You know the problem: You’ve had a product in production for years with recurring failures you can’t seem to stop. Your people say it’s inherent to the product, and there’s nothing you can do about it. Production continues and so do the failures. You get to watch a sizeable chunk of your profits continue to move from the production line to the scrap bin.
There’s good news here, folks: You don’t have to let this continue. This article describes how Composite Structures used root cause failure analysis to identify and eliminate recurring Apache main rotor blade failures, failures the company previously thought were inherent to its high tech blades and unavoidable.
Composite Structures Division in Monrovia, California, is a build-to-print manufacturer (that means they build products to their customers’ designs). The product we’re discussing here is the Apache helicopter main rotor blade, a product that had been in production approximately 10 years at the time the events in this article occurred.
Rotor Blade Vulnerability and Survivability
The Vietnam War marked the U.S. Army’s first wide-scale use of helicopters, but the Army’s iconic Huey suffered from a serious weakness: its vulnerability to small arms fire. The Army learned the hard way that if a single rifle bullet struck a Huey rotor blade, the blade would disintegrate, resulting in loss of the helicopter and its crew.
The AH-64 Apache. This modern combat helicopter’s main rotor blades can withstand direct hits from both small arms fire and anti-aircraft high explosive projectiles.
After Vietnam, the Army wanted a tougher helicopter, and it turned to McDonnell Douglas for the Apache. One of the primary requirements was greatly-improved main rotor blade survivability.
To meet this requirement, the Army and McDonnell Douglas designed a much tougher rotor blade for the Apache. The new blade has redundant load paths with four adhesively-bonded steel spars. The idea is that if the blade is hit by anything up to a 23mm high explosive round, the four spars keep the blade together, and the helicopter can continue to fly. The concept works; during the Gulf War, an Apache blade took a direct hit by an Iraqi 23mm warhead and made it home.
The Failures
Composite Structures manufactured Apache main rotor blades to a McDonnell Douglas design using a typical layup and bonding process. This involved assembling metal components and applying adhesive under clean room conditions, and then baking the assemblies to cure the adhesive.
Although the company had decades of experience manufacturing other composite structures for a variety of aerospace assemblies, the Apache rotor blades were problematic from the outset of the program. The blades experienced adhesive failures. This occurred shortly after the baking process. It also occurred in service (after blades were on the helicopters). Half the blades failed before they left the Composite Structures plant. Once in service, the failures cut the blades’ service life to about 800 hours (they were supposed to last 2000 hours). Amazingly, both the Army and Composite Structures endured this problem during the first decade of the Apache’s life. McDonnell Douglas and Composite Structures took the position that the rotor blades were state-of-the-art, custom-engineered components, and a 50% yield was all that could be expected.
After the program had been in place for about a decade, new management took over the Composite Structures operation. The new management team’s charter was to improve profitability, and the 50% rotor blade rejection rate naturally attracted attention. Composite Structures’ new leaders wanted to eliminate the rotor blade failures, but the reaction to this initiative was frustrating:
- McDonnell Douglas seemed disinterested in correcting the problem. In fact, when Composite Structures showed McDonnell Douglas that blade disbonds were its most frequently occurring and highest cost nonconformance, McDonnell Douglas took the position that Composite Structures’ quality had deteriorated under the new management team.
- Composite Structures own people were not interested in fixing the problem. They were convinced that a 50% yield was “pretty good,” and the blades’ high tech design drove the low yield.
Composite Structures’ new management did not accept the above reactions. After talking to the in-house blade team, the company’s leadership realized they had little experience in root cause failure analysis. Composite Structures needed failure analysis training, and the company brought in an outside training organization for this purpose.
The Failure Analysis
Undeterred by the McDonnell Douglas reaction and after receiving root cause failure analysis training, Composite Structures pressed ahead. The company was determined to eliminate the rotor blade adhesive failures. With the tools provided by their failure analysis training, Composite Structures prepared a fault tree analysis to identify all potential failure causes. The fault tree’s hypothesized causes included contamination, adhesive anomalies, component nonconformances, process issues, shelf life issues, and design issues.
After using the fault tree to identify hundreds of potential causes in the above areas, the team objectively evaluated each. Here’s what happened:
- The company inspected its clean room assembly area for food, silicone-based spray lubricants, and other contaminants. Although they found and eliminated several potential contaminant sources, these actions had no effect on the blade failure rate. Overall, the company felt it had improved its contamination discipline, but the failures continued at about the same rate.
- The team disassembled and inspected several failed blades. None of the disbonded blades’ components exhibited any dimensional nonconformances. There was nothing to fix in this area.
- The team followed 12 blades through the manufacturing process. The blades were built with conforming parts in accordance with all process requirements. Within 2 days of the autoclave operation, however, 5 blades experienced adhesive failures.
After completing the above actions, the team felt confident that the failures were not process, component conformance, or contamination driven. Having ruled out these potential failure causes, team focused on the design.
During a meeting with McDonnell Douglas, Composite Structures explained that the failures were in the adhesive, indicating inadequate strength. McDonnell Douglas stated that this would not occur if the adhesive was wide enough. Composite Structures asked McDonnell Douglas how wide the bond joint had to be, and the answer was “at least .440-inch.”
The AH-64 Apache Main Rotor Blade Bond Joint. If the area marked by the scribe lines drops below 0.440 inches, the adhesive bond can fail.
Armed with this information, Composite Structures x-rayed the failed blades (the bond joint was not visible in assembled blades, but it could be seen in an x-ray). To everyone’s surprise, every failed adhesive joint was less than the magical 0.440-inch, and the team realized it had found its “smoking gun.”
Composite Structures next examined the blade drawing tolerances and another surprise emerged: The design allowed adhesive widths as low as 0.330 inches. Composite Structures shared this information with McDonnell Douglas. McDonnell Douglas agreed that their drawings allowed the bond width to go below 0.440 inch, but it took the position that Composite Structures should have known this and controlled the adhesive width to tighter dimensions than those required by the engineering design. McDonnell Douglas again stated that Composite Structures’ quality had deteriorated, and (in their opinion) that was why the adhesive widths were below 0.440 inches.
Composite Structures felt that McDonnell Douglas was ducking its responsibility, and that the rotor blade design was deficient. They were insulted by the poor quality accusations coming from McDonnell Douglas, but to address the charge, the Composite Structures team inspected blades produced earlier in the production program. (Composite Structures was the Army’s rotor blade depot repair facility, and it had older blades in-house for repair.) X-rays showed that the earlier blades displayed the same bond width variability as the current production runs. McDonnell Douglas finally conceded that there had been no quality deterioration at Composite Structures, and in fact, later complimented the Composite Structures failure analysis team for the thoroughness and success of their root cause failure analysis.
Corrective Actions
McDonnell Douglas advised Composite Structures to define and then control new manufacturing tolerances to meet the minimum bond width dimensions, but they would not change the existing rotor blade drawings (in other words, McDonnell Douglas dumped the problem on Composite Structures). Composite Structures did as McDonnell Douglas requested. The blade adhesive failure rate dropped to zero (the failures were completely eliminated), and the blades delivered to the Army met their blade life specification requirement.
In addition to finding and fixing the above root cause, during the course of the investigation, Composite Structures unearthed many other potential causes that could have induced an adhesive failure. Although none of these caused the failures that led to the investigation, Composite Structures implemented corrective actions to prevent these other potential failures.
Lessons Learned
What did Composite Structures learn from this experience? In a word, plenty. Here’s what emerged from this root cause failure analysis:
- Don’t accept the argument that state-of-the-art products necessarily have low yields. It may be that the people responsible for fixing the problem simply don’t know how to find what is causing it. Appropriate root cause failure analysis training will address this issue. The team that solved this problem consisted of the same people who had built the blades while the failures were occurring. They simply needed management direction to fix the problem, and a toolkit that allowed them to do so.
- When failures occur, assembly errors or nonconforming parts aren’t always the root cause. The part may completely conform to engineering drawings, yet still fail. When this situation exists, it’s time to re-evaluate the design.
- Identifying and ranking recurring nonconformances brings more visibility to improvement opportunities, but this increased visibility may be perceived as deterioration in quality. Organizations seeking to identify and eliminate recurring nonconformances need to educate everyone involved (including customers) to prevent adverse perceptions.
- Don’t accept complacency. Even if your customers accept poor quality, you don’t have to. It’s still costing you money, even if your customers aren’t complaining.
Composite Structures Results
Composite Structures did an outstanding job in finding and eliminating the cause of the Apache main rotor blade adhesive failures. They fixed a problem that had been plaguing their production efforts for 10 years, and they reduced cost, improved quality, and increased customer satisfaction as a result. The Army appreciated Composite Structures’ efforts immediately, and McDonnell Douglas ultimately did, too.
In addition to finding and fixing the actual cause of the main rotor blade adhesive failures, Composite Structures identified many other potential failure causes. A rigorous review of these potential failures resulted in the implementation of many other corrective actions.
Do you have recurring nonconformances in your organization? Are you hearing that such recurring failures are inherent to the process and there’s nothing that can be done? Would you like to eliminate these cost drivers and quality detractors? Please call us to learn more about our Root Cause Failure Analysis training programs. We can tailor our training to your needs, and your employees can start applying these skills immediately! Root cause failure analysis is an important part of any quality improvement and cost reduction program. It worked for Composite Structures and it can work for you.
Editor’s Note: Joe Berk, a Principal Member of our Engineering Faculty, has 30 years of engineering, management, and consulting/training experience. He teaches our courses on RCFA, FMEA, quality management, delivery performance improvement, cost reduction, engineering statistics, design of experiments, statistical process control, engineering economics, and more. He has published nine books on subjects related to the courses he teaches. His courses have been enthusiastically received by companies in diverse industries including IT and telecom, aerospace, defense, ordnance, electronics, electro-optical, energy generation and transmission, automotive, biomedical, marine, consumer goods, water treatment, and metal fabrication.