In operational terms, the early de Havilland Comets distinguished themselves for confirming what many already knew: that the oval aircraft window was a design principle neither to be trifled nor dispensed with. An example of aircraft design lore whose breach over observance was best demonstrated by a string of well-publicised accidents in the early 1950s caused by catastrophic metal fatigue.
Less well known is that the aircraft introduced the concept of what most aircraft fatigue experts today term structural health monitoring, where the notion of damage tolerance alongside the pressurised fuselage and the wing-integrated jet engine was introduced for the first time on a high flying airliner, in addition to the hapless square windows.
© Roger Ressmeyer/Corbis
"The Comet accident implied remarkable things, such as the fact that we have ovoid windows today in pressurised fuselages and that we perform a major full scale fatigue test for each new type of aircraft such as the Boeing 787 and the A400M," says Christian Boller, professor and director at the Fraunhofer Institute's department for non-destructive testing in Saarbrücken and Dresden, Germany.
Much of the recent European research effort to develop advanced sensor technology falls within the Structural Monitoring with Advanced Integrated Sensor Technologies (SMIST) project of the EU-funded 6th Framework programme.
The 36-month €5.88 million ($8.3 million) project, which was launched in May 2005, selected nine sensor and monitoring technologies of different natures and examined each for their suitability to meet all of the research objectives and specifications. Although now coming to an end, the project was established to develop the most advanced sensing technologies as an integral part of an aircraft's structure, exploiting SHM capabilities in an effort to reduce maintenance costs, increase aircraft availability while making significant weight savings.
The main objective was to develop and validate monitoring technologies capable of enabling innovative structural design for both metals and composites while the main innovation of the project was the equipping of real in-service aircraft structure with an integrated sensing function by converting advanced sensor technologies into a SHM system.
Boller, who has driven much of the SMIST effort, explains the rationale for SHM, firstly, in terms of load monitoring: "Today we design our aircraft for necessary in-service load factors and then build in a safety margin on top to cover all we do not know in terms of structural behaviour.
"The higher the in-service loads - either maximum static or maximum spectrum in-flight - the heavier we have to make the aircraft. So, the more we know, the lighter we can build without compromising safety through any structural fracturing. This is what aircraft designers have learned over the past decades with success in designing aircraft structures that are damage tolerant."
Its rationale in terms of maintenance cost reduction resides chiefly in the fact that a technology that can detect invisible strain or cracks through, say, light-conducting fibres embedded in, or bonded onto the aircraft's structure act very like the human nervous system. Fractures, cracks or delaminations occur and destroy the fibres, interrupting the light flow. This interruption, in turn, allows the anomaly to be isolated.
The monitoring technologies that were presented as SMIST candidates were: fibre optic Bragg gratings (see box right), sensitive coatings, environmental degradation monitoring sensors, µ-wave antennas, acoustic-ultrasonics, comparative vacuum measurement, acoustic emissions, imaging ultrasonics and eddy current foil systems.
Boller says SHM, through the ability to isolate variance, could enhance operability significantly. "It may solve the problem of inspection in areas of the aircraft which are difficult to access, components that can take many hours of dismantling just for a cursory check that may result in 'no failure found'."
He adds that SHM could also allow for the reduction of inspection intervals at no additional cost or time, promoting the idea of "enhanced" damage tolerance. This, he says, would apply not to the whole aircraft structure but more to damage critical components which are highly loaded, door frames, say, or wing attachment boxes.
Such an approach could also help detect damage to specific rivet lines, something most structural engineers will remember from the infamous high-cycle Boeing 737-200 Aloha Airlines accident in 1988 whose unprecedented loss of integrity and resulting explosive decompression remains unsurpassed. Boller believes few would contest that this accident gave birth to the damage monitoring debate.
Critical Crack Length
"The regulations currently say that if you detect a crack then you have to repair it but we really need to develop a SHM system to determine critical crack lengths through damage tolerance analysis. Basically, how large a crack needs to be before it requires repair," says Boller.
For Hans-Juergen Schmidt of consultancy Aerostruc, a veteran fatigue calculation expert with 38 years of experience and responsible for Airbus aircraft fatigue test programmes from the launch of the European airframer, there is too much emphasis on cracks - far too scientific an approach rather than a jobbing engineering approach. "While we don't design it to crack, a modern aircraft is however designed to withstand certain level of cracking - that was always the basis for the development of inspection programmes for metallic structures, to detect fatigue, corrosion, accidental damage," says Schmidt.
He adds that one of the most important things that helped evolve design philosophy was to assess fatigue damage analysis not exclusively through static loads but through examining operational loads.
This came into regulatory force in 1978 with the entry into service of the A310, one of the first aircraft designed and certificated against the US Federal Aviation Regulations (FAR) Part 25.571 that required an evaluation of the strength, detail design, and fabrication, showing that catastrophic failure due to fatigue, corrosion, or accidental damage could be be avoided throughout operational life.
Boller says there is no question that SMIST has advanced the industry's knowledge, although there is a caveat here. In SMIST's sister TATEM - or technologies and techniques for new maintenance - project which aims to build and integrate a future integrated health management architecture's elements at a subsystem, system, aircraft and fleet level, those researching the feasibility of SHM looked closely at the maturity of these health monitoring technologies and systems, possibly systematically for the first time.
The aim of the four-year €40 million research project, which began in March 2004, was to demonstrate the means to achieve a 20% reduction in airline maintenance related costs within 10 years and a 50% reduction over 20 years.
To achieve this, TATEM is focused on developing maintenance-free avionics signal processing techniques to convert aircraft performance data into information about utilities, actuation, engines as well as structural systems' health sensor technology in an effort to reduce no-fault found alarms and generate decision instructions for maintenance engineers.
"We asked the technology providers and what we found was that the technologies are not yet at the required maturity level we would expect them to be - ranging from a realistic technology readiness level (TRL) of 5 to a more optimistic TRL7," says Boller.
"TATEM really confirmed what may be currently observed with the Boeing 787 programme, that SHM technology has been promised for implementation for a while but this has been much more gradual than it was said to happen."
Boeing last year talked about the possibility of incorporating structural health monitoring as early as the 787-3 variant, then due to enter production in 2009. Although previously saying it would not use built-in sensors because the technology was not thought sufficiently mature, Boeing now says: "If we were to incorporate the capability at some point, it would likely be no earlier than the -3 or the -3 timeframe. When that time comes and the technology is mature, we'll analyse the capability and see if it's economically beneficial for us and our customers to incorporate it into our models."
Francesco Camerlingo, who heads Alenia Aerospace's structural health monitoring research efforts within SMIST, says health monitoring for the all-composite Boeing aircraft will at its inception rely on essentially tried and trusted traditional fatigue calculation methodologies.
"For the 787, structural integrity will be known through a mix of rig testing which will provide information on how the aircraft will perform in-service. Once the first aircraft enter into service the structural integrity of composite aerostructures will then rely on scheduled inspection and maintenance."
For Schmidt, composite is definitely not the only game-changer in town. While his studies have shown that in some specific areas such as the upper fuselage panels above the window belt, SHM allows significant weight savings of around 15% - with similar reductions gained on the lower wing panels, he dismisses the lemming-like rush to develop a go-further plastic aircraft that will involve "enormous problems simply because of its level of innovation".
He recommends looking at other, albeit less exotic, materials due to their level of industrial maturity. Alcoa and Pechiney, both developers of aluminium alloys, have for example achieved weight reductions of around 20% while the fibre-metal GLARE laminate on the rear upper fuselage of the A380 has allowed Airbus to develop expertise in the less risky bonded material technology. Schmidt says he would not be surprised even to see far greater use of hybrid-materials in future aircraft developed by perhaps an Embraer, Bombardier, the Chinese or the Russians.
He also expects that Airbus's next new aircraft - the A350 widebody - will tentatively employ structural health sensors in areas such as the upper shell fuselage and lower wing skin as well as in the door structures where damage during loading can easily occur.
For Boller, however, some of the issues that remain over the general question of SHM include determining which components on an aircraft are worth monitoring in terms of operator benefit, developing sensor systems with the highest reliability compared to single transducers, the appropriate location of the sensor, electromagnetic interference, multi-plexing, sensor reliability, durability and possibly a variety of other issues.
Another is whether embedded optical fibre itself adds weight and if it could compromise the composite material itself. Yet another challenge is whether SHM be adopted on a large scale without becoming too cost prohibitive. "Even TATEM has been struggling to put a value on that so far," says Boller.
Notwithstanding the nature of the technology itself, one general concern remains over how these SHM technologies are to be integrated within the wider aircraft health monitoring avionics architecture that already monitors environmental systems and engine monitoring? "In that context TATEM will possibly provide a link for the very first time," he says.