Advances in aircraft technology generally go unheralded except in the specialist press. If a new box of electrical gadgets appears in the cockpit and makes the aircraft safer or a design upgrade on the engines makes it a little faster, it is of minimal interest to the traveling public.
But an airplane made of plastic? Moreover, a plastic that heals flaws within itself? That is something likely to worry the socks off the traveling public.
Composite materials, now widely used in commercial and military aircraft (as well as more prosaic items such as the fender supports of the Dodge Viper or Tiger Woods’ golf-clubs) are not exactly “plastic” in the layman’s understanding of the term — think of cheap ball-point pens — but highly engineered materials with some incredible properties.
The source of such an advanced piece of engineering can, oddly enough, be found naturally in a most familiar material: Wood. It too is a composite material, made up of fibers of cellulose bound together by lignin. At one time, perfectly sound aircraft — not least the famous De Havilland Mosquito — were made entirely of wood. Composites in aircraft therefore are not new in principle; the difference is the variety and strength of the current varieties.
A composite material typically consists of relatively strong, stiff fibers in a tough resin matrix. Better known as man-made composite materials used in the aerospace and other industries are carbon- and glass-fiber-reinforced plastics (CFRP and GFRP respectively). These consist of carbon and glass fibers, both of which are stiff and strong for their density, in a polymer matrix that is tough but neither particularly stiff nor strong. By combining materials with complementary properties in this way, a composite material with most or all of the benefits (high strength, stiffness, toughness and low density) is obtained with few or none of the weaknesses of the individual component materials.
Generally speaking, composites can be made into more complex shapes than can metals. This not only reduces the number of parts in a component but also the need for fasteners and joints. The advantages are twofold. Fasteners and joints may be the weak points of a component — a bolt needs a hole which is a stress point and therefore a potential breaking point — and fewer fasteners and joints can mean a shorter assembly time. Shorter assembly times, however, need to be offset against the greater time likely to be needed to fabricate the component in the first place.
The really attractive aspect of composites in aircraft is the lighter weight and therefore huge fuel savings: 20 percent claimed for the Boeing 787 Dreamliner.
Initially, composites accounted for a very small percentage of the weight of an aircraft — only two percent for example in the US F15 fighter but 18 percent in the F18 fighter. In the new Eurofighter, 40 percent of the weight is made up of carbon-reinforced composite material.
The first significant use of composite material in commercial aircraft was by Airbus in 1983 in the rudders of the A300 and A310, then in 1985 in the vertical tail fin. In the latter case, the 2,000 parts (excluding fasteners) of the metal fin was reduced to fewer than 100 for the composite fin, lowering its weight and production cost. Following these successes, composite materials were used for the entire tail structure of the A320. Currently about 22 percent by weight of the A320 is composite material.
The Boeing 777 comprises nine-percent composites by weight, compared with a whopping 50 percent for the Boeing Dreamliner. The superior strength of the composite fuselage will allow higher pressurization in the passenger cabin, making it easier to control temperature, humidity and ventilation and, of course, save on fuel.
Composite materials are also more durable than aluminum, because of corrosion and fatigue benefits, as well as a dramatic reduction in the number of fasteners. The structure of the Dreamliner is essentially one giant macromolecule. Almost everything is fastened through cross-linked chemical bonds reinforced with carbon fiber and wrapped in composite material.
The biggest challenge to producing aircraft-standard composite units is the tooling required. The sheer size of the tools and problems with heat on them — they have to be made of a special iron-nickel alloy called Invar to minimize distortion and maximize accuracy of the composite parts — posed no small number of challenges during the early 787 production process.
Perhaps the least believable but very real development in composite technology is the self-healing material. This simple but ingenious construction uses a process similar to the bruising and bleeding/healing processes in humans and animals. Aerospace engineers at Bristol University in the UK developed the system that potentially can be applied wherever fiber-reinforced polymer (FRP) composites are used. The material’s innovative aspect involves filling the hollow, glass fibers contained in FRP composites with either resin or hardener. If the fibers break, the resin/hardener oozes out, enabling the composite to recover up to 80-90 percent of its original strength — comfortably allowing a plane to function at its normal operational load.
The traditional material used in aircraft, aluminum, is a very tolerant material and can take enormous punishment before it fails. It can be dented or punctured and still maintain structural integrity. Composites are not like this. If damaged, they require immediate repair. This is difficult and expensive. Furthermore, an aircraft made entirely from aluminum can be repaired almost anywhere. This is not the case with composite materials, particularly as they use different and more exotic materials. It is likely that even though the pressures of pricing are encouraging manufacturers to look at composites as a building material, they will be used more in military aircraft — these are constantly maintained — than in commercial aircraft, designed to need less maintenance.
Damage to aircraft is much more frequent than the traveling public realize. It was spectacularly demonstrated when on Jan. 19, 2009 US Airways flight 1549, an Airbus A320, hit a flock of Canada geese, lost thrust in both engines and had to ditch in New York’s Hudson River.
There has been no technological solution to birds flying into the engines of aircraft at all, although various attempts ranging from ultrasonic emitters through loud bangs and plastic hawks positioned at strategic parts of airports are among some of the solutions tried.
Dr. Esteban Fernández-Juricic of Purdue University, Indiana, has looked deep into how birds interact with humans and concluded: “The tolerance of different bird species to humans appears to be related to the distance at which they detect disturbance.” It seems that some birds cannot “detect” a screaming passenger jet in time to avoid flying into its engines. Definitely “work in progress” here.
Not all impacts by debris — organic or otherwise, ends in the fortunate way that flight 1549 did. Air France Concorde flight 4590 crashed killing all 113 on board on July 25, 2000. France’s Accident Investigation Bureau concluded that a stray part lost by a plane that had taken off five minutes earlier punctured one of the Concorde’s tires and triggered the chain of events that caused it to burst into flames and crash. Fortunately, there now exists a British-made system that all but eliminates any failure to detect runway debris. It is QinetiQ’s Tarsier Foreign Object Debris (FOD) detection system.
The Tarsier system is based on high-resolution millimeter-wave radar coupled with automatic day and night cameras. Continuous monitoring by the radar quickly identifies the location of an object and directs the cameras to provide visual confirmation of hazards on the runway. Whilst originally designed for FOD detection, Tarsier can also detect wildlife, such as birds, on the runway and inform the airport safety management. The system is automated and fully operational 24/7, in all weather conditions. The system went live at Vancouver International Airport in early 2007 and in March 2008 it was selected by BAA for London Heathrow. It has been deployed at Dubai and Doha airports and recently scooped the Innovation Award at Jane’s Air Traffic Control awards ceremony in Amsterdam.