Washington (CNN) -- The failure of General Electric engines on four jet aircraft overseas during the past two years has prompted the National Transportation Safety Board to issue an "urgent" recommendation to increase inspections of the engines on U.S. aircraft.
None of the incidents resulted in crashes, injuries or fatalities. But in all four cases, engine parts penetrated the engine housing.
Such "uncontained engine failures" are particularly dangerous because flying engine parts could puncture fuel or hydraulic lines, damage flight surfaces or even penetrate the fuselage and injure passengers.
At issue are General Electric CF6-45/50 series jet engines, older engines found on a small number of jets.
FAA officials said 373 of the engines are in service in the United States, on a fewer, but unknown, number of planes. The engines are used on some Airbus A300s, Boeing 747s, DC-10s, MD-10s and U.S. Air Force KC-10s.
A GE spokeswoman said most of the engines are used on cargo planes.
On four occasions, the NTSB said, a rotor imbalance caused rotor disks to fail, leading to the uncontained engine failures.
FAA spokeswoman Laura Brown said the FAA is aware of the problem and issued a rule in March requiring inspections of the engines within 50 flights, and repeat inspections every 175 flights thereafter. It also is working on a rule that would add testing of the rotor disks for cracks.
But on Thursday, the NTSB issued its urgent recommendation, saying the FAA should require inspections every 15 flights until the disks can be replaced with improved parts.
The FAA said the action "will maintain the safety of the fleet," and that it will decide whether to alter the inspection schedule after completing examinations of the engines involved in the recent incidents.
GE spokeswoman Deborah Case said GE issued a service bulletin last August advising operators to inspect and monitor the engines.
The NTSB recommendation follows these four incidents:
-- July 4, 2008: A Saudi Arabian Airlines (Saudia) Boeing 747-300 experienced an engine failure after takeoff from Jeddah, Kingdom of Saudi Arabia.
-- March 26, 2009: An Arrow Cargo McDonnell Douglas DC-10F, about 30 minutes after takeoff from Manaus, Brazil, experienced loss of oil pressure in one engine. The pilots shut down the engine and diverted to Medellin, Colombia.
-- December 17, 2009: A Jett8 Airlines Cargo Boeing 747-200F was passing through 7,000 feet when the crew members heard a muffled explosion. With one engine losing oil pressure, the airplane returned to land at Changi, Singapore.
-- April 10, 2010: An ACT Cargo Airbus A300B4 experienced an engine failure while accelerating for takeoff at Manama, Bahrain. The crew declared an emergency, aborted the takeoff, activated the fire-suppression system, and evacuated the airplane.
The NTSB is participating in or leading investigations of the four incidents.