(FORTUNE Magazine) – As if the terrible airline crashes last year weren't bad enough, Federal Aviation Administration figures show that the number of near misses took off 36% in 1994. No one seems to know why.

The FAA records a near midair collision (NMAC) when planes pass within 500 feet of each other. Controllers try to separate planes three to five miles in distance and at least 1,000 feet in altitude.

"I don't think anyone can explain the increase with assurance," says Richard Swauger, technology coordinator at the National Air Traffic Controllers Association. "Even if we were to lose lots of controllers, I still wouldn't necessarily say that the number would increase again."

Before the big leap last year, the number of near misses had fallen steadily each year since 1988, even as airlines logged more hours in flight (see chart). David Harrington, a manager in the FAA's flight standards office, attributes the drop to the traffic collision avoidance system (TCAS) adopted in the early Nineties. One feature: Pilots now use cockpit-based radar to augment data from controllers.

Some controllers blame the sudden blip on the airlines, since service cutbacks at secondary hubs often force more traffic into fewer flyways. And crashes set off a subsequent increase in incident reports. "Our hot line rings more often after accidents," says Harrington.

Satellite navigation due soon promises to make near misses even rarer. Until then? "I just hope this is not a trend," he says.

-- RONALD B. LIEBER