The US National Transportation Safety Board (NTSB) has issued four urgent safety recommendations to the FAA following uncontained engine failures in four recent incidents.
The first recommendation covers a blade borescope inspection every 15 flight cycles for the GE CF6-45 and -50 high pressure turbine rotor until a redesigned disk can be installed.
CF6-45 and -50 model engines power McDonnell Douglas DC-10-15/30s, Boeing 747-200s/300s and the Airbus A300B.
The recommendations apply to the low pressure turbine (LPT) stage three (S3) rotor disk, which the NTSB says "can fail unexpectedly when excited by high-pressure (HP) rotor unbalance".
The second recommendation covers a re-design of the disk to withstand unbalanced vibration forces from the high pressure rotor.
The remaining two recommendations to the FAA would require operators to perform a fluorescent penetrant inspection on the LPT S3 during every shop visit, and install the newly designed disk "at the next maintenance opportunity".
The four recommendations come after four recent incidents involving CF6-45 and -50 powered aircraft. On 4 July 2008 a Saudi Arabian Airlines Boeing 747-300 experienced an engine failure on climb out from Jeddah, Saudi Arabia.
The next incident occurred 26 March 2009 when an Arrow Cargo McDonnell Douglas DC-10F in Manaus, Brazil experienced a loss of oil pressure in one engine, prompting an in-flight shut down.
Following that shut down, on 17 December 2009 the crew of a Jett8 Cargo Boeing 747-200F heard a muffled explosion and immediately applied left rudder and lost pressure in one engine. The aircraft landed at Changi Airport, Singapore.
The final incident that led NTSB to issue the urgent recommendations occurred 10 April 2010 when an ACT Cargo Airbus A300B4 experienced an engine failure on takeoff roll from Manama, Bahrain, prompting a rejected takeoff and an activation of the aircraft's fire suppression system.
"Engine cases are not designed to contain failed turbine disks," says NTSB. "Instead, the risk of an uncontained disk failure is mitigated by designating disks as safety-critical parts, defined as the parts of an engine whose failure is likely to present a direct hazard to the aircraft.