Structural Monitoring Systems

Often this can require significant downtime to get access to such an area with a handheld testing machine.

SMS has developed a method of detecting cracks in aircraft structures using its patented Comparative Vacuum Monitoring (CVM) technology, which could see an inspection take a matter of minutes through the use of in-situ sensors, instead of hours and potentially days using traditional maintenance and inspection methods.

CVM was originally invented by Perth based former pilot Ken Davey, who developed a process for remanufacturing cathode ray tubes (CRTs) in the early 1990s.

“One day he saw that the vacuum pressure in a CRT was increasing quickly, and traced this back to a leak through a crack in the tube’s glass,” explained Mark Vellacott, managing director of SMS.

Davey had reason to look further into this process’s possible application for monitoring aircraft structures.

On December 31 1968, a Vickers Viscount from MacRobertson Miller Airlines crashed on descent into Port Hedland and killed some of Davey’s colleagues.

The origin of the crash was a crack in the spar that supported one of the Viscount’s wings. Davey had flown the same aircraft, VH-RMQ, only the day before.

Since Davey’s eureka moment, SMS has now developed a range of CVM sensors which can be placed in fatigue critical hotspots and are sensitive enough to pick up cracks as they initiate.

These sensors contain two sets of minute ‘galleries’ that are sealed against the metal. When a check needs to be done, the sensors are plugged into a handheld unit, the PM 200. When active, this provides a constant partial vacuum through one of the galleries while the alternate galleries are kept at atmospheric pressure. If a crack is present and has breached under the sealed area between the vacuum and atmospheric gallery, the PM 200 will show a positive reading, and will then warrant a full inspection.

“What we’re trying to do at the moment is alleviate the 95 per cent of the time when people pull an aircraft apart, go do an inspection and don’t find anything and then put it back together again,” said Vellacott. SMS has now developed a range of sensors which can be placed in fatigue critical hotspots and are sensitive enough to pick up cracks as they initiate.

The advantage of the sys-tem is that once the sensors are installed, they do not have to be directly accessed for an inspection, but can be simply inspected using the PM 200. This removes the long process of disassembling a certain part of the aircraft in order to conduct a non-destructive test on an area such as a pressure bulkhead, and also the human factor issues associated with the use of complex NDT (non-destructive testing) equipment in hard to access areas.

“This isn’t going to help you if you have an existing crack, but what it will do is help you from a maintenance outcome viewpoint if you don’t have to pull the aircraft apart to do the inspection in three to five minutes rather than spending two days of aircraft downtime, pulling toilets and galleys out. ” The real advantage for aircraft operators will come from regular use of the sensors to conduct service bulletin inspections and streamline aircraft maintenance practices across large fleets. Vellacott sees this as the major value proposition of CVM.

For the moment the company is focused on working with manufacturers and operators to retrofit the technology into existing aircraft as an alternative means of compliance (AMOC) for hard to inspect frequent inspections, although “The future of this technology as we see it is that it will be incorporated into both new and existing aircraft to enable improved maintenance practices,” said Vellacott.

In 2007, Boeing included CVM as one of the methods for checking metal fatigue cracking in its Common Non-Destructive Inspection Manual. This has effectively made CVM able to be used as an alternative for existing inspections and be available for any new inspection directives.

SMS has been working with Boeing to get CVM certified as an AMOC to current service bulletins. Vellacott says that a full production installation trial with Boeing and the FAA was completed recently and that the approval should occur for the first fleet application in the near future.

This process has been driven by strong interest from the airlines, including three operators in the US who have already committed to a requirement to use CVM. “The US airline fleet is quite old at the moment and a number of those aircraft won’t be replaced for some time, and there’s certainly a lot of pressure on the airlines to reduce the maintenance costs of those ageing airframes. So we think we’re hitting the market at the right time with our technology,” said Vellacott.

As well as the work with Boeing, SMS has also had a long working relationship with Airbus. “We started the relationship with Airbus back in 2000,” says Vellacott.

“They spent about two years just evaluating the technology in a laboratory environment.

They then had a specific requirement in 2003 for the use of CVM to assist the qualification and full scale fatigue testing of the A380. ” This led to what Vellacott says is one of the largest applications of CVM so far.

“We have had about 150 CVM sensors monitoringareas on the A380 full scale fatigue test rig, and it was used by Airbus for qualification of the GLARE material which counts for about a third of the surface area of the fuselage structure. ” As an aluminium-epoxy laminate, GLARE is hard to inspect using traditional NDT methods, such as eddy current monitoring. But SMS was able to develop a sensor which could be integrated into GLARE joints and was highly effective in monitoring the development of cracks in the material. The data gathered from these tests helped qualify GLARE in time for the inaugural flight of the A380.

Airbus has expressed great confidence in CVM for both laboratory testing and aircraft maintenance, and announced in April last year that it considered CVM as “technology ready” for inclusion in commercial aircraft maintenance programs. Airbus has also recently presented it to a group of Airbus aircraft operators and is said to have attracted major interest.

Wider than current applications, Vellacott says that both SMS and Airbus are looking to future development of CVM. “We’re also working at the moment with Airbus to look at opportunities for incorporating this into new aircraft designs. We have a joint development agreement with Airbus which gets to an endpoint in 2008, and as part of that agreement we have agreed to licence the technology to them. ” As well as the two largest aircraft manufacturers, SMS has also been working with Bombardier and Embraer who have ordered sensors for use in aircraft structural testing. Both manufacturers are also interested in the maintenance applications of CVM, and Bombardier has recently said that it “sees a future for the technology in reduced maintenance costs. ” Although a lot of work has gone into the use of CVM in commercial airliners, SMS has similarly strong links with the Australian military, including conducting a number of trials with the RAAF and Defence Science and Technology Organisation (DSTO).

“We’ve had a long association with DSTO,” said Vellacott. “We’ve been doing a lot of durability testing, a lot of laboratory based trials for the type of material thicknesses and material types you see on large military transport aircraft. ” In 2007, CVM was cleared for operations on all ADF aircraft by the Director General of Airworthiness, and has been flown on some RAAF P-3 Orions and Army Black Hawks to monitor representative areas. SMS is also targeting fitting the sensors on board large military transport aircraft for crack monitoring.

SMS has also been working on the development of its sensors for a number of different aircraft with a number of air forces across Europe and also the US. This has included the development of sensors for Sea King helicopters being operated by the British Royal Navy and Royal Air Force.

The company has also been involved in trials of CVM on a US Air Force Hercules, and has been conducting tests of the product at the Air Force Research Laboratories at Wright-Patterson Air Force Base. Vellacott considers that this will lead on to other applications with the USAF.

“It’s early days, but the fact that we’re under contract through their research laboratory at Wright-Patterson is significant. ” SMS has also been working with EADS Military Air Systems for the application of CVM as part of the Integrated Vehicle Health Management System for a UAV. The system, which is currently still in the prototype phase, would allow operators to be able to take real-time readings on the development of cracks or detection of composite damage while the UAV is in flight.

Such technology is also being developed for integration into commercial aircraft health monitoring systems, with SMS set to demonstrate it to Airbus and Embraer this year.

Longer term, SMS even sees the possibility of being able to build sensors and vacuum galleries into aerostructures, both metal and composites. The company is currently in talks with some suppliers on this.

With the potential for huge operational gains for both military and commercial applications, CVM could change the maintenance regimes for aircraft in a major way. With the hard yards done and approvals and endorsements on their way, CVM should soon start keeping aircraft out of the hangar.