General Electric is working on a longer term redesign of a CF6-50 low pressure turbine (LPT) nozzle lock which it aims to complete for introduction during November.
The redesign follows three uncontained failures which struck Continental Airlines-operated McDonnell Douglas DC-10s in April and September. These were later found to be caused by failure of the stage two LPT nozzle lock studs. As a result, the nozzles rotated with the force of the gas exhaust, and effectively machined their way through the casing. All of the failures occurred on high time engines, leading to suspicions that this was a "time and temperature" phenomenon, says GE.
The US Federal Aviation Administration, meanwhile, issued an airworthiness directive (AD) earlier this month calling for inspections over the next two months of lock assemblies for engines with less than 5,501 service hours. The AD says that if any studs are missing or are loose, all locks on the nozzle must be changed before the next flight.
In the meantime, the FAA also ordered that new LPT borescope plugs be installed on the three rows of the affected nozzles. The plugs were originally developed for the CFM56 and are longer than the current CF6 unit for the -50 series. GE says the plugs should be installed throughout the fleet within 30 days. The longer term solution, now being refined by GE, involves a purpose designed plug along with new nozzle locks.
Source: Flight International
