The National Transportation Safety Board today issued two urgent safety recommendations to the Federal Aviation Administration (FAA). The first recommendation asks that the FAA require operators of aircraft equipped with a particular model engine to immediately perform blade borescope inspections (BSI) of the high pressure turbine rotor at specific intervals until the current turbine disk can be redesigned and replaced with one that can withstand the unbalance vibration forces from the high pressure rotor. The second recommendation asks the FAA to require the engine manufacturer to immediately redesign the disk. The NTSB issued an additional recommendation for a requirement that operators perform a second type of inspection and another recommendation related to the engine manufacturer regarding the installation of the replacement disk.
All four recommendations apply to the low pressure turbine (LPT) stage 3 (S3) rotor disk in the General Electric (GE) CF6-45/50 series turbofan engines that can fail unexpectedly when excited by high-pressure (HP) rotor unbalance.
An uncontained engine event occurs when an engine failure results in fragments of rotating engine parts penetrating and exiting through the engine case. Uncontained turbine engine disk failures within an aircraft engine present a direct hazard to an airplane and its passengers because high-energy disk fragments can penetrate the cabin or fuel tanks, damage flight control surfaces, or sever flammable fluid or hydraulic lines. Engine cases are not designed to contain failed turbine disks. Instead, the risk of 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.
In its safety recommendations to the FAA, the NTSB cited four foreign accidents, which the NTSB is either investigating or participating in an investigation led by another nation, in which the aircraft experienced an uncontained engine failure of its GE CF6-45/50 series engine.
The date, location, and circumstances of these four events (none had injuries or fatalities) are as follows:
On July 4, 2008, a Saudi Arabian Airlines (Saudia) Boeing 747-300 experienced an engine failure during initial climb after takeoff from Jeddah, Kingdom of Saudi Arabia. This investigation has been delegated to the NTSB.
On 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, Columbia. This investigation has been delegated to the NTSB.
On December 17, 2009, a Jett8 Cargo Boeing 747-200F airplane was passing through 7,000 feet above ground level (agl) when the flight crewmembers heard a muffled explosion and immediately applied left rudder. With one engine losing oil pressure, the airplane returned to land at Changi, Singapore. The NTSB is participating in the investigation that is being led by the Air Accident Investigation Bureau of Singapore.
On April 10, 2010, an ACT Cargo Airbus A300B4 experienced an engine failure while accelerating for takeoff at Manama, Bahrain. The crew declared an emergency, rejected the takeoff, activated the fire suppression system, and evacuated the airplane. The NTSB is participating in the investigation that is being led by the Bahrain Ministry of Transportation - Civil Aviation.
The four recommendations to the FAA are as follows:
- Immediately require operators of CF6-45/50-powered airplanes to perform high pressure turbine rotor blade borescope inspections every 15 flight cycles until the low pressure turbine stage 3 disk is replaced with a redesigned disk that can withstand the unbalance vibration forces from the high pressure rotor. (Urgent)
- Require operators of CF6-45/50-powered airplanes to perform fluorescent penetrant inspections of CF6-45- 50- low pressure turbine stage 3 disks at every engine shop visit until the low pressure turbine stage 3 disk is replaced with a redesigned disk that can withstand the unbalance vibration forces from the high pressure rotor.
- Immediately require General Electric Company to redesign the CF6-45/50 low pressure turbine stage 3 disk so that it will not fail when exposed to high pressure rotor unbalance forces. (Urgent)
- Once General Electric Company has redesigned the CF6- 45/50 low pressure turbine (LPT) stage 3 disk in accordance with Safety Recommendation , require all operators of CF6-45/50-powered airplanes to install the newly designed LPT S3 at the next maintenance opportunity.