CAPE CANAVERAL, Fla. - To prevent another catastrophe, NASA will replace braking mechanisms on all its space shuttles after discovering some of the gears were installed backward.

Shuttle program manager Bill Parsons said Monday he had launched an investigation into why the rudder speed brake gears, all old original parts in the shuttle tails, were never inspected in more than two decades of flight. If one of the improperly installed gears had been in a high-stress position, it probably would have led to the destruction of the spacecraft at touchdown, he said.

"Bottom line is, it was not good," Parsons said.

The rudder speed brake is used to guide and slow the shuttle as it comes in for a landing. If even one of the four sets of gears that operate the mechanism jams, then the spacecraft could not land safely.

As it turns out, the reversed gears found recently in Discovery were in the least stress-prone position and never failed. But one of the replacement gears, a spare set that was also installed backward, would have ended up in a much more high-stress location in the tail.

All the rudder speed brake gears in NASA's inventory, dating as far back as the 1970s, are being X-rayed to see whether they were properly built, and to look for rust and microcracks, already spotted on some gears.

Parsons said new or refurbished gears should be installed in time for shuttle flights to resume next March, after a two-year grounding following the Columbia tragedy. The extra work might put NASA a week or two behind, but "I think we'll be able to make that up," he said.

Discovery will fly first because the work is further along. Atlantis must be ready to quickly go to the Discovery crew's rescue at the international space station, however, if need be during an emergency.

The installation problem surfaced late last year and prompted NASA to delay the next shuttle flight from fall 2004 to spring 2005.

"Because of the way these gears go together, you can actually make a mistake and put them in incorrectly, and there was not a good process back in the timeframe" to catch mistakes, Parsons said.

He said the maker of the rudder speed brake mechanisms, Hamilton Sundstrand in Rockford, Ill., now had better quality control.

Call the Plumber?

At the same time, NASA is inspecting the plumbing in each of its three remaining shuttles. The hoses in question are also original parts, and some are starting to leak, Parsons said.

"As we deal with aging vehicle kind of issues, we will find other things along these lines as well, I'm sure," he said.

Parsons said engineers were making good progress on the inspection booms and wing-repair kits that will be required on all future shuttle flights.

Columbia was destroyed and its seven astronauts were killed during re-entry last year because of a hole in the left wing caused by a piece of insulating foam that broke free at liftoff.