Structural Monitoring Can Pinpoint Impending Failures

Air Safety Week, July 18, 2005

A new technology from down under is vacuuming up cracks

A quick head count of airworthiness directives (ADs) discloses that around one-fifth of them are directed at loss of aircraft structural integrity, either through corrosion, cracking or potential overload. This rough figure is probably not representative of the size of the problem, however, as unpublicized Service Information Letters (SILs) or Service Bulletins (SBs) would normally address myriad other minor issues that haven't as yet reached a critical stage requiring a regulatory decree.

Suffice it to say that when airplanes are built, even when overbuilt in terms of robustness for longevity's sake, avoiding the loss of structural integrity over time is a significant cost factor in maintenance, repair and overhaul (MRO). Problems that don't surface visually need to be revealed before they reach significant proportions. The venerable technique is generically referred to as non-destructive inspection (NDI). One case that springs to mind is that of China Airlines Flight 611, an aging Boeing B747 that suffered catastrophic structural failure over the Taiwan Straits on May 25, 2002.

The Aviation Safety Council (ASC) of Taiwan concluded that a 180- centimeter (or 5.9-foot) crack on the under-fuselage had been hidden from view externally by a tailstrike repair doubler for some 25 years (ASW, March 7, p. 1). It was hidden internally by an accumulation of dust and other detritus. When it finally let go, it resulted in an inflight breakup at 35,000 feet, killing 206 passengers and 19 crew. The ASC report included the observation that: "According to the regulations of Boeing's Repair Assessment Program, the aircraft would have undergone inspections of that repair before reaching 22,000 flights. However the accident occurred when the jet had completed its 21,398th flight, a few cycles of landing and take-off away from the maintenance program's maximum threshold."

An Australian company, Perth-based Structural Monitoring Systems (SMS), has devised a patented methodology to reliably detect such unseen defects. It has signed a contract with Airbus for licensed use of its Comparative Vacuum Monitoring (CVM) technology for integration with new and existing Airbus airframes, including the A380. It is also working with Boeing on retrofitting the technology into older aircraft. SMS hopes to be able to roll it out to the U.S. manufacturer's fleet in 2006.

Before looking at how it works and what it can do, what could be the advantages? Firstly designers could re-engineer areas that must be designed to cope with cyclical fatigue (like wing-roots) for weight-savings of around 15 percent. Weight-savings can obviously increase payload or reduce fuel carriage - which in turn can reduce airline fuel bills. The global airline industry spends $12.9 billion annually on structural maintenance.

The National Aeronautics and Space Administration (NASA) estimates that structural integrity monitoring through embedded devices such as CVM could reduce airframe and maintenance costs by as much as 35 percent. At present, paring weight is not really an option because provisions must be made in load- bearing structures for multiple redundant load-paths. Design standards accommodate the unseen and unforeseen degradation of a primary component due to cracking or corrosion. But even in existing overbuilt structures, the ability to detect cracking and degradation in a timely manner would provide the same sort of safety buffer that modern engines have via the Spectrographic Oil Analysis Program (SOAP). At a certain latter point in its life, thanks to a combination of use, abuse and exposure to the elements, an airframe will transition from fail-safe to fail-sure. NDIs and non-destructive tests (NDTs) can offer only a modicum of reassurance that something isn't failing sight unseen. These kinds of in-depth examinations also occur only periodically. With CVM, airline maintenance personnel shouldn't have to keep their fingers crossed behind their backs in between major overhauls. When something untoward occurs, CVM technology is designed to reveal it without delay.

It's not out of the question that modern aircraft will develop dangerous cracking. Australia's second airline, Ansett, went out of business following two lengthy groundings by the country's regulator. The carrier's maintenance group had totally missed the target dates on two important service bulletins, one involving a potential crack in the tail of its 767s and another addressing engine pylon cracks. Once an audit had picked up these oversights, the carrier was chagrined to find that, yes, it did have dangerous fleet-wide cracks in both areas that were in need of repair and, yes, these cracks had been there for quite a while.

SMS is looking to have an offline monitoring system hooked up to the monitoring technology of widebody aircraft some time next year (i.e., a "plug- innable" system with embedded sensors). The following year it hopes to have an online monitorable system interfaced with the aircraft's Health and Usage Monitoring System (i.e., it would be able to provide downloadable data wirelessly). If it proves to be a reliable technology, it may well be incorporated in the Federal Aviation Administration's (FAA) AAR-400 aging aircraft program.