Environmental issues and the demands of safety and reliability drive airliner design as much as technology

Ever since the first powered machines flew at the start of the 20th century, aviation has been driven by the quest to improve aircraft efficiency. With extraordinary persistence, often surmounting seemingly impossible technical barriers, man has created flying machines that, as the first century of aviation draws to a close, have achieved a level of perfection that could only be imagined at its outset.

In terms of safety, reliability and efficiency, modern commercial aircraft, and the system in which they operate, represent one of man's ultimate technological successes.

Consider the daily routine that attends air transport. On a normal morning in Europe, 25,000 aircraft carrying 400,000 passengers take off and reach their destinations without incident. Each of those aircraft had to meet extremely demanding safety - and hence engineering - standards before being allowed to fly. The standards are just as tough for the airlines that operate them, the air traffic control (ATC) system that directs them and the certification authorities charged with ensuring the rules are obeyed.

The aviation industry contains a basic paradox: it has to be highly conservative, to ensure the aircraft and infrastructure are safe and reliable, but also dynamic, to incorporate the new technologies needed to cater for the relentless growth in demand for air transport.

A glance at any issue of Flight since its launch in 1909 reveals that development has been incessant and is set to continue. What has changed is that many of the so-called advances featured in earlier issues were destined never to see the light of day, whereas it is likely that most reported on today will enter service.

Technology costs

This reflects the high investment required to develop new technologies. Not only must new designs meet increasingly tough regulatory and environmental demands, but manufacturers have to ensure that any new technology "pays its way" on to the aircraft. Today, new technology is only affordable if it demonstrates cost and operational savings. History is littered with evidence that the aerospace industry ignores either one at its peril.

The 1980s' propfan engine, despite its promise of huge fuel savings, failed on the grounds of operational unsuitability, while the huge investment in automatic landing systems in the 1960s saw only minimal sales to airlines, which did not need them for most of their operations.

Whereas in the 1940s and 1950s manufacturers (usually with government backing) were able to produce new designs almost on an annual basis, today they cannot afford to field an aircraft without scrupulous attention to the exacting requirements of the marketplace.

So the major changes in configuration and technology which made air shows so exciting in the pre-1960s are absent from the commercial aircraft of today. Instead, one configuration has become the norm, in which two or four engines are mounted under a swept wing attached to a single or twin-aisle fuselage with a two-pilot cockpit. Manufacturers now base their aircraft ranges around standard fuselage sizes. Airbus Industrie has derived an entire range of twin-aisle airliners from the fuselage of the original A300, while Boeing has carried essentially the same single-aisle cabin cross-section from the 1950s' 707 through to the 1980s' 757.

Airbus consistency

For Airbus, the consistent application of new technologies has been a hallmark of its first 25 years. The European consortium was the first to introduce fly-by-wire flight controls on a commercial aircraft, allowing it to market a range of airliners with control characteristics so similar that only minimal pilot retraining is needed to convert from its smallest airliner, the 100-seat A318, to the largest, the planned 500-seat A3XX. Airbus also pioneered the use of composites in commercial aircraft primary structures.

Boeing could justify fly-by-wire controls and composite primary structures for its all-new 777 widebody twin, but its next major programme, the Next Generation 737, had to balance new technology with the need to keep development costs low and time to market short. Advances were constrained to a new wing, new avionics and improved engines.

Of the efficiency improvements achieved in the half-century since jet airliners were introduced, around 60% have come from the engine, and the rest from airframe configuration and technology, increases in aircraft size and advances in operational efficiency.

While developments in engine technology promise more efficiency gains, attention is shifting to airframe advances and the dramatic increases in computational power becoming available, both on the ground to enable an improved understanding of aerodynamics, and onboard, to allow smarter and lighter aircraft systems.

It is accepted that fuel efficiency improvements will be harder to achieve in the new century than they have been in the past few decades. With customers willing to pay only for technologies which bring economic advantage, benefits must outweigh costs, and airlines are more conscious than ever that fuel efficiency gains are often wiped out by airspace inefficiencies. ATC has become part of the efficiency equation and is pressed to improve performance.

Europe has the most densely used airspace in the world, with aircraft often unable to fly direct routes and wasting fuel in the process. US airspace is rapidly approaching gridlock. There are many initiatives under way to solve ATC problems based around new communications and navigation technology. These should result in overall reductions in route lengths that contribute more to fuel savings than any near-term airframe or engine technology development.

Airlines, however, are increasingly subject to environmental constraints. While fuel prices are relatively low, it is inevitable that prices will go up as environmental constraints penalising the use of fossil fuels begin to bite. A recent report on aviation's effect on the global atmosphere highlights the dilemma - there is no near-term alternative to kerosene as the best aircraft fuel and aviation will face increased penalties for its use.

This creates a need not only for improved fuel efficiency, but also for engines that pollute less. Concerns that contrails contribute to high-altitude cloud production and add to the greenhouse effect may lead to aircraft being routed around areas where contrails are produced - increasing fuel consumption and placing more demands on efficiency.

Environmental issues also introduce a new factor into the overall efficiency equation - the energy balance of introducing new technologies. For example, could boundary layer suction use more fuel than is saved by the resulting drag reduction?

Whatever the future holds for civil aviation, there can be no doubt the new century will see developments that will make today's aircraft seem antiquated. But those advances will be driven as much by the need for a cleaner planet as by the desire for improved efficiency to reduce operating costs.

Source: Flight International