Tankers in Unknown Territory

April 1, 2010

Despite recent optimism that the Air Force’s KC-X tanker replacement program may be finally moving forward, an operational next generation tanker fleet is years away. The nation’s ability to project power around the world will therefore depend to a tremendous degree on keeping geriatric KC-135 Stratotanker and KC-10 Extender tankers airworthy.

This is a troubling situation, but the Air Force is not taking it lying down. The service is pushing full speed ahead on a variety of initiatives designed to keep its tankers viable until their replacements are ready.

The Total Force currently operates 474 tankers. Air Mobility Command states a need for 520 to 640 replacement tankers. This is tacit acknowledgment that there is already a gap in force structure, so even when KC-X aircraft begin entering service, existing KC-135s will remain on the job.

As things stand, roughly 236 of the Eisenhower-era KC-135s could remain in the Air Force fleet until 2045.

For Gaddis Gann, tanker health is both a daily and strategic concern. Gann is the chief engineer at Tinker AFB, Okla.’s 827th Aircraft Sustainment Group—the KC-135’s depot, where workers constantly evaluate the airworthiness of the jet aircraft, perform major structural repairs and overhauls, and work up maintenance specifications for the rest of the fleet.

The KC-135 is rolled out at the Boeing plant in 1954. In the background is the KC-97 it replaced. (Boeing photo)

Beefing Up the Schedules

A lot of this is “unknown territory,” he said. “Nobody’s ever flown a modern jet transport for 80 years.”

The need to keep ancient tankers in service has raised difficult sustainment questions which don’t have easy answers, Gann said. When will corrosion set in? Is parts obsolescence manageable? Will components stay healthy

Most commercial aircraft are sold or replaced every 20 to 30 years, he noted. To keep the tanker fleet viable out to the 2040s, a range of checkups, modernization, replacement efforts, and maintenance practices is in motion to keep the healthiest of the refuelers in the air.

Sustainment officials say they’ve made progress toward keeping aircraft on flight lines and out of the shop. Much of the “low hanging fruit” problems with the 135 have been worked out and now depot workers are attacking tougher issues, said Col. Robert Torick Jr., commander of the 827th Aircraft Sustainment Group.

The initial stage of inspection, which Torick called “triage,” is crucial to figuring out what’s wrong from an engineering perspective and to diagnose problems early. At this point, depot workers can move into repairs, buildups, operations checks, and other processes.

Inspection regimes have been beefed up at the unit level, Gann said, so the depot has a better idea of what is wrong with a given airframe by the time it rolls in for its five-year depot checkup.

But parts shortages and obsolescence issues are persistent, difficult to solve problems. “It continues to get worse,” said Col. Douglas Cato, commander of Tinker’s 76th Aircraft Maintenance Group, on solving the “severe” spare parts puzzle. Parts from the “Boneyard” at Davis-Monthan AFB, Ariz., provide some relief, but the replacement pieces don’t always fit.

While the various airframes have common components such as gear boxes or flaps, the big issue is the assembly techniques used in the 1950s. “They did not have laser precision assembly techniques; they were assembled by master mechanics,” Gann said.

The net result is that without significant rework, the chance of a structural part from a retired E model fitting one of the R models that remain in service is rare. The additional work required to make components fit in turn raises costs.

As a result, Cato said, the maintenance community is pushing for the Defense Logistics Agency to increase “ship sets” of spare parts on hand before aircraft are disassembled, to better mitigate risk and cut down on maintenance flow times.

Process improvements will also be critical to tanker sustainment, Cato said. The target for the depot is to cut the number of flow days that it takes to move a tanker through the depot from about 200 today to 130 by Fiscal 2012.

“We’re working hard to get there,” he said. In addition to solving parts logistics issues and speeding up technical solutions, Cato said there will be integrated maintenance stands arriving at the depot, which will simplify working around the aircraft. A new facility will also allow more dock space, as Cato said they plan to ramp up from 54 aircraft in service annually this year—with further expansion set for service leadership approval.

Giving maintainers and depot workers the tools they need to solve problems is just as important as improving processes, he added. “I talk about the mechanic being like a surgeon,” Cato said. “Every time the surgeon reaches for an instrument, somebody is slapping it in his hand. It should be the same way when the mechanic reaches for a part. They shouldn’t have to want for anything.”

To the lay observer, Gann said, it is not apparent that the average KC-135R is a vastly different structure than a typical commercial jetliner. Yet it is. The tankers were assembled before the development of many modern mechanical engineering processes.

Despite the availability of parts from retired KC-135Es, a fundamental problem for the fleet’s survivability is the difference in construction. Techniques used in manufacturing are not up to spec with modern aircraft, and even sealants must be replaced with better materials in many instances.

F-22 Raptors line up to take on fuel from a KC-135 during a mission over the Pacific Ocean. More than 200 KC-135s could remain in service until 2045. (USAF photo by SSgt. Andy M. Kin)

Late Finds

What makes fixing tankers so difficult now and depot work so important, Gann noted, is that many components which were never intended to be replaced are now failing—parts buried in hard-to-access parts of the aircraft. “One of our big problems is ‘late finds’ [requiring] structural repairs,” he added.

Depot officials are working with the Air Force Research Laboratory to develop new technologies to evaluate the health of hard-to-access structural components. Many of these are only visible by X-ray or through visual inspection. Catching problems early can prevent them from devouring precious man-hours.

Less time in depot translates to more time in the air, several officials stress. “We’ve tried really hard to identify areas that might be problems,” Gann said, such as wind-break production fittings. Without attention, fittings can loosen and lead to fuel leaks which can lead to unscheduled depot maintenance stops—where an aircraft will arrive at depot for a fix the unit isn’t equipped to handle, he said. Currently, about 15 to 20 tankers arrive at the depot each year for unscheduled fixes. With proper diagnostics, the numbers could come down.

Sustainment of the KC-135 fleet already clocks in at around $2 billion a year, said Gen. Arthur J. Lichte, then AMC commander, last fall. Around 2018, the Air Force will look at possibly “re-skinning” the aircraft, putting in new wires, flight controls, and cabling. At this point, costs will rise exponentially, predict most AMC analysts. The cost to sustain the Stratotankers could surpass $6 billion a year by 2018, based on predictions from past economic service life studies, Lichte said.

Every year of delay in the KC-X program is also a problem, and costs AMC an additional $55 million through increased maintenance requirements for the legacy tankers and additional time the old aircraft spend out of service.

The primary challenge over the coming years will be structures, stress corrosion, and cracking, Gann said. Stratotankers built in the 1950s were selected for high strength and low weight ratio. These elements were not necessarily related to durability or resistance to corrosion, but they were great for carrying heavy fuel loads. As a result, maintainers and depot workers have been trying to systematically replace certain components, such as fuel bladders, wiring, and flight controls, in an effort to return the parts to original condition. This never-ending work-set doesn’t even address needed modernization efforts.

Airmen with the 379th Expeditionary Maintenance Squadron replace a landing strut on a KC-135 tanker in Southwest Asia. (USAF photo by SSgt. Robert Barney)

In an effort to get ahead of the age curve, the KC-135 program office at Tinker is undertaking an aircraft teardown program between now and Fiscal 2015, Gann said. The effort involves the rigorous disassembly of three KC-135R airframes at the depot over the next several years, taking apart large sections, looking at areas such as wing boxes, the aircraft’s internal components not usually examined in the course of flight line maintenance, and other key points on the aircraft.

While such programs have been undertaken before, this effort is taking a more intensive look inside the aircraft. “Hopefully it will give us an indication of things to come,” he added.

Another key to extending the tanker fleet’s life is good management of the assets, mobility officials contend. Tankers are regularly rotated in and out of areas such as Hawaii and Japan, where the potential for corrosion over time is high if they are overexposed to sea salt.

Even tankers located at training sites, such as Altus AFB, Okla.—where touch-and-go landings are a regular occurrence—are watched closely to make sure the landing gear and components don’t get too heavily taxed, Gann added.

Keep Them Flying

Modernization of components and fleetwide upgrades are critical to ensuring the tanker fleet will fly in the years ahead. Global Air Traffic Management modifications wrap up on the KC-135R models in Fiscal 2011, and then Block 45 upgrades will get under way. They include new displays, replacement of leftover “steam gauges” in the cockpit, digital panel installations, and other modifications.

Air Mobility Command plans dictate that the fleet will remain in the force until 2045. These Extenders, which have yet to undergo a fleetwide modernization, may require upgraded avionics, particularly for their boom control units.

Skin and corrosion issues will have to be examined around 2020, which will increase parts costs. Another source of concern: The KC-10 fleet is averaging up to 71,000 flying hours a year—much higher than its anticipated 52,000 hours annually when it arrived in the force.

The Air Force’s long-serving tankers will continue to test the maintenance community. The aircraft are so old there is no one solution or problem to tackle.

“It’s the unknowns of the future, … being able to balance any potential structural issues, particularly cracking, with systems issues,” said Gaddis Gann, chief engineer at the 827th Aircraft Sustainment Group, Tinker AFB, Okla.

He noted the KC-135 fleet could just as easily be grounded for a gear box problem as a structural crack—and his job is to make sure the components which could fail are inspected at the right times.

“[It’s like] you’ve got a 50-year-old car, and you now have to drive it another 35 years,” he said. “There are a lot of things we have to do. … It’s hard to pinpoint a single challenge.”

No one denies this is a difficult job, but a nation that utilizes airpower around the globe must keep its tankers flying.