Geriatric ward

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Sustainment has become a major issue because military aircraft are remaining in service longer - many are likely to serve for more than 50 years

Stewart Penney/WRIGHT-PATTERSON AFB

Retaining elderly military aircraft in service is a worldwide issue as air forces shift away from new aircraft programmes and place increasing emphasis on through-life upgrades. Long term work at the US Air Force Research Laboratory (AFRL) will apply science and technology along with improved business practices to sustain the USAF's fleet. Over 75% of the USAF's aircraft are more than 25 years old, and many are expected to be in service more than 50 years.

"We have turned around from being reactive to proactive, but it's not easy to apply," says Mike Zeigler, ageing aircraft structures leader within the AFRL's air vehicles directorate.

The AFRL's sustainment programmes can be broadly split into two areas - ageing aircraft and systems modernisation.

Ageing structures are more expensive to operate and maintain, says Zeigler, and suffer unforeseen problems such as corrosion and cracking, which can ground fleets and affect the USAF's readiness and responsiveness. At the same time, technology - bringing extra capabilities - needs to be inserted into today's aircraft to meet tomorrow's needs, he adds.

The AFRL has relationships with a host of US bodies - including the navy, army, Federal Aviation Administration and NASA - and internationally with Australia, Canada and the UK. It takes part in a four-power agreement with France, Germany and the UK and in the NATO ageing systems working group.

Sustainment is further split into system payoffs and system level focus goals, says Zeigler. The former is aimed at a 10% cut in direct operating and support (O&S) costs while improving operational capability by 10%.

System-level goals are to reduce maintenance man-hours per flight-hour by 10%, maintenance by 15% and material cost by a similar amount. Applying these to Boeing's figures for the KC-135 in-flight tanker's economic service life, the AFRL believes the fleet's direct O&S costs will shrink by $7.2 billion, says Zeigler. Assuming that USAF studies do not change plans, the KC-135 is to remain in service until 2035.

Economic service life

Determining economic service life is a "big question", says Zeigler, and it becomes more complex when other nations operate an aircraft. If the USAF decides to retire its first generation Lockheed Martin C-130 Hercules, for example, it would adversely affect US allies that operate the airlifter. Retiring a large part of the worldwide fleet would likely lead to an increase in spares costs and reduce depot maintenance provision. Support costs tend to grow exponentially, says Zeigler, because, for instance, parts are no longer manufactured.

AFRL's cost-cutting initiatives target cost growth rather than actual costs, he adds, and reducing actual costs would probably require a complete rethink in maintenance provision.

Demonstrations cover three areas: cost reduction, modelling and simulation, and capability enhancement. Major fixed-wing programmes and demonstrations to 2007 include composite repair of aircraft structures; the fatigue and corrosion insensitive airframe; structural life extension demonstration; dynamic load alleviation; low observable structural repair; and diagnostics and prognostics.

The ageing aircraft structures integrated technology thrust programme (ITTP) supports the Sustainment ITTP. Key elements are developing structural integrity methodology; advanced non-destructive inspection (NDI) systems; corrosion prevention, assessment and control and repair and replacement technologies. The goals, says Zeigler, are to ensure USAF safety and readiness to reduce the cost of ownership; to manage corrosion, cracking and damage and to extend fleet lives.

Inserting new technology

The AFRL aims to insert new technology - structural or systems - into aircraft during maintenance, be it at inspection or depot level which reduces down time and improves availability. Each platform has a force structure maintenance plan that defines the policy for keeping enough aircraft in service to maintain a capability. Safety remains a key issue. The AFRL makes a major impact in the maintenance, depot and inspection schedules, says Zeigler, in timing and work performed.

Fatigue and corrosion management are important drivers. Corrosion prevention is receiving a lot of attention as environmentally unfriendly chromate-based protective paint is being outlawed. Whatever is used to replace chromates must have the same capability.

The AFRL is working on prevention strategies and "innovative" repair and modification processes. The work applies to metallic and carbon-fibre composite structures. Non-destructive inspection (NDI) is crucial to finding and understanding damage. Once any damage is discovered, the AFRL and the air force need to understand its extent and the repairs that are needed. Can a component be maintained in place until the aircraft next receives depot maintenance, or should it be rectified immediately? "As a science and technology organisation," says Zeigler, "the AFRL tries to cut through to the underlying issues, to identify problems and decide whether science and technology should be applied to the problem."

Maintenance work, particularly at depot level, is often delayed for long periods when unforeseen problems are found and require spares that are not readily available. Delays in sourcing and acquiring these spares can reduce the number of aircraft available to the frontline and can hold up the maintenance of other aircraft if a maintenance bay is occupied by an aircraft awaiting parts. Zeigler asks whether it is possible to provide parts more quickly, or in a different way. He says the AFRL can use its science and technology base to consider new materials, or revise a design so that a component can be produced in weeks rather than months.

It is important to integrate the work across the AFRL's directorates to ensure compatibility. This also helps ensure that there are "no surprises" when technologies are scaled up from laboratory test pieces to full-scale repairs, says Zeigler. Bonding is one area that the AFRL is keen to see used more often in repair schemes. "It has been used before, but not as much as we would like," says Zeigler. "We're working across all areas to get bonding more accepted."

One bonding success story is the Lockheed C-141 Starlifter, the wings of which were suffering cracking around weep holes. Traditional repair would have required additional skin thicknesses being riveted to the wings, but traditional fasteners could not be used because of the proximity of fuel tanks. Bonding repairs take composite patches, which are bonded - or glued - to the structure around the damage, or, says Zeigler, where a problem is predicted. "It is safe and economic." he adds, "but glue is glue, so what happens when it gets old?" To determine the bond's long-term properties, the AFRL will perform "rot tests" with Australia's Defence Science and Technology Organisation.

Anti-corrosion initiatives

Similarly, says Zeigler, anti-corrosion initiatives take place in the materials and manufacturing directorate as well as in air vehicles. "We want to manage corrosion damage like we do with cracks." Corrosion is costly and time-consuming to fix, says Zeigler, "but if we manage it like we do cracking, then we can plan maintenance, repair and replacement when it is feasible". This means during scheduled maintenance rather than being surprised by the problem and having to ground the aircraft, he adds. In a two-pronged attack on corrosion, one element is considering its effects on structural integrity, the other looking at advanced protection.

Non-destructive evaluation (or NDI) aids the discovery of corrosion and cracking, says Zeigler. The materials directorate has developed the MAUS, a hand-held device that can be used in the field (most NDE/NDI equipment is too bulky and heavy to be used away from maintenance depots). This capability needs to be extended to systems that can "see through" several layers, such as lap joints where the skins are laid one over the other, says Zeigler. At present, lap joints are peeled back - which requires taking out the rivets or bonding and resecuring afterwards - to check for corrosion. If this check can be done in the field, any components needed can be ordered and will be waiting for the aircraft when it arrives at the maintenance base.

Other challenges are composites, large unitary structures and the design and maintenance of flight control systems, particularly software.

Composites can be inspected using ultrasonics, but this becomes more difficult and "presents a real challenge" when the material is thicker, says Zeigler. Currently, this imposes an up-front penalty because structures are "over-designed and then inspected rigorously".

Unitary structures - those machined from a solid block rather than built up with smaller components - are a good manufacturing technique, he says, but present a challenge in the longer term. "If they get damaged, is it cost-effective to replace such components? It is a supportability question. We will probably find a middle ground that is good for manufacturing and for support."