WASHINGTON (AP) _ The Federal Aviation Administration has ordered that 518 Piper Malibu airplanes be operated only in good weather following a series of unexplained accidents in which the aircraft broke up in flight.

Seven of the six-seat, single-engine aircraft have broken up in flight since May 1989, including one that fell apart during a rainstorm a week ago near Bronson, Fla., killing a family of four.

The FAA said Monday it was taking emergency action before the completion of investigations into the accidents in an effort to prevent further incidents. An agency source had said over the weekend that the action would be taken, and outlined its scope.

The FAA ordered owners of the $300,000 Piper Malibu to place a placard on the instrument panel of each Malibu saying, ''Flight in instrument flight rule (IFR) meteorological conditions is prohibited.''

''Flight into areas of known or forecasted moderate or severe turbulence should be avoided,'' the FAA said in issuing its formal order Monday.

The FAA order also prohibits use of the aircraft's autopilot or vertical trim control for altitude changes.

It also requires the removal of the vertical speed and altitude control panel from the Malibu's control panel, along with a series of other steps restricting operations of the aircraft.

The Piper Malibu, introduced in 1983, can fly at altitudes of up to 25,000 feet at speeds of up to 225 knots.

Piper scheduled a news conference for Thursday at its headquarters in Vero Beach, Fla., to discuss the status of the Malibu.