The Federal Aviation Administration -- which has been investigating a series of unexplained business jet crashes -- ordered yesterday that a major component in the automatic pilot of about 700 Learjets be disconnected immediately until new parts can be installed.

The order, called an airworthiness directive, comes after a months-long probe of the popular Gates Learjet Series 20, the top-selling business jet. According to FAA officials in Kansas City, some of the unexplained accidents apparently are related to the pitch control on the automatic pilot.

Pitch is a term meaning the angle at which an airplane's nose is pitched into the sky. If that angle gets too high, then the wings are incapble of providing enough lift to keep the plane in the air. That loss of lift is referred to as a stall, and several of the accidents were high-altitude stalls. r

Larry Mailer, an FAA engineer, said that "Severe stall was resulting because of the pitch-up attitude in the autopilot." The investigation is continuing. Mailer said, but the autopilot remains the key suspect "until other evidence becomes available."

The effect of disconnecting the pitch axis is that a Learjet will have to be "flown by hand" instead of flown on automatic. The disconnection must take place before any Series 20 flies again, Mailer said. Now parts must be installed within 60 days. During that 60-day period, lower top speeds will be required because of the disabled automatic pilot. Also during that period, a series of transistors in the plane's control system will have to be inspected.

Mailer said that the new parts are, in his view, only an interim solution to the pitch-axis problem. "A more complex modification will be forthcoming," he said.

The autopilot problem is the second airworthiness directive to come out of the FAA's examination of the Learjet accidents. Last summer, the FAA ordered pilots to change their procedures and speeds while approaching airports prior to landing.

Futhermore, a third directive on changes in the control system is expected as a result of the Learjet investigation, Mailer said.

The accidents in the recent past have led to another discovery about Learjets. Some airplanes -- the FAA discovered seven -- had disconnected overspeed warning horns. The horn makes a terrific noise when the jet exceeds its permitted top speed.

The FAA conclusion was that some pilots liked to push the high-performance jet beyond the margins without having the horn sound and disturb an executive in the passenger cabin.

Gates Learjet officials could not be reached for comment.