Guy Norris/LOS ANGELES
The US Federal Aviation Administration has issued new airworthiness directives (AD) covering urgent inspections and modifications to General Electric CF6-50 engines following an uncontained failure on an Air France Boeing 747-300 on an engine that already complied with the current AD.
The failure on 4 February involved a CF6-50E2-powered 747-300 after the aircraft landed. The engine's second stage low pressure turbine (LPT) nozzle lock assemblies had been visually inspected and the borescope plug replaced with a new design.
The engine had not, however, been ultrasonically inspected or equipped with additional bolts on the LPT casing, as outlined in recommended actions contained in GE's service bulletins. These actions are mandated by the latest AD, and pave the way for the eventual replacement of a new CF6-80C2-based nozzle lock assembly design throughout the affected fleet of 1,600 engines, most dating from the 1970s and early 1980s.
The problems stem from three uncontained failures on CF6s which struck Continental Airlines McDonnell Douglas DC-10s from April to September 2000. The turbine break-ups were traced to failures of the second stage LPT nozzle lock studs which machined their way through the casing by the force of the gas exhaust.
Last October the FAA issued an AD ordering operators to inspect the locks on engines, and to replace missing or loose studs before the next flight. It also ordered that the new LPT borescope design be installed on the three rows of the affected LPT nozzle areas.
The latest AD embraces both earlier actions, GE's recommended service bulletin actions, and plans for longer-term corrective modifications. It includes installation of the solid borescope plug for engines that have not already complied with the earlier AD, initial and repetitive ultrasonic inspections of second stage LPT nozzle assembly lock studs made from Waspalloy. It also calls for lock assemblies to be replaced with the new design before further flight if they are found to be cracked, loose or have missing studs. Additional locks should also be bolted on "if no indications of nozzle rotation are found, as an interim action to allow time to arrange for a shop visit within 3,500 cycles in-service".
The AD also calls for inspection of the area around the borescope plug for buckling or cracks, inspection for loose/missing nozzle locks and LPT case cracking around the added locks every 750 hours time-in-service. It also calls for replacement of the nozzle lock assembly with the new design before further flight if any LPT case buckling or cracks, or evidence of nozzle segment rotation, are found.
Source: Flight International
