Freedom to fly

GRAHAM WARWICK / WASHINGTON DC

In the third of our series, we look at how civil aviation could diversify as it evolves - if airspace and infrastructure allow

When the Wright brothers made their first flights, only seagulls shared the air above the sand dunes of Kitty Hawk. Wilbur and Orville did not have to ask a controller for clearance to take off, and there were no neighbours to object to the noise as the 1903 Flyer made its historic first flight.

Today, thousands of aircraft of all sizes are airborne at any moment. The skies are far from full, but they are strictly controlled. As civil aviation has grown, so has the complexity of the airspace system within which it operates. That complexity is now a becoming a barrier to the further development and diversification of civil aviation. A slow modernisation of the airspace systems of Europe and the USA is under way, but many believe the growth projected for air transport demands a radical transformation.

Air traffic is expected to grow not only in volume, but also in variety. This is likely to stress the airspace system in ways that are only now becoming apparent.

If international hub-to-hub traffic grows as some predict, long-range large-capacity aircraft will be required. The favoured concept is an 800-1,000-seat blended wing body, which promises to be far more efficient than today's large aircraft, but poses far greater issues with ground manoeuvring and passenger handling than the 550-seat Airbus A380.

While the prospect for a 300-seat supersonic airliner has receded beyond the foreseeable future, overwhelmed by economic and environmental issues, studies continue on supersonic business jets. With a smaller aircraft it appears feasible the sonic boom can be reduced to the point where supersonic overland flight becomes viable. This could unlock the door to high-speed long-range flight.

Low-noise aircraft will be essential if the full capacity potential of existing airports is to be realised. If the goal of reducing the perceived noise levels by a factor to four within 25 years is achieved, then most aircraft noise should be contained within the airport boundaries, allowing 24h operation. The target 20dB reduction over today's aircraft will require substantial progress in minimising airframe, as well as engine, noise and could require new configurations.

Increased capacity

Runway-independent aircraft could increase the throughput of existing airports by taking over short-haul flying and freeing runways for long-haul large-capacity aircraft. Studies indicate that 40% of departures from US hub airports, carrying 20% of the passengers, fly less than 550km (300nm). Transferring that traffic to vertical/short take-off and landing aircraft could increase airport throughput by 25% and provide half as much delay reduction as the construction of a new runway.

In the USA, research into runway-independent aircraft is centred on designs seating 80-120 passengers, and cruising at 350kt (650km/h) over ranges up to 1,100km, that can be competitive with today's regional airliners. Advanced rotorcraft such as tiltrotors and compound helicopters appear to offer the combination of high cruising speed and altitude and low-speed terminal area manoeuvrability required to make the runway-independent aircraft concept feasible.

A network of small aircraft serving small airports is being promoted as an alternative to surface transport, at least in the USA, where the interstate highways are increasingly congested and high-speed rail has yet to catch on. There are over 5,000 small, and underused, airports in the USA, but their effective use requires that large numbers of single-pilot aircraft be able to operate safely in almost all weathers in non-radar airspace and around non-tower airports.

If the technology and procedures can be developed, then the conditions would exist for the creation of "yellow cab" air taxi and same-day package delivery services linking small communities. This would create a market for small four- to eight-passenger aircraft and rotorcraft that would be affordable and easy to fly, but the American dream of an aircraft in every garage still appears to be many decades distant.

As trade becomes increasingly global, innovative cargo aircraft concepts are being considered to meet the expected demand for rapid air delivery of freight. Studies have encompassed massive semi-buoyant airships, "flat-bed" freighters with roll-on/roll-off fuselages, and huge wing-in-ground-effect (WIGE) craft that are essentially flying ships. Even more exotic concepts include "trains" of unmanned cargo aircraft plying the oceans, possibly flying in formation like geese to achieve drag reductions from wingtip vortex interaction.

Pelican proposal

An example of a possible future freighter is Boeing's proposed Pelican ultra-large transport aircraft, a WIGE craft designed to carry 178 ISO-standard shipping containers. With a wingspan exceeding 150m (500ft), and almost 122m long, the Pelican is intended to carry a 1.3 million kg (2.8 million lb) payload 5,500km, cruising low over the water at 240kt. Unlike traditional WIGE craft, the Pelican is designed to also fly at altitudes above 20,000ft over land.

Unmanned air vehicles will almost certainly find commercial roles, if they can be integrated into the civil airspace system. The safe simultaneous operation of manned and unmanned aircraft is a challenge that has yet to be met, although experiments are already underway. If it can be, the potential civil applications of UAVs are almost limitless, and range from low-altitude security surveillance to high-altitude telecommunications relay. The spectrum of possible vehicle types is almost as wide, and ranges from unmanned rotorcraft to solar-powered stratospheric airships able to stay aloft for months.

Eventually, civil airspace will have to be opened to air-breathing spaceplanes, if the goal of routine affordable access to space is to be achieved. Manned or unmanned, these vehicles will be designed for aircraft-like operations. While the next generation of reusable launch vehicles is likely still to be rocket-powered, there are several promising hypersonic air-breathing engine concepts under development that could enable spaceplanes to take-off and land horizontally and fly like aircraft when inside the atmosphere.

Such diverse aircraft will only be able to share airspace if air traffic management evolves far beyond its present reliance on ground-based communication, navigation and surveillance. The evolution has begun, but progress is too slow and too fragmented for such future visions to become reality any time soon. The need for change is widely recognised, but economic pressures and regulatory obstacles are slowing the pace.

The navigation element of the equation is nearest to solution, but is still incomplete. The US global positioning system has been operational since 1995, but full use of satellite navigation awaits deployment of space- and ground-based augmentation systems that improve the accuracy, availability and integrity of the signals to the levels required by civil aviation. A new generation of navigation satellites is also needed before commercial aircraft have the safety-of-life service they need. The US space-based wide-area augmentation system should be commissioned this year, three years late, but its ability to provide precision approach guidance will be limited until the second GPS civil frequency, the L5 safety-of-life signal, becomes operational - now scheduled for 2015. The companion ground-based local-area augmentation system is planned to become operational in late 2006, but low-visibility landing guidance will not be available before late 2008.

Galileo help

Some help may be provided by the planned European Galileo global navigation satellite system, which is due to be fully operational early in 2008, if complex public-private financing and operating arrangements can be agreed. Galileo will provide a second, civil-controlled, source of position, velocity and timing, including a safety-of-life signal. The system's developers hope this will be enough to persuade civil aircraft operators to re-equip with receivers combining Galileo with GPS.

The communications piece of the puzzle is coming together, but slowly. Datalinks are the key, with applications ranging from replacing voice communications between controllers and pilots, to uplinking weather graphics to the cockpit and downlinking cabin video to security agencies. The first datalink services are being fielded, but their widespread use faces two challenges: the need for greater bandwidth and the need for international interoperability.

The first datalinks have relatively narrow bandwidths, suitable for controller-pilot communications but not sufficient for the extensive air-to-ground information-sharing envisaged for the collaborative, distributed air traffic management system of the future. But the regulatory hurdles that must be overcome before a datalink is approved for international use are immense. There are no fewer than three VHF datalink (VDL) standards under development or being deployed, plus other datalinks for surveillance.

Under contract to the US Federal Aviation Administration, Boeing last month demonstrated the feasibility of using its Connexion broadband satellite communications link to improve air traffic control. The company's concept for a future air traffic management system is built around a high-capacity common information network linking flight planners, traffic managers and flight crews in real time, and Connexion with its 128kbit/s uplink and 3Mbit/s downlink is an ideal medium. But with the 31.5kbit/s VDL Mode 2 only now being deployed in Europe and the USA, it could be years before broadband links are approved for air traffic control.

Approval is only one step, as a significant proportion of airspace users must equip accordingly if the benefits of any new technology are to be realised. This is particularly true for the surveillance leg of the stool. Few doubt the benefits of automatic dependent surveillance - broadcast (ADS-B), which enables aircraft to share position data with controllers and other aircraft, but those benefits only accrue if the majority of airspace users equip appropriately. This drove the decision to use the existing Mode S surveillance link for ADS-B despite a limited bandwidth that virtually ensures a new higher-capacity link will be required within a relatively few years.

Security debate

Adding to the debate over datalinks is the new focus on civil aviation security, following the terror attacks of 11 September 2001. The ability to see in real time what is happening on board an aircraft has great appeal to security agencies, but requires higher bandwidths than current datalinks can provide. Then there is the potential requirement that the ground be able to take over control of a hijacked aircraft. That possibility, however remote, may call for a high-integrity link that does not exist today.

Boeing, with its air traffic management initiative, is challenging the traditional piece-by-piece approach to airspace modernisation. The company has taken a top-down systems engineering approach, beginning by defining all the potential user demands and refining them into a number of "cornerstone" requirements, which it is now attempting to harmonise internationally, with the goal of drawing up the specification for a seamlessly global next-generation air traffic management system.

It seems a distant hope, but with air transport expected to resume its growth path within the next year, the limitations of today's airspace and infrastructure will stand in the way of civil aviation achieving its full potential.