Every aircraft has them; those hard-to-reach nooks and crannies that must be cleaned and repaired during regular maintenance. On the Fairchild-Republic A-10 Thunderbolt II — better known by the nickname ‘Warthog’ — those spaces are the aircraft’s eight center wing fuel cells.
As the photo of an A-10 fuel tank shows — actually, this is just the port half of the tank — the fuel cell openings are barely large enough for an average technician to get his arm into. So imagine just how difficult it can be for the same technician to remove sealant from the fuel cell’s double rows of fasteners, in order to do non-destructive testing or make repairs!
Despite this difficulty, technicians do work inside the A-10’s fuel cells. However, the mechanical abrasion they must employ to remove sealant is both difficult and not entirely effective. Too little force and sealant can be left behind. Too much, and the internal structure can be damaged.
A laser-based system
In an effort to solve this problem, the Pacific Northwest National Laboratory (PNNL) in Richland, WA, (www.pnl.gov) and General Lasertronics Corp. (GLC) of San Jose, CA, (www.lasertronics.com) have successfully developed a laser-based removal system. Specifically, this approach employs a handheld Nd:YAG laser system, equipped to safely and completely remove sealant from A-10 fuel cells.
“The laser system causes ‘thermal mechanical ablation’ at the surface. In simple terms, you heat up the sealant using the laser, and it vaporizes, breaks away from the substrate, and/or pops off,” says Norm Olson, PNNL’s program manager of engineered systems. “It does this without getting the 2024-T3 or 7075-T6 aluminum alloy too hot, so there is no substrate damage. Also, because a 25-cfm purge airflow blows from the hand-held end effector ‘tool’ toward the ablating surface, the potential for ignition of sealant or fuel vapors in the tank is completely eliminated. The purge air also blows the vapors and flaked sealant away from the work area. The airborne vapors and sealant are easily evacuated from the fuel tank using conventional tank purge equipment. The result is complete sealant removal in a manner that is safe, effective, and easy to use in confined spaces.”
Nuts and bolts
The GLC-developed laser system uses either a 200- or 500-watt portable Nd:YAG (Neodymium-doped Yttrium-Aluminum-Garnet) laser system.
The Nd:YAG’s beam has a 1,064-nanometer wavelength, which is pulsed at a predetermined frequency. Extremely high-powered, short pulses are the most effective for heating up and ablating the sealant without damaging the metal underneath.
“In our first test series, we used a 200-watt laser,” Olson tells AMT. “It took about a minute to remove the sealant on and around a fastener. In the second series, it took less than 30 seconds using a 500-watt laser.” For the 200-watt test, the laser was a US Laser Corp. Model 406Q. It is a lamp-pumped, Nd:YAG, Q-switched device, which we operated at 13 kHz from a 24-kw power source. The 500-watt test was done with a diode-pumped, Nd:YAG, Q-switched Cutting Edge Optronics (a Northrop Grumman subsidiary) laser operating at 10 kHZ. The newer-generation diode-pumped laser is more efficient so it only needed a 10-kw power source.”
In the GLC system, the laser source is located about 100 feet from the work site, with the hand-held tool being connected to it by a long, flexible umbilical hose. This hose contains an optical fiber to transmit the laser pulses and provides the air to purge the vapors, fuel, and debris off the work surface.
The hand-held tool for these tests even had a video camera included. The camera allows the technician to see around corners; either on a monitor or a head-mounted display. The laser tool is also configured to rapidly scan the pulsed beam over the target area. This prevents the user from staying too long at a single point on the work surface.
A large vacuum return hose is also used inside the fuel cell. It has a 400-cfm waste collection and filtration stage that removes ablated sealant debris and related vapors, keeping the working surface clean. After all, this is a fuel cell we are working in!
When it comes to sealant removal, the only difference between the 200- and 500-watt laser systems was how long the job took. Either way, the laser beam completely removed sealant measuring about 1/8 inch in depth from the nut fasteners. Again, the photos tell the tale: The fasteners were left clean and accessible without suffering any damage.
“We did the test with inexperienced operators, to provide useful results to MROs and other potential users,” Olson says. “The only variable they had to master was to keep the beam’s focal length within a range of plus or minus 3/16 inch for maximum efficiency, which each did after a little practice. The only safety requirement was protective clear eyeglasses. These glasses filter out the laser light without obstructing the technician’s vision.”
When compared to mechanical sealant removal techniques, laser removal is faster, more effective, and safer to the aircraft and the technician. The equipment is also durable. For instance, the 200-watt system has been used in GLC demos since 2000 without laser-induced damage. That’s about 3,000 cumulative hours. Meanwhile, the newer 500-watt laser system uses laser diodes that are warranted for 10,000 hours of operation.
So what’s the downside? Cost. “Using a laser works out to about $1,000 per watt in terms of equipment,” says Olson. “This means that a one-off (non-volume sales) 200-watt laser system will cost you $200,000, and a one-off 500-watt system will cost about half a million.”
To date, this cost has kept the laser removal system off maintenance shop floors. That’s a shame, because such equipment could pay for itself very quickly. For the A-10 Warthog requirement, cost savings could pay for a system after less than 10 aircraft, and that does not count the benefit of a much faster maintenance throughput.
So what’s the solution? One idea would be for MROs in a given region to band together to purchase and then share a laser removal system. Another idea would be to establish third-party laser removal companies, who would travel from MRO to MRO as required to provide a coating removal service, or as needed to rent the equipment for the MRO to use.
“Laser removal works; there’s just no doubt about it,” Olson concludes. “Now we just have to find a way to get it into the market, and into aircraft repair shops.”