Foreign Object Damage:

The continuing story of US Airways Flight 1549

The power plants
The equipment used on US Air Flight 1549 was a narrow body Airbus 320 (A320) powered by two CFM56-5A engines. The cases on these tough engines contained the FOD damage as designed, resulting in no collateral damage to the fuselage from disk and blade shrapnel. I was able to discuss the CFM56 engine with Deborah Case of General Electric (GE) Aviation. The CFM56 Series engines are used on commercial, corporate, and military aircraft, powering more than 1,706 aircraft worldwide, and accumulating more than 46 million flight hours in the process.

The CFM56 Series engines are built by CFM International, a 50/50 joint company of GE and Snecma (SAFRAN Group) of France.

The CFM56 engines on the A320 were certified by both the European Joint Airworthiness Authority (JAA) and, subsequently, by the FAA. The engines were originally certified to take a hit in the core engine from a flock of seven 1.5-pound birds and run for five minutes at a takeoff power setting. Current standard for the CFM56-5 is a hit from one 2.5-pound bird followed by five 1.5-pound birds, with a maximum allowable 25 percent loss of thrust. CFM exceeded these requirements, and actually tested the engine using three 2.5-pound birds. The CFM56-5 engines on Flight 1549, even though tested to the higher standard, still failed that morning in January. Tough as they are, bird strikes are problematic for all aircraft engines. Engine failure caused by bird strikes that result in complete loss of power is relatively rare. This seems like a contradiction, but not so because engines are designed to withstand bird strikes.

It is this Flight 1549 failure that is most concerning to OEMs, pilots, passengers, and the regulatory agencies. The FAA recently amended the certification standard for future engines by raising the weight of the bird to 8 pounds. Some of the feedback to the FAA suggested that weight is too low. Some argue that the weight should be raised to a realistic level that would include birds as large as the Canada Geese that brought Flight 1549 down in the Hudson.

It was obvious from news media footage and statements from the NTSB that both of Flight 1549’s engines were a mess. Large pieces of the cowlings were missing, and on initial examination the NTSB reported that while missing obvious organic matter (code for Canada Goose liver pate), they had found evidence of soft body impact, and “found dents on both the spinner and inlet cowling.” In addition, the inlet guide vanes were fractured, and eight outlet guide vanes were missing. Both engines have been sent to the GE Aviation facilities in Cincinnati, OH, for disassembly, examination, and analysis. Having seen the damage bird strikes can cause to jet engines, I know the nondestructive testing (NDT) crews will be busy for quite some time while poring over these two engines.

The ongoing NDT inspections
If you have read the March AMT article on the “Effective Use of Borescopes” by James Careless, you know there are some great new NDT technologies out there. While it can’t be known which inspection equipment was being used for the US Airways plane, the actual technologies and applications will depend on the distinctive characteristics of the accident and the specific components under inspection.

Most likely, the investigation will include remote visual inspection (video borescopes) and ultrasound, as well as eddy current and digital X-ray to ensure that as much data as possible can be gleaned from this accident. Jeff Anderson, aerospace expert at GE Sensing & Inspection Technologies, points out that composite structures do not necessarily retain impact damage marks and was concerned about the extensive damage that the A320 airframe and engines had already sustained. He was well aware that the inspection crews must be careful not to further stress those areas like the pylons, engine mounts, and adjacent fuselage. These areas will also hold data critical for understanding the consequences of the bird strike and engine failures of Flight 1549.

To help in the investigation process, the FAA made its entire bird strike database available on a public website Friday, April 24. The FAA has determined that it can release the data without jeopardizing aviation safety. Over the next four months, the FAA will make changes to improve the search function and make it more user-friendly. It also plans to work with the aviation community to find ways to improve and strengthen bird strike reporting.

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