Instead of hearing the helpful voice of ground control crackling over the radio, pilots winging over California recently heard only the wind in the wires.

A power failure on Aug. 9 at an Oakland air traffic control center left more than 60 aircraft in flight over northern California and the Pacific Ocean with no voice contact to air traffic controllers for almost an hour. Pilots were left to maneuver in busy airspace without the radio and radar guidance they normally need to stay on course.Pilots are trained to cope with such surprises, and no accidents resulted

from the outage. But Oakland was not an isolated incident. Since January, there have been 12 major power interruptions at Federal Aviation Administration Air Route Traffic Control Centers (ARTCCs) across the country.

The interruptions have ranged from as little as six minutes, at the Boston ARTCC on Jan. 19, to as long as almost 26 hours, at Houston on June 22. Other lengthy interruptions occurred this year at the Kansas City center, which lost power for over nine hours on Jan. 22, and Atlanta, which went down for seven hours on April 29. On April 6, the New York center lost power for four hours. A month later, on May 25, New York went down again, this time for two hours and 40 minutes.

The failures aren't mysterious. Like much of the equipment guiding aircraft over U.S. airspace, the power systems at about half of the nation's 21 ARTCCs are over 30 years old, said Karen Cronin, director of the FAA's National Airspace System Transition and Implementation Programs.

"We have seen a lot of problems in the power systems that are related to their old age," Ms. Cronin said. She said the FAA sends a team of engineers to the site of each power failure, "who try to come up with suggestions to see how we can handle these better. But we've never seen the same problem twice, even though we've been trying to look for patterns. It's like a car getting old - everything starts to fail at the same time."

The federal government has a solution to the problem: a multimillion-dollar effort dubbed the ARTCCs Critical/Essential Power System Modernization Program.

Under the program, new power systems are being installed at 25 locations, including all 21 ARTCCs, by the Exide Electronics Co., which has a $350 million contract for the program with the U.S. Air Force (some of the sites belong to the military).

Eight of the systems have already been installed at ARTCCs, with a ninth system, at the ARTCC in Leesburg, Va., due to be completed in September. The FAA hopes the remaining centers will have their new power systems in place by February 1997, Ms. Cronin said.

The basic design of the upgraded power systems will be similar to the old version.

Bill Weeks, a power engineer in the FAA's Engineering and Environmental Safety Division, said while power system design has basically remained unchanged since the 1960s, new solid-state electronic components make modern power systems much more reliable and easier to maintain.

Mr. Weeks said that solid-state engine controls and voltage regulation systems, such as those used in the FAA's new power systems, can fine-tune power voltages and frequencies with the degree of precision demanded by ultra- sensitive computer systems.

The new system, which in addition to the 21 ARTCCs is also being installed at two offshore FAA air traffic control facilities, will triple critical power capacity at each center. The FAA's new power system will also add additional power backup, so the odds of an air traffic control center shutting down completely are reduced, Ms. Cronin said.

Yet while the new power program is critical to keeping air traffic flowing, in the short run, the cure sometimes proves worse than the disease. Ms. Cronin said at least some of this year's power failures at ARTCCs are traced to the 12- to 15-month installation process for the system. In fact, she said, the reason Oakland went down was because part of its power system was off-line, being upgraded to the new equipment.

The morning of the failure, one of the Oakland center's three critical power centers had been taken off-line for testing, leaving the bare minimum of two centers to handle the critical power load. Then, for reasons FAA engineers don't yet understand, a circuit breaker in one of the two remaining critical power centers tripped, Ms. Cronin said. The final power center, overwhelmed by the resulting load, shut down as well.

At the same time, "the center never lost commercial power, so the transfer switch between the commercial power and the back-up generators never happened," Ms. Cronin said. "The power control system never got any indication that there was something wrong, because the failure happened downstream in the system."

FAA engineers are now trying to determine what caused the short in the first critical power center, Ms. Cronin said. At the time the system's circuit breaker tripped, a computer card in the governing power control system began smoking, she said, "and people are looking to see if that card was related to the circuit breaker trip."

The FAA has elaborate implementation plans in place to smooth the transition to the new power as much as possible, Ms. Cronin said. Meanwhile, the personnel manning the eight ARTCCs that are up and running with the upgraded system are very pleased with it, she said.