It is not uncommon for engineers to develop a new compound for maintenance or manufacturing purposes, but then find that they cannot control processes well enough to produce large enough batches to make it cost-effective. That was the case recently when the U.S. Air Force (USAF) developed a replacement material for tape around access panels on the B-2 bomber.
The project to develop the material was about to be cancelled until the Air Force Research Laboratory (AFRL) stepped in with a $2.8 million program to bring the fabrication processes under control.
Manufacturing technology engineers from AFRL’s Materials and Manufacturing Directorate, working with the B-2 Systems Group and material-processing experts, solved a critical material scale-up problem that directly affected the operational maintainability, and therefore combat availability, of the B-2 bomber fleet.
The program reduced the production schedule for the new material from 26 weeks to 12 weeks and implemented an improved test method that saves eight calendar days per batch. The program also enhanced risk mitigation, improved material durability, reduced maintenance costs, and lowered material costs. Maintenance actions previously requiring a week of aircraft downtime for repair now require as little as 30 min. Aircraft on which the new material is used have shown significantly lower maintenance man-hours per flight hour and have maintained a fly rate more than double that of the rest of the fleet.
Earlier this decade, the B-2 Systems Group initiated a program to improve the B-2 fleet mission capability rate by replacing tape that covered access panel gaps and fasteners with a material called Alternate High Frequency Material (AHFM).
The process of removing and replacing the tape was very laborious. The tape, assumed to be made of or painted with a radar-absorbing material (AFRL engineers would not say), had to be physically scraped off to open the panels. Upon completion of the work, the gaps and fastener heads were filled with a caulking-type material, which had to be cured. Some of the tape, which comes in a roll several inches wide, had a pressure-sensitive adhesive already applied; some of it needed a secondary adhesive. Then a primer was applied, followed by paint on top of that.
“We’ve always known this would be a maintainability problem, but performance outweighed that issue,” said Doug Carter, Senior Materials Engineer at AFRL.
Use of AHFM has been shown to meet performance requirements while eliminating a number of maintenance steps. For example, AHFM exposes the gaps and fasteners for easy removal and replacement of access panels without any material restoration required.
Flight tests demonstrated the effectiveness of the AHFM design, but upon material scale-up for fleet-wide implementation, consistent batch-to-batch performance could not be obtained at the Northrop Grumman Palmdale, CA, hangar where the aircraft are maintained. PPG Industries was responsible for preparing the batches.
“This is a very common problem,” said Carter. “In this case, we were going from a small 50-gal batch to a 500-gal batch. The chemical composition of the material didn’t change, but everything about the process [of manufacturing a large batch] was different. The process was done at a different location and the type of equipment was different. It was not inspected properly because they didn’t know what to look for in the manufacturing processes. Each batch had to be individually tuned.”
Not only did it take 26 weeks to manufacture a 500-gal batch, the yield rate was only 50%, meaning that half the batches did not pass qualification testing. Weeks more were then spent tuning the out-of-compliance batches until they were usable.
Because of the troubles, a planned fleet-wide AHFM implementation on all 21 B-5 Spirits was postponed. It was at that point that AFRL stepped in with a Rapid Response Process Improvement program tasked with solving the problem.
“We gathered a team of industry and academia along with AFRL and identified all the variables that might affect the final properties [of the material], everything from the actual process of how the material was mixed to the final application of the material—how it was sprayed on the aircraft, the environment, temperature, and humidity,” explained Carter. “We varied the variables and looked at the effect on the performance of the material. We determined which variables greatly affected the final product and determined how tightly we needed to control the tolerances.”
Low-hanging fruit such as the proper mixture of materials was addressed in the first six months of the two-year effort. Greater challenges such as determining the proper humidity for material curing took longer.
At this point, AHFM is installed on eight bombers.
Now that the material scale-up kinks have been worked out, AHFM is expected to find application on “future weapon systems that have similar materials,” said Carter. He would not name a specific aircraft, though the stealthy F-22 Raptor is a likely prospect.“They have the data and information to do it properly the first time,” he said.