The US Federal Aviation Administration hopes to halve the number of uncontained engine failures by requiring improved inspection of high-energy rotating components. Enhanced inspection procedures will be introduced in the first quarter of 1998, beginning with the "highest-priority" components - large fan hubs - says Jay Pardee, manager of the FAA's engine and propeller division.
Pardee says that a study of turbofan/turbojet failures over ten years revealed that "-the most serious threat is uncontained disk failures". Engine failures are the cause of 12-13% of all aircraft accidents, he adds, and "high-energy disks are the single largest contributor".
Although the uncontained-failure rate has dropped by 50% over the past ten years, it is still too high, says Pardee. A high-energy disk fails every eight to 14 months, and with the expected increases in aircraft numbers and departures, "-there will be additional failures per year over the next five to ten years if we do nothing".
Working with engine manufacturers, airlines and overhaul centres, the FAA plans to make the best non-destructive-inspection procedures standard across the industry. Enhanced fluorescent-penetrant visual inspection of hubs and disks will be required during engine shop visits. Improved, standardised, processing steps will increase the probability of detecting the smallest flaw, says Pardee.
Supplementary non-visual inspections will be needed in hard-to-see areas, including rotating eddy-current probes for tie-bolt holes and specially shaped ultrasonic probes for blade-root slots. The cost to airlines of the new inspection requirements is unknown.
Beginning with the highest-energy components (large fan hubs), manufacturers are drawing up service bulletins for the enhanced inspections. These will be given to the FAA by early January, to be made mandatory. Other rotating components will be tackled sequentially, based on their energy levels, with high-pressure turbine disks likely to be next on the FAA's list.
Pardee says that analysis of engine failures over the past ten years indicates that 50% would have been avoided through enhanced inspections. Describing the improved inspections as a "safety net", he says that work is under way to eliminate the root causes of flaws.
In the longer term, says Pardee, the FAA wants engine manufacturers to move towards damage-tolerant structures and multiple- load-path disk designs. The fan hubs of Boeing 777 engines already have multiple load paths.
Source: Flight International
