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.