Veteran’s Assistance: 5-Axis Machining

New materials and improved part manufacturing help maintain and upgrade aging military aircraft.

The U.S. Defense Department spends a significant portion of its $700 billion annual budget on advanced technology. However, operating alongside stealth fighters, remotely piloted drones and space-based defense systems are aircraft that are more than twice as old as many of the pilots who fly them.

“The military has not modernized its Cold War fleet at the rate expected,” said Dr. Loren B. Thompson, COO of Lexington Institute, Arlington, Va., a defense and national security research center. “There are literally thousands of old bombers, tankers, fighters and cargo aircraft that have been kept beyond their originally planned lives.” Considerations of part obsolescence, fatigue and corrosion will “force the government to spend a great deal of money keeping them airworthy,” he added. “There is demand for tens of millions of dollars in replacement parts not available from traditional suppliers or OEMs.”

Posted with CTE’s October cover story are a few video reports—one on parts for legacy military aircraft, and two others on the KC-135 Stratotanker. Watch the video reports here.

Golden Oldie

A prime example of an aging but essential aircraft is the Boeing KC-135 Stratotanker, an aerial refueling tanker that expands the flight range of military aircraft around the world. The first flight of a KC-135 took place on Aug. 31, 1956, and it was introduced to service in June 1957. The last plane was delivered in 1965. Average age of the 417-plane fleet is about 48 years, according to Gaddis Gann, chief engineer, KC-135, for the Aerospace Sustainment Directorate at Tinker Air Force Base outside Oklahoma City.

At Tinker, KC-135 tankers go through a regularly scheduled overhaul, inspection and repair process called programmed depot maintenance. Services performed during PDM include replacement of worn or damaged parts, sometimes substituting new materials that enhance performance or offer better resistance to corrosion and metal fatigue.

KC-135 designers used the best materials and assembly techniques available in the 1950s. Over the years, however, it has become apparent that some of the high-strength-to-weight-ratio alloys of that time are prone to corrosion and stress- corrosion cracking, where corroded metals can fail suddenly when subjected to tensile stresses.

When stress-corrosion cracking occurs in a structural member, it must be replaced. “Typically, if it is an aluminum part we will look for new-generation aluminum,” Gann said. As long as the basic characteristics of the material are similar, it will not alter the aeroelastic characteristics of the airplane. “That way, it can be a form-fit-function, drop-in replacement, eliminating some qualification testing,” he said.

Break Time

A good example is a series of “production break” KC-135 parts that join the outboard wing to the main wing section. Each aircraft requires 12 of the fittings, some up to 6 ‘ long. After finding evidence of corrosion and stress-corrosion cracking on some of the legacy parts, Gann said, “we made a decision to change all the production breaks on the fleet.” The original production breaks were made of 7079 aluminum; in the new parts, 7050 aluminum, which is more resistant to stress-corrosion cracking, is used.

Courtesy of Air Force photo by Margo Wright

On the depot maintenance floor of the nearly mile-long Oklahoma City Air Logistics Center at Tinker Air Force Base, a KC-135 is stripped of paint, engines and much of its interior as mechanics overhaul the workhorse aircraft. The KC-135 was introduced in 1957 and last produced in 1965.

Courtesy of Air Force photo by Tech. Sgt. Angelique Perez

A KC-135 tanker aircraft refuels an F-15 Eagle fighter.

The original production breaks were forged, but the new parts are machined and not forged because the new alloy has enough strength. Cut from 800-lb. billets, the finished parts weigh just 35 lbs. The shop’s machine tools include Cincinnati and SNK 5-axis machines.

According to Danny Tornello, manufacturing machining section chief, 551st Commodities Maintenance Squadron, 5-axis technology is required to machine a production break “because there isn’t a flat surface on it.”

Gann said the replacement parts are not typically machined to original dimensions because the planes were manufactured in the days before CAM files and 5-axis machines. “They were basically hand built,” he said. “As you go from airplane to airplane, you find there is a little bit of difference for each one. In the old days, they installed a forged part and it was kind of hand-massaged in.”

As a result, when the shop manufactures a replacement part, it leaves extra material that the print would not necessarily call out. The part is then fit-checked and trimmed to match the individual aircraft. Holemaking takes place after the fit-check. “Holes on different airplanes are not going to match,” Gann said. “They may be as much as a quarter or half a hole off.” Accordingly, the parts are machined blank and holes are drilled to mate with existing holes on the individual aircraft.

Reverse Engineering

Glenn Berglan, maintenance flight chief, 551st Commodities Maintenance Squadron, said there is little documentation for many of the old parts and “we have to do a considerable amount of reverse engineering and develop the dimensions using an old part.”

“We don’t have any digital data on this airplane,” Tornello said. “To reverse engineer a part, we use a Leica laser tracker to scan an old sample and build a model for our Catia V5 programming to create toolpaths for our machine tools. [When the first production break was machined,] we did a destructive first article. We sawed one in half and made sure the grain structure was good. Then, engineering bought off on the first production item of every dash number (different version) that we made.”

Courtesy of Air Force photo by Margo Wright

On the depot maintenance floor of the Air Logistics Center, a wing production break on a KC-135 tanker is flagged for safety.

When a large quantity of parts are required, as in the case of the production breaks, the Defense Logistics Agency (DLA) calls for bids and contracts from outside shops. However, early in installation of the production breaks, there were delivery delays. “That is where Glenn and his folks jumped in and helped us with individual dash numbers,” Gann said.

“By machining the missing parts, we were a short-term bridge,” Berglan said. “We kept the aircraft moving until the parts started arriving from suppliers.”

The machining operations at Tinker also provide quick response to unanticipated needs. “With a 50-year-old airplane, you’d think we’d know everything,” Gann said. “But we still get surprises every day. Occasionally, we will run into a failure. We must have parts to get the airplanes out of here and we don’t always have time for the supply chain to react.”

Tornello cited the example of a 3 “-dia., 4340 steel shaft, about 6 ” long. The parts are installed in pairs in the tail section of the KC-135 aircraft, in the torque box component that holds the horizontal stabilizer.

During a PDM, Tinker staff discovered that all such shafts on incoming planes were corroded or worn. Waiting for delivery of the parts from contractors through the DLA would have delayed the PDM process. “So we started generating an emergency work order yesterday (in early August). We were able to find material and have it delivered to the shop this morning, and we are starting production on 50 of them that will last on the PDM line here through October,” Berglan said.

Keeping Them Healthy

Depending on the progress of tanker replacement programs and other factors, some KC-135s may still be in service in 2040. The first tanker replacement program began in 2002 and a contract was awarded to Boeing.

After an investigation revealed corruption in awarding the contract, it was canceled in 2005. Another replacement program, called KC-X, was awarded to a joint venture of Northrop Grumman and the European Aeronautic Defence and Space Co. (EADS), based on Airbus A330 tankers, in February 2008.

Courtesy of Air Force photo by Margo Wright

In the Oklahoma City Air Logistics Center shop, Mike Bow (right) and engineering technician Tom Lange examine a sample wing production break machined on the SNK.

Courtesy of Air Force photo by Margo Wright