Federal investigators are studying additional safety inspections of the type of Brazilian-made commuter airliner that crashed in Jacksonville, Fla., Dec. 6 and killed 13 people, they said yesterday.

Part of the plane's horizontal stabilizer, or tail section, fell off the plane shortly after takeoff, but it still is not known what initiated the failure, the investigators said. A plane is uncontrollable without a functioning horizontal stabilizer.

The airplane involved was a 19-seat Embraer 110, or Bandeirante. About 130 are in service in the United States, 89 on several scheduled commuter airlines.

The Federal Aviation Administration ordered the tail sections of many of those planes inspected in December after preliminary investigation by National Transportation Safety Board specialists determined that the stabilizer had separated from the body of the aircraft and that earlier modifications to strengthen the tail had been recommended by the plane's manufacturer.

FAA officials said nothing alarming was discovered in those inspections.

The board's continuing inquiry into the Bandeirante's construction has raised questions about whether those earlier inspections were sufficient to discover possible structural problems. Part of the difficulty, board sources said, is that the area requiring inspection is small and difficult to check.

The argument against further inspection is that, because board investigators do not know what caused the failure, they do not know what to look for.

Board and FAA specialists have returned from a visit to the Embraer factory in Brazil, where they checked the aircraft's certification records. The plane also has a certificate of airworthiness from the FAA, but it is based largely on previous work done by the Brazilian government.

The board can only recommend further inspections; the FAA then would have to decide whether to order them.