to let a machine land the vehicle instead of himself, and I'm personally a lot happier just to be able to land it myself."

A shuttle is a highly automated, $2 billion machine, but NASA engineers intentionally excluded from shuttle computers the ability to lower landing gear. Shuttle officials said there was no need for the landing-gear deployment to be included in the software because astronauts were to land the ships manually.

During most of a shuttle's descent from orbit, the computers handle the thousands of simultaneous calculations needed to adjust the orbiter's flaps and rudder and get it low enough and moving slowly enough for a safe runway landing.

But the shuttle's commander always takes control of the space plane as it starts its final turn to line up with the runway. The pilot, meanwhile, lowers the landing gear by pressing a button in the shuttle cockpit.

Once the plane is on the runway, the flight crew also applies the brakes and deploys the drag chute.

Former chief of space flight Bill Lenoir, who suffered from severe space sickness in 1982 during his only shuttle flight, worried about the long-term effects of weightlessness on shuttle commanders and pilots.

Even after relatively short stays in space, astronauts often become lightheaded or dizzy from the return to gravity as a shuttle drops back into Earth's atmosphere.

"Knowing what we know today about human physiology, we have to assume that the crew cannot land," he said last November.

In anticipation of such an emergency, Lenoir, a scientist by training, wanted to test and certify the shuttle's so-called "autoland" system. When shuttle planners chose this week's Discovery mission, they began fine-tuning the computer software and creating the flight simulations that Walker and Cabana needed to practice automatic landings.

"We trained pretty extensively for it," Walker said. "We just about doubled our landing training."

In addition to practicing normal, hands-on landings and letting the computer land the craft by itself, Walker also let the computer take the simulator or the training plane down to within a few feet of the runway and then switched to manual before touching down - just in case the autoland program malfunctioned at the last moment.

"We were getting pretty good at it," he said.

During the actual test, Walker was to have let Discovery land by itself on the dry lake bed at Edwards Air Force Base in Southern California.

Shuttle commanders and pilots lobbied against the test, however, and it was scrubbed when Jeremiah Pearson, the ex-Marine pilot, replaced Lenoir as director of space flight earlier this year.

Pearson said the test wasn't needed because, once the space station is in orbit, fresh shuttle crews can be sent aloft to retrieve long-term visitors.

Walker will now take full control of Discovery as it makes the final turn for home at the end of its planned week-long mission - just as shuttle commanders have done for the 50 landings before his.

"Someday in the future there will be a need for it," Jim Wetherbee, who commanded the most recent shuttle mission, said in explaining the test's cancellation. "(But) we need to demonstrate the need for it first."