Investigators discovered a propeller defect on a commuter airplane that crashed April 5 in Georgia, killing former Sen. John Tower, a Federal Aviation Administration official said Saturday.

The propeller malfunction prompted the FAA to order inspections on aircraft with similar propellers, said agency spokesman Paul Steuke.In addition, he said there had been three separate incidents in which commuter aircraft pilots cited propeller problems.

Steuke said the propellers either failed to respond to the pilots' command to reverse, or changed direction independently. That could cause problems for the pilot as one propeller pulled harder than the other.

"We issued a directive to all the operators to immediately inspect all those types of propellers," Steuke said. "They are presently doing so."

About 250 planes are thought to be involved in the inspection, which usually takes about three hours.

The planes are the Embraer 120, the Aerospatiale ATR42, the DeHavilland DHC-8, the Saab-Scania SF340A and 340B and some Construcciones Aeronautics models. Their propellers were made between 1990-1991.

Steuke said he couldn't say if a propeller problem caused the Embraer 120 with Tower on board to crash in Brunswick, Ga., last month. The National Transportation Safety Board investigation continues.

The NTSB is "being careful not to assign a cause to the accident because the investigation in ongoing," Steuke said.

Tower, his daughter, Marion, and astronaut Manley Lanier "Sonny" Carter were among the 23 people killed when Atlantic Southeast Airlines Flight 2311 crashed on a flight from Atlanta to Brunswick.