A space shuttle is likely to crash every five years under NASA's current schedule because key power units will fail, predicts a NASA-funded study that was disputed by a space agency official.

The study said there was a 1-in-70 chance on any flight that the shuttle's three auxiliary power units, which control flaps needed to land, will all fail and cause the shuttle to crash, the San Jose Mercury News reported Tuesday.The National Aeronautics and Space Administration hopes to launch about 14 shuttle missions a year once it resumes its full schedule in the early 1990s, said agency spokesman Steve Nesbitt. That would mean 70 missions every five years.

The $2 million study conducted by a consultant was among those commissioned in the wake of the Challenger disaster on Jan. 28, 1986, to examine risk assessment by NASA.

"I know they're quoting 1 in 70, (but) in my opinion, the unit is much more reliable than that" based on results of NASA testing, said Chester Vaughan, chief of the shuttle's power and propulsion division at the Johnson Space Center in Houston. "I think we have a hell of a good unit."

The study's finding quantifies the potential dangers of just one of the shuttle's two dozen critical systems, the Mercury News reported.

According to NASA tests, the power units are far more reliable than the study indicates, and the agency has not changed plans to resume launches this fall, NASA officials said. The space agency said there is a plan to double the shuttles' lifespan from 20 missions to 40.

However, independent experts and scientists close to NASA told the Mercury News they were startled by the report.