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Thursday, September 1, 2005

Sandia Labs and the Science of Non-Destructive Inspection

Dale Smith, Senior contributing editor

If Missouri's motto is "Show Me," then New Mexico, or at least Albuquerque and the areas around the FAA Airworthiness Assurance NDI Validation Center at Sandia National Laboratories (AANC), should have the motto, "Prove It To Me," especially for those in the business of developing new tools, techniques, and procedures for inspecting aging aircraft. Companies, universities, and the FAA bring their latest innovations to this facility in hopes of earning AANC's stamp of approval.

"Our function is bringing together the researchers, users, and regulators so that if there is a need and a new technology to better meet that need, [it] can be made available to the end user," explained Dick Perry, manager, airworthiness assurance, AANC. "We're not the world's foremost experts on these technologies," he added, "we are the ones who created the processes that can prove whether or not something does what it should do.

"We give people the kind of information they need to make a decision on an economic advantage or a safety advantage. For example, if the people want an alternate way to comply with an airworthiness directive, they come to us for a validation of the process," he said. "Can the regulators accept it? Can the manufacturers accept it? Will they put it in their inspection manuals? Will the people in the field actually use it? They are all things we look at."

Perry said that the work that AANC is doing now is in direct response to the Aloha Airlines accident in 1988, when one of its Boeing 737s lost a section of upper fuselage. The FAA established the Aging Aircraft Initiative and that's when the FAA Aging Aircraft NDI Validation Center (AANC) was formed. In 1995, it was renamed the Airworthiness Assurance NDI Validation Center to better reflect the center's continuing airworthiness mission of "providing validation of NDI [non-destructive inspection], maintenance, and repair processes with comprehensive, independent, quantitative evaluations of new and enhanced inspection, maintenance, and repair techniques."

"The main thing we deal with is operational fatigue, a part under a lot of repetitive-type loads as opposed to overloads," Perry explained. "People anticipate this kind of damage. But in the case of Aloha Airlines, they didn't anticipate it in the same terms. Boeing knew that the lap splices were subject to fatigue damage, and needed inspection for cracks at a particular time in their life. This particular type of damage is caused by pressurization cycles, and the inspections at that time were based on the time on the airframe.

"Unfortunately, Aloha Airlines was flying their 737s as commuters averaging 45 to 50 minutes a flight," he continued. "Boeing thought a typical 737 flight would be one an a half to two hours, so the cycles built up a lot faster than the total hours. The resulting fatigue damage was what Boeing anticipated, it just happened a lot sooner, before the inspections could identify the problem. Today, we base inspection intervals on total cycles as well as time."

Perry said that all of the damage- and inspection-tolerance practices today are based on two things: how small a crack can you find? And, how large a crack do you have to find to be effective? "With a good knowledge of the loads on the airplane and how cracks develop, you can then figure out what you need to do to identify damage before it becomes serious," he said.

A big part of AANC/Sandia's workload is validating the effectiveness of new NDI technologies and procedures, and like any effective basis for research, the first necessity is to create a set of controls. "We can't determine if a new tool or process can find a crack or corrosion if we don't know it's there," he said. "So one of the first things we did was to create 40 interchangeable panels with three rows of 20 rivets each. Then we `grew' real fatigue cracks in them. We know what kind of cracks are at each rivet, so we can correctly evaluate a tool's or a technician's ability to find them.

"With these panels we can consistently use the same protocols and procedures, again and again," Perry said. "So when a new piece of equipment comes along we will know how well it works. By looking at what the equipment is capable of finding and what it is not able to find, we can make direct comparisons to existing technology."

And the selection and variety of tests aren't restricted to the test panels. That's easy. Where they really shine is their ability to put equipment and procedures to the test on real airplanes. Sure, anybody else can do that, but the major difference is, when you inspect airplane `A' out on the ramp, you have no way of knowing what you should be finding. So you don't know if the equipment is wrong, the process is wrong, or the problem isn't really there.

A situation that was recently illustrated is a problem a major inspection equipment manufacturer was having validating a new technology. "They had gone to Mojave to test a new digital radiography unit on some airplanes in storage out there," Perry explained. "Well, after a couple of weeks sweating in the desert, they hadn't found what they expected to find. Was the machine wrong or were they just looking in the wrong place? They were very discouraged. So they came here to repeat the tests on our 737. It is probably the best-characterized airplane in the world as far as what is wrong with it," he continued. "We haven't found anything new for four years, and there have been a lot of people looking at it. We know where the cracks are and how big they are. We know where the corrosion is and what kind it is. So if someone is looking for a particular type or size of damage, we can tell them exactly where to look. In three or four days they had everything they were looking for."

And being able to find exactly what they need to validate or test their equipment or procedures is what it is all about. "We know exactly where the flaws are. If you can't find them, there's a problem. It's not because they aren't there," Perry said.

As airframes become older, the size and frequency of cracks, delaminations, disbonds, and whatever else you can think of grows exponentially. That's why operators are always on the lookout for more accurate and economical ways to find what evil lurks under layers of paint. Right now, the folks at AANC are working with major universities and commercial industry to validate a variety of new inspection technologies. While they are all pretty cool, there are a few that are going to change the ways inspections are done in the future, including phased-array ultrasonics, pulsed thermography, and sonic IR.

"Phased-array ultrasonics is the same technology your doctor uses to look at soft tissue," Perry said. "The company has developed specialized software and a new angled probe to control the wave pattern. It makes cracks and flaws really easy to see. The company came out and asked us to validate that it really could show what they wanted to see. Boeing is very interested in this one. It's fast and accurate, and not only can you see the crack, you can actually measure the size of the crack with it."

Perry explained that the new pulsed thermography equipment uses a flash lamp to uniformly heat a given area of the airframe surface. As the area cools, the temperature changes at varying rates depending on the materials, their thickness, and their contact with other sub-surface parts. Rivets go all the way through and because of their thermal properties they conduct the surface heat away quickly so they look cooler faster than the surrounding metal.

Metal straps that are properly bonded to the surface skin will cool at a different rate than straps where the adhesive has failed. By using an infrared camera, inspectors can capture the various cooling rates and create an accurate picture of where the glue has separated from the joint.

"This is much faster and easier than previous methods," Perry said. "It was developed originally by Wayne State University. The Air Force wanted it to inspect composite parts. We originally created the tests to verify and validate the capabilities of the equipment from the different manufacturers. Then we conducted the tests and turned the results over to the Air Force for the final decision. We then assisted the winner in developing the specific inspection procedures and tailoring the equipment for Air Force needs."

Sonic IR is another new inspection technology that is showing great promise. "It was originally created for the automotive industry to test engine blocks for cracks after casting," he said. "Again, Wayne State University worked on the initial development and then the FAA got involved and the Air Force became interested in using it to inspect different cast parts on jets."

Perry explained that this process involves putting an ultrasonic signal into the part. If there is a crack, the sound waves will cause the sides of the crack to vibrate together and generate heat and that heat can be seen with an infrared camera. "We're finding out more and more about it every day," he added. "Three major airlines have expressed interest in replacing some liquid penetrant and magnetic particle inspections with Sonic IR. Reduced part preparation, [paint stripping and specialized cleaning], reduced processing and evaluation time, reduced floor space requirements, and the elimination of chemicals are a few of the potential benefits. It shows great promise for detecting cracks in large homogeneous parts such as engine discs, turbine blades, and landing gear components," Perry said.

The pros at AANC/Sandia are also involved in leading-edge research on inspection and repair technologies like automatic health monitoring of repairs, self-healing composite structures, piezoelectric sensors, Eddy current pancake probes, and more. We may not see these technologies in the shop for many years, but be assured, they are coming.

With all the advancements in technology and capability, still the most important part of any inspection or maintenance task is the person doing the job, and AANC/Sandia is working on verifying and validating the effectiveness of the procedures we use every day. "We don't just validate hardware," Perry said. "It's also for the instructions technicians use."

One recent experiment was designed to look at the effects different types of instructions had on the ability of a visual inspector to find a common set of defects on AANC's 737. "We had around 40 inspectors come here from the airlines, FedEx, and one or two outside maintenance companies to take part in the test," explained member of the technical staff Mike Bode. "We had them perform a visual inspection on the aircraft. We wanted to find out if the type of instructions they received had any influence on the outcome. Some were told to do a general inspection and others were given specific zonal instructions on where to look and what to look for. What we found was that there were significant variations in the degrees to which the procedures were followed."

"After the inspections were completed, we provided feedback to the various inspectors," Perry added. "Some of them weren't as good as they thought they were."

"We had guys here, that by all accounts, were very, very good inspectors," Bode said. "Good inspectors who missed some big stuff for no apparent reason, they just happened to miss it."

While the researchers are still poring over all of the data they collected from the visual inspection experiment, Bode feels that the bottom line will be that the majority of inspectors are not always going to find the numbers of defects that maintenance people would like them to find. There are things that just get missed because of the inherent limitations of visual inspections. "I believe our data will support that assumption," he said. "And what I hope is that the industry will look at this and do two things: give mechanics more appropriate instructions to improve their effectiveness and have more people doing more frequent inspections where they may be appropriate to justify our continuing high confidence in the visual inspection process

If Missouri's motto is "Show Me," then New Mexico, or at least Albuquerque and the areas around the FAA Airworthiness Assurance NDI Validation Center at Sandia National Laboratories (AANC), should have the motto, "Prove It To Me," especially for those in the business of developing new tools, techniques, and procedures for inspecting aging aircraft. Companies, universities, and the FAA bring their latest innovations to this facility in hopes of earning AANC's stamp of approval.

"Our function is bringing together the researchers, users, and regulators so that if there is a need and a new technology to better meet that need, [it] can be made available to the end user," explained Dick Perry, manager, airworthiness assurance, AANC. "We're not the world's foremost experts on these technologies," he added, "we are the ones who created the processes that can prove whether or not something does what it should do.

"We give people the kind of information they need to make a decision on an economic advantage or a safety advantage. For example, if the people want an alternate way to comply with an airworthiness directive, they come to us for a validation of the process," he said. "Can the regulators accept it? Can the manufacturers accept it? Will they put it in their inspection manuals? Will the people in the field actually use it? They are all things we look at."

Perry said that the work that AANC is doing now is in direct response to the Aloha Airlines accident in 1988, when one of its Boeing 737s lost a section of upper fuselage. The FAA established the Aging Aircraft Initiative and that's when the FAA Aging Aircraft NDI Validation Center (AANC) was formed. In 1995, it was renamed the Airworthiness Assurance NDI Validation Center to better reflect the center's continuing airworthiness mission of "providing validation of NDI [non-destructive inspection], maintenance, and repair processes with comprehensive, independent, quantitative evaluations of new and enhanced inspection, maintenance, and repair techniques."

"The main thing we deal with is operational fatigue, a part under a lot of repetitive-type loads as opposed to overloads," Perry explained. "People anticipate this kind of damage. But in the case of Aloha Airlines, they didn't anticipate it in the same terms. Boeing knew that the lap splices were subject to fatigue damage, and needed inspection for cracks at a particular time in their life. This particular type of damage is caused by pressurization cycles, and the inspections at that time were based on the time on the airframe.

"Unfortunately, Aloha Airlines was flying their 737s as commuters averaging 45 to 50 minutes a flight," he continued. "Boeing thought a typical 737 flight would be one an a half to two hours, so the cycles built up a lot faster than the total hours. The resulting fatigue damage was what Boeing anticipated, it just happened a lot sooner, before the inspections could identify the problem. Today, we base inspection intervals on total cycles as well as time."

Perry said that all of the damage- and inspection-tolerance practices today are based on two things: how small a crack can you find? And, how large a crack do you have to find to be effective? "With a good knowledge of the loads on the airplane and how cracks develop, you can then figure out what you need to do to identify damage before it becomes serious," he said.

A big part of AANC/Sandia's workload is validating the effectiveness of new NDI technologies and procedures, and like any effective basis for research, the first necessity is to create a set of controls. "We can't determine if a new tool or process can find a crack or corrosion if we don't know it's there," he said. "So one of the first things we did was to create 40 interchangeable panels with three rows of 20 rivets each. Then we `grew' real fatigue cracks in them. We know what kind of cracks are at each rivet, so we can correctly evaluate a tool's or a technician's ability to find them.

"With these panels we can consistently use the same protocols and procedures, again and again," Perry said. "So when a new piece of equipment comes along we will know how well it works. By looking at what the equipment is capable of finding and what it is not able to find, we can make direct comparisons to existing technology."

And the selection and variety of tests aren't restricted to the test panels. That's easy. Where they really shine is their ability to put equipment and procedures to the test on real airplanes. Sure, anybody else can do that, but the major difference is, when you inspect airplane `A' out on the ramp, you have no way of knowing what you should be finding. So you don't know if the equipment is wrong, the process is wrong, or the problem isn't really there.

A situation that was recently illustrated is a problem a major inspection equipment manufacturer was having validating a new technology. "They had gone to Mojave to test a new digital radiography unit on some airplanes in storage out there," Perry explained. "Well, after a couple of weeks sweating in the desert, they hadn't found what they expected to find. Was the machine wrong or were they just looking in the wrong place? They were very discouraged. So they came here to repeat the tests on our 737. It is probably the best-characterized airplane in the world as far as what is wrong with it," he continued. "We haven't found anything new for four years, and there have been a lot of people looking at it. We know where the cracks are and how big they are. We know where the corrosion is and what kind it is. So if someone is looking for a particular type or size of damage, we can tell them exactly where to look. In three or four days they had everything they were looking for."

And being able to find exactly what they need to validate or test their equipment or procedures is what it is all about. "We know exactly where the flaws are. If you can't find them, there's a problem. It's not because they aren't there," Perry said.

As airframes become older, the size and frequency of cracks, delaminations, disbonds, and whatever else you can think of grows exponentially. That's why operators are always on the lookout for more accurate and economical ways to find what evil lurks under layers of paint. Right now, the folks at AANC are working with major universities and commercial industry to validate a variety of new inspection technologies. While they are all pretty cool, there are a few that are going to change the ways inspections are done in the future, including phased-array ultrasonics, pulsed thermography, and sonic IR.

"Phased-array ultrasonics is the same technology your doctor uses to look at soft tissue," Perry said. "The company has developed specialized software and a new angled probe to control the wave pattern. It makes cracks and flaws really easy to see. The company came out and asked us to validate that it really could show what they wanted to see. Boeing is very interested in this one. It's fast and accurate, and not only can you see the crack, you can actually measure the size of the crack with it."

Perry explained that the new pulsed thermography equipment uses a flash lamp to uniformly heat a given area of the airframe surface. As the area cools, the temperature changes at varying rates depending on the materials, their thickness, and their contact with other sub-surface parts. Rivets go all the way through and because of their thermal properties they conduct the surface heat away quickly so they look cooler faster than the surrounding metal.

Metal straps that are properly bonded to the surface skin will cool at a different rate than straps where the adhesive has failed. By using an infrared camera, inspectors can capture the various cooling rates and create an accurate picture of where the glue has separated from the joint.

"This is much faster and easier than previous methods," Perry said. "It was developed originally by Wayne State University. The Air Force wanted it to inspect composite parts. We originally created the tests to verify and validate the capabilities of the equipment from the different manufacturers. Then we conducted the tests and turned the results over to the Air Force for the final decision. We then assisted the winner in developing the specific inspection procedures and tailoring the equipment for Air Force needs."

Sonic IR is another new inspection technology that is showing great promise. "It was originally created for the automotive industry to test engine blocks for cracks after casting," he said. "Again, Wayne State University worked on the initial development and then the FAA got involved and the Air Force became interested in using it to inspect different cast parts on jets."

Perry explained that this process involves putting an ultrasonic signal into the part. If there is a crack, the sound waves will cause the sides of the crack to vibrate together and generate heat and that heat can be seen with an infrared camera. "We're finding out more and more about it every day," he added. "Three major airlines have expressed interest in replacing some liquid penetrant and magnetic particle inspections with Sonic IR. Reduced part preparation, [paint stripping and specialized cleaning], reduced processing and evaluation time, reduced floor space requirements, and the elimination of chemicals are a few of the potential benefits. It shows great promise for detecting cracks in large homogeneous parts such as engine discs, turbine blades, and landing gear components," Perry said.

The pros at AANC/Sandia are also involved in leading-edge research on inspection and repair technologies like automatic health monitoring of repairs, self-healing composite structures, piezoelectric sensors, Eddy current pancake probes, and more. We may not see these technologies in the shop for many years, but be assured, they are coming.

With all the advancements in technology and capability, still the most important part of any inspection or maintenance task is the person doing the job, and AANC/Sandia is working on verifying and validating the effectiveness of the procedures we use every day. "We don't just validate hardware," Perry said. "It's also for the instructions technicians use."

One recent experiment was designed to look at the effects different types of instructions had on the ability of a visual inspector to find a common set of defects on AANC's 737. "We had around 40 inspectors come here from the airlines, FedEx, and one or two outside maintenance companies to take part in the test," explained member of the technical staff Mike Bode. "We had them perform a visual inspection on the aircraft. We wanted to find out if the type of instructions they received had any influence on the outcome. Some were told to do a general inspection and others were given specific zonal instructions on where to look and what to look for. What we found was that there were significant variations in the degrees to which the procedures were followed."

"After the inspections were completed, we provided feedback to the various inspectors," Perry added. "Some of them weren't as good as they thought they were."

"We had guys here, that by all accounts, were very, very good inspectors," Bode said. "Good inspectors who missed some big stuff for no apparent reason, they just happened to miss it."

While the researchers are still poring over all of the data they collected from the visual inspection experiment, Bode feels that the bottom line will be that the majority of inspectors are not always going to find the numbers of defects that maintenance people would like them to find. There are things that just get missed because of the inherent limitations of visual inspections. "I believe our data will support that assumption," he said. "And what I hope is that the industry will look at this and do two things: give mechanics more appropriate instructions to improve their effectiveness and have more people doing more frequent inspections where they may be appropriate to justify our continuing high confidence in the visual inspection process.

THE RIGHT TIME TO REPLACE AN AGING

AIRCRAFT?

There are countless reasons to replace an airplane and they include the need for greater range and speed, a larger cabin, a desire to have the latest avionics suite, and everything in between. And they are all pretty cut and dried: You need `X' so you buy `Y' airplane to do it. But what about the old-age factor? That's a reason of a different color, and it's usually a shade of gray.

While a new interior, avionics upgrade, and a coat of paint can make most airplanes seem `new' there are some things you can't hide. Namely a dramatic increase in maintenance costs with a corresponding decrease in availability. "The data, when plotted together, shows that aging has a profound impact on maintenance costs," explained David Wyndham, partner, Conklin & deDecker. "The aircraft not only need more routine and unscheduled maintenance, the cost and availability of spare parts can become an issue."

Wyndham said that while he has no statistical data to support this conclusion, an aircraft's wiring is one area where ongoing maintenance can be extremely problematic. "You spend money replacing a fuel gauge and then the sensor and you still haven't fixed it, so the problem must be in the wiring somewhere," he added. "And replacing a wiring bundle isn't an afternoon's job. It can be a lot of work and troubleshooting."

Whether it's chronic wiring problems or just the natural progression of decades of operation, growing maintenance costs are the leading reason companies decide to retire an aircraft. "Unfortunately, there is no clear formula that spells out when an aircraft should be withdrawn from service," Wyndham said. "Instead, operators need to keep track of three key factors: mechanical dispatch reliability, aircraft availability, and maintenance cost per flight hour.

"Trip reliability needs to be kept very high," he said. "But, the amount of money and time it takes to maintain an aircraft goes up progressively as it gets older. So at some point, depending on your operation, you may need three older aircraft to do the work of one brand new one. While upgrading to a new aircraft is expensive, you don't need as many of them to do the job," he added, "because they will spend a lot less time in the shop."

"Depending on how the aircraft is used, mechanics can track which are the most frequently replaced parts, and, more importantly, why do they need replacing?" he said. "Maybe it's not the part, but something else that's causing the problem." And the cost to fix that problem may very well be out of line with the overall value of the airplane.

So when do you hold your aircraft's retirement party? "It all comes down to the economics of it," Wyndham concluded. "It's not that the airplane is unsafe to fly, but more a matter of how much extra money do you want to spend just to get a little more use from it."

For more information on Conklin & deDecker's services, phone: 508-255-5975 or Web: www.conklindd.com.

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