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With all forms of aviation experiencing higher costs, ever keener competition and greater demand for across-the-board efficiency, minimising time on the ground has become an increasingly important aspect of any operation – military or civilian. And with aircraft remaining in service longer, the combination of aging fleets and growing cycle times puts inevitable pressure on maintenance regimes.

Leaking fuel systems represent a notorious major drain on both available time and budgets. As Matthais Block, market manager of Alcatel Vacuum Technology, puts it, "fuel leaks are a thorough nuisance, since it’s incredibly difficult to estimate how long it will take to repair them." Unsurprisingly, it is an issue that is currently coming under renewed scrutiny.

Fuel leaks principally arise from stress, structural defects and degraded seals or sealants as aircraft age and, especially in those subject to particularly high-cycle deployment, detecting them and locating their source can be a constant struggle. These problems are particularly severe with military aircraft, where elevated performance-specific stress further exacerbates the effect of routine operational activity – though for civil aviation, airframe design, maintenance programming and routing can also conspire to make things worse.

“Minimising time on the ground has become an important aspect of any operation – military or civilian.”

Aircraft routinely operated in hot and humid conditions, for example, are particularly prone to loss of sealant elasticity and even for those which are not, exposure to bad weather or repeated hard landings can have a detrimental effect, as the stress loads absorbed by their wings affect the structural integrity of wet-wing tanks.

The nature of modern fuels can also be a factor, since the construction of fuel tanks in some older model aircraft still in use renders them susceptible to corrosion. The leak issue is, undeniably, a multifaceted one.

Detecting fuel leaks

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By GlobalData

Fuel can travel a long way before it becomes a noticeable drip. Channels that form under stress-damaged aging sealant can duct the leak far from its point of origin – and to complicate matters still further, there can often be more than one source.

Leak detection is often a meticulous and painstaking task for any detective but advances in technology have increasingly helped ease this burden. A number of approaches are available, principally based on visual inspection, pressure measurement or the use of tracer gases, each having its own particular niche in a market that demands fast and accurate location allied to high certainty and reliability to minimise ground time.

Recent developments in tracer gas systems in particular have brought unprecedented precision to the leak detection arena, providing maintenance engineers and repair specialists with instruments capable of delivering more than 1,000-fold improvement in sensitivity, compared with standard blow-back methods.

“Fuel leaks principally arise from stress, structural defects and degraded seals or sealants as aircraft age.”

Small molecule gases, such as hydrogen – in a non-flammable, non-toxic 5:95 hydrogen : nitrogen mix – and helium are capable of accurately determining the smallest of leaks in fuel systems. And just as importantly, to verify a finished repair, prior to refuelling.

With re-leaks and new leaks all too common after such work, the move is clearly a welcome one. Moreover, the rise of intrinsically safe detectors has allowed location and repair to be undertaken with some fuel still in the tank, shaving vital hours off an already often protracted process.

Tests on the Adixen Sensitor Extrima, for instance – one of the latest-generation systems to use dilute hydrogen tracing – have shown the potential to cut repair time by half, in some instances.

Fuel leaks and sealant technology

Sealant technology itself is an area that has seen a spate of innovation, inspiring a new range that offers significantly enhanced adhesion, temperature stability, reliability and ease of application, aimed at meeting modern aerospace needs.

Products such as Royal Adhesives and Sealants Hardman WS 8020 RC – a rapid cure manganese dioxide cured polysulfide – are typical of an evolving range of state-of-the art sealants simultaneously designed to improve quality and reduce costs. Cutting curing time in half – to just 12 hours – as CEO Ted Clark explains "allows for faster aircraft assembly and reduces the time for repair and maintenance which will reduce our customers’ overall costs." It also features a new standard of flexibility and bond-strength over a range of metal and composite aircraft substrates, a wide temperature window, good chemical resistance and a high tolerance to stress and weathering.

Other developments, such as the recently launched PR 2007 from PPG Aerospace and PRC-DeSoto have addressed the issue of aircraft weight, offering up to 30% lower weight than typical preceding sealant technologies. According to Bill Keller, PPG Aerospace’s global segment manager, with the thousands of fuel tank fasteners on an aircraft, sealant application can represent considerable weight. In today’s increasingly fuel-conscious climate, sealant technology can make savings in more ways than one.

“Channels that form under stress-damaged and aging sealant can duct the leak far from its point of origin.”

The skills issue

Fuel system inspection and repair is widely recognised as a specialist skill, and often different aircraft require their own particular approaches, hence the continuing high degree of outsourcing, not least because of the high cost of maintaining a separate cadre of mechanics on-staff. Unsurprisingly, much of the necessary training relates to correct sealant application, something that many of the product manufacturers have themselves contributed to extensively, with the establishment of standards for both approved techniques and user certification schemes.

"Simply slapping new sealant over old doesn’t constitute a repair," aviation consultant Mike Hampson says. "That’s something that we’ve seen done in the past. Whether you’re dealing with legacy aircraft, or the latest designs, there are always going to be problems if you don’t use the right people," he says.

Although there are good initiatives around, it is something that he believes needs to be watched carefully in the future. "The aerospace industry is going through a bit of a strange phase at the moment, adapting to the changing times and a lot of what once seemed unassailable, isn’t any more. The challenge is going to be making sure that the skills issue continues to get addressed."

In his report "An Independent Review into the Broader Issues Surrounding the Loss of the RAF Nimrod MR2 Aircraft XV230 in Afghanistan in 2006" Charles Haddon-Cave QC notes: "a prevailing attitude that leaks in aviation fuel systems are an inevitable fact of life." Fuel leaks may not quite be ubiquitous, but they are certainly endemic – though fortunately few end in such disaster or even in the sort of drama that recently forced a Thomas Cook 757 into an emergency landing moments after take-off from Turin. Haddon-Cave also said that many of these problems can be reduced by adopting a proactive approach; with recent advances in detection equipment, sealants and training, it seems doing so is now becoming much easier to achieve.