State of the Art?
Back to the photo I showed a week or so ago…
An Apache Rotor Blade Bond Joint
The photo above shows a bonded section of an AH-64A Apache helicopter main rotor blade in the area where you see the blue Dykem. It’s where the blade manufacturer and the Army experienced numerous disbonds, and it’s the problem the blade manufacturer had to solve.
An AH-64A Apache at Fort Knox, Kentucky
Before delving into the failure analysis, let’s consider the Apache rotor blade’s design and its history. The Apache helicopter has what are arguably the most advanced rotor blades in the world. They can take a direct hit from a 23mm ZSU-23/4 high explosive warhead and remain intact. During the Vietnam war, a single rifle bullet striking a Huey blade would take out the helicopter and everyone on board. When the Army wrote the specifications for the Apache, they wanted a much more survivable and much less vulnerable blade.
Vietnam-Era Huey Helicopters
The Apache helicopter prime contractor designed a composite blade with four redundant load paths running the entire rotor blade length. The blade’s advanced design uses titanium, special stainless steels, and honeycomb, but those four redundant load paths were the key to its survivability. If one section of the blade took a hit with a 23mm warhead detonation, the three remaining load paths held the blade together. That actually happened once during the first Persian Gulf war, and the Apache helicopter made it back to its base. It’s an awesome design, but it had a production weakness.
Apache Rotor Blade Sectional View Showing Four Spars
Let’s also consider the nature of the Apache production approach. Three entities are important here: The US Army (the Apache customer), the prime contractor (who designed the helicopter and its blade), and the blade manufacturer. The blade manufacturer was a built-to-print manufacturing organization. They built the blade in accordance with the helicopter prime contractor’s technical data package.
The manufacturing process consisted of laying up the blade in a cleanroom environment using special fixturing, bagging the blade components in a sealed environment, pulling a vacuum on the bag, transporting the blade to an autoclave, and then autoclave curing. The autoclave cure was rigidly controlled in accordance with the prime contractor’s specification.
During production startup, many of the blades had a high rejection rate after the autoclave cure. The bond joint (where the stainless steel longitudinal spars overlapped, as shown in our photo above) frequently disbonded. Eager to get the blade into production, the blade manufacturer, the prime contractor, and the Army pushed ahead. They believed that due to the “state of the art” nature of the Apache blade’s design, a less-than-100% yield was inherent to the process. The disbond failures continued into production. To cut to the chase, the blade manufacturer continued producing the blade for the next decade with an approximate 50% rejection rate. To make matters worse, blades in service on Apache helicopters only had about an 800-hour service life (the specification called for a 2,000-hour service life).
By any measure, this was not a good situation. The blade manufacturer had attempted to find the disbond root cause off and on for about 10 years, with essentially no success. While not happy, the Army continued to buy replacement blades, and they continued to send blades back to the prime contractor from the field for depot repairs. The prime contractor sent the blades back to the blade manufacturer. In retrospect, neither the prime contractor nor the blade manufacturer were financially motivated to fix the disbond problem.
After a change in ownership, the blade manufacturer realized the in-house blade disbond rework costs were significant. The new management was serious about finding and correcting the blade disbonds. Using fault-tree-analysis-based root cause analysis techniques, the company identified literally hundreds of potential failure causes. The failure analysis team found and corrected many problems in the production process, but none had induced the blade disbonds. The failures continued. Surprisingly (or perhaps not surprisingly, considering the lively spares and repair business), the helicopter prime contractor did not seem particularly interested in correcting the problem.
After ruling out hundreds of hypothesized failure causes, one of the remaining suspect causes was the bondline width where the longitudinal spars were bonded together. That’s the distance marked on the macro photo with scribe marks on the blue Dykem (the photo I showed you earlier, and the one at the top of this blog entry). During a meeting with the helicopter prime contractor, the blade manufacturer asked if the bondline width was critical. The prime contractor, evasive at first, finally admitted that this distance was indeed critical. The prime contractor further admitted that if the distance was allowed to go below 0.440 inch, a disbond was likely.
Armed with this information, the blade manufacturer immediately analyzed the prime contractor’s build-to-print rotor blade drawings. To their surprise, tolerance analysis showed the blade’s design allowed the bondline width to go as low as 0.330 inch. The blade manufacturer inspected all failed blades in house, and found that every one of the failed blades was, in fact, below 0.330 inch. It was an amazing discovery.
The blade manufacturer immediately asked the prime contractor to change the drawings such that the bondline width would never go below 0.440 inch. The prime contractor refused, most likely fearing a massive claim from the blade manufacturer for a technical data package deficiency spanning several years. The prime contractor instead accused the blade manufacturer of a quality lapse, stating that this was what allowed the bondline width to go below the 0.440 inch dimension.
The blade manufacturer explained the results of their tolerance analysis again, and once again pointed out that the blade design permitted the disbond-inducing condition. When the prime contractor refused to concede the point (and again accused the blade manufacturer of a quality lapse), the blade manufacturer took a different tack. As repair facility, the blade manufacturer had blades in house for depot repairs from various points during the Apache program’s life (including the 12th ever blade built, which went back to the first year of production). All of these earlier failed blades had the same problem: They conformed to the technical data package, but their bondline width was below 0.440 inch.
The blade manufacturer, faced with an ongoing 50% rejection rate, decided to hold the blade’s components to much tighter tolerances than required by the prime contractor’s technical data package. By doing so, the blade manufacturer produced conforming blades with bondline widths above 0.440 inch. After implementing this change, the blade disbond rejection rate essentially went to zero.
So what’s the message here? There are several:
- Don’t accept that you have to live with yields less than 100%. You can focus on finding and fixing a failure’s root cause if you are armed with the right tools. Don’t accept the “state of the art” argument as a reason for living with ongoing yield issues.
- Don’t think that simply because the product meets the design (i.e., there are no nonconformances) that everything is good. In many cases, the cause of a recurring failure is design related. Finding and addressing these deficiencies is often a key systems failure analysis outcome.
- If you are a build-to-print contractor, be wary. The design agency may not always be completely open to revealing design deficiencies.
- It’s easy to become complacent and accept a less-than-100% yield as a necessary fact of life. In some cases, the yield is not just a little below 100%; it’s dramatically less than 100% (as occurred on the Apache rotor blade production program for many years).
- There are significant savings associated with finding and fixing recurring nonconformances. You can do it if you want to, and if you have the right tools.
You know, the wild thing about this failure and the Mast Mounted Sight failure mentioned a week or so ago is that the two companies making these different products were literally across the street from each other. The Mast Mounted Sight was a true show stopper…it stopped production and it probably delayed the start of Operation Desert Storm. The Apache blade didn’t stop production…it was just a nagging, long-term, expensive rework driver for the Army and the blade manufacturer. Which one was more expensive? Beats me, but if I had to guess, I’d guess that the ongoing (but non-show-stopping) nature of the Apache rotor blade failures carried a heftier price tag.
Do you have recurring inprocess failures that you’d like to kill? Give us a call at 909 204 9984…we can help you equip your people with the tools you need to address these cost and quality drivers!