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.

What is CVM?

Retired airline pilot Ken Davey invented Comparative Vacuum Monitoring in 1994 to provide a “Structural Health Monitoring System” for aircraft. In 1968, Davey had been flying a Vickers Viscount the day before its starboard wing catastrophically failed in flight, resulting in the loss of all on board. The cause was an undetected crack through the main wing spar. CVM is a measure of the differential pressure between the “vacuum” side, next to the surface to be checked, and the non-applied side, which has its openings to “atmospheric.” If no flaw is present, the vacuum will remain at a stable level. If a flaw develops, air will flow through the passage created from the atmosphere to the vacuum side. A very simple analogy might be trying to get a suction cup to stick to a smooth surface, but traversing a crack. It would stick reliably to glass, but not to a crack in the glass. Sensors may either take the form of self-adhesive polymer “pads” or may form part of the component. A transducer measures the air flow between the passages in the sensor.

A basic system consists of three main components:

  • An inert sensor that can be adhered to (or embedded within) the structure during manufacture;
  • A regulated vacuum source to apply and modulate a low vacuum;
  • A measuring device.

While flat, self-adhesive polymer sensors are most commonly used, sensors can be designed and manufactured to conform to two and three-dimensional surfaces. Sensors can be made in a range of materials to suit even extremely hostile environments.

Sensors can be embedded within the mass of a structure or encased within bonded joints and lap joints to monitor for internal failure. This design can enable significant cost savings when compared to retrospective installations.

Several variations of the basic system are available:

  • Kits for structural fatigue testing in which multiple test sites can be continuously monitored from a single vacuum source and data acquisition computer.
  • Unique real-time crack propagation monitoring systems.
  • Portable systems designed for use on aircraft, where only the passive sensor is installed on the aircraft and the self-contained portable instrument is manually connected for periodic inspections, greatly improving inspection times.

Operator training is fairly easy and covers preparation of the surfaces, installation of the sensors, and the use of the monitoring instrumentation – which is completely automated, according to the manufacturer. The company plans to establish a training program for operators as the commercialization effort progresses.

Of significance, based on damage under doublers found in the wreckage of the China Airlines plane, this technology could detect cracks under doublers. An integral sensor can be sandwiched between the doubler plate and the surface with the cracking problem. Mark Vellacott, chief executive officer of the company, is bullish on the technology, and its potential to significantly reduce the cost of structural maintenance inspection programs. “We are well advanced in the process of qualifying CVM systems for use by both civil and military operators,” he says. The company already has installed 120 sensors in the A380 full-scale fatigue test rig in Germany, and it hopes to have inspection applications for aircraft fleets in 2006.

>>Contact: Vellacot, e-mail [email protected]; see also http://www.smsystems.com.au<<

FAA’s Aging Aircraft Program

After the explosive decompression and near-crash of a Boeing 737 in the Hawaiian Islands in 1987, Congress passed the Aviation Safety Research Act of 1988 (Public Law 100-591). This act increased the scope of the Federal Aviation Administration (FAA) mission to include research into the causes, effects, and mitigation of fatigue and environmental degradation of aircraft structures. In response, the FAA developed the National Aging Aircraft Research Program (NAARP).

Traits of Comparative Vacuum Monitoring

Comparative Vacuum Monitoring has special qualities because of its sensing and condition-monitoring capabilities. The sensor can:

  • Detect sub-1mm cracks in processed metal surfaces;
  • Measure the physical crack;
  • Operate successfully on peened, painted or otherwise treated surfaces;
  • Monitor welded joints;
  • Operate in situ on aircraft;
  • Monitor riveted fastenings and sandwiched structure (both external and internal surfaces);
  • Operate in varied atmospheric conditions, altitudes and hazardous environments; and
  • Measure crack initiation and propagation without stopping the operating equipment for inspection.

Source: SMS