Sustainable aviation

Citation

(2003), "Sustainable aviation", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 5. https://doi.org/10.1108/aeat.2003.12775eac.001

Sustainable aviation

Sustainable aviation

Keywords: Avionics, Conferences

A conference on this theme held at the Royal Aeronautical Society notably included "The Environmental Effects of Civil Aircraft in Flight" from the UK's Royal Commission on Environmental Pollution. This report began with the cause for concern which in this case focuses on in flight effects rather than ground level pollution, etc. Four sections are detailed: trends in air transport; environmental effects of aircraft in flight; technical possibilities; and policy and action.

Trends in air transport

Passenger-kilometres flown from UK airports increased from 125 billion in 1990 to 260 billion in 2000. The most rapid growth has been in international travel, though domestic flights have also risen. The huge expansion of the no-frills carriers has contributed to this growth. In the last 5 years, the passenger- kilometres carried by these operators have more than doubled, and account for about 20 per cent of the total passenger-kilometres flown from UK airports. Until 2001 the actual growth rate followed one of the higher growth scenarios although there was a drop below that trend in 2001 which was accentuated following the terrorist attacks in the USA. Many analysts believe this downturn to be temporary. Air freight has been growing even faster than passenger transport, rising by an average of 8.7 per cent per annum between 1992 and 1998 with this trend expected to continue.

Some of the environmental effects of flight can be addressed through technology but this takes time. Even so, the gradual replacement of older aircraft with newer designs ought to have a positive environmental impact, although these are not designed primarily for their environmental features.

The benefits of the air transport industry include directly providing jobs for 180,000 people in the UK and contributing some £10.2 billion to the gross domestic market. Many different industries are involved in the aviation sector; engines, airframes, airlines, air freight, etc. The industry is regulated by bodies such as ICAO, the UK CAA, etc.

Currently, the development of airframes is split between serving two market views; fragmentation and consolidation. The first is that a larger number of direct flights will occur between local airports so creating a requirement for a large number of smaller, high speed aircraft. The consolidation view sees further development in hub airports, flights between which would be provided by larger high capacity aircraft. Over the past 40 years, fuel burn and emissions of carbon dioxide and oxides of nitrogen have reduced dramatically for new aircraft, owing to increased efficiencies of design of engines and airframes.

The number of passenger movements is increasing faster than the number of aircraft movements because of the development of bigger aircraft. Also, most flights do not fly at full capacity, which causes difficulty in interpreting figures for emissions per passenger-kilometre, which are often calculated on the basis of a full aircraft.

Environmental effects of aircraft in flight

The main concerns are: climate change; stratospheric ozone reduction; regional pollution; and local pollution. The focus here, however, is on the possible larger-scale impacts of aviation, on surface UV radiation through changes in atmospheric ozone and on climate.

During flight, aircraft engines emit carbon dioxide, oxides of nitrogen, oxides of sulphur, water vapour, hydrocarbons and particles, the last-named consisting mainly of sulphate from sulphur oxides and soot. The majority of these occur far above the earth's surface.

At the top of the atmosphere, the solar energy absorbed by the earth/atmosphere is balanced by the emission of longer wavelength thermal radiation (heat). However, the thermal radiation emitted from the near surface region is absorbed by greenhouse gases, which then re-emit back towards the surface, keeping it warm.

The impact of aircraft emissions can be very different depending on whether they are in the upper troposphere or the lower stratosphere. The height of the troposphere varies with latitude. In the tropics, the tropopause is higher than the normal range of subsonic cruise altitudes, but in polar regions it is usually at the lower end of this range. Supersonic aircraft generally cruise at levels in the range 17-20 km which is always in the stratosphere. Jet streams are typically located at the tropopause in regions where there are abrupt transitions in the horizontal between the troposphere and the stratosphere.

When the moist high temperature air from a jet engine mixes with the ambient cold air, saturation can occur and the moisture can condense onto particles in the atmosphere, and in particular those present in the exhaust. The result is a condensation trail or contrail.

Many of the emissions from aircraft change the absorption of solar radiation and the absorption and emission of thermal radiation. They may therefore, affect climate. To estimate the relative and absolute importance of various activities and emissions of climate, "Aviation and the Global Atmosphere" was published in 1999. This uses a globally averaged measure known as radiative forcing, of the imbalance in solar and thermal radiation caused by the sudden addition of the activity or emission.

Implications from these studies show a major large-scale environmental problem associated with the expansion of aviation forcing climate change. One aspect of climatic changes is global warming and a convenient, but approximate measure of the tendency to produce global warming is the concept of radiative forcing.

Technical possibilities

Three possible areas for development are examined: airframes, that is for conventional aircraft, the fuselage, wings and tail; engines and fuel; and operation, including air traffic management and routeing.

For a long range aircraft, the majority of its load at take-off is its fuel. The rate of fuel burn varies widely between the different phases of flight. The high rate of fuel burn during take-off and initial climb represents a disproportionate fuel usage for short flights. The average fuel used per passenger- kilometre for flights of varying length is calculated.

Take-off and landing become less significant as the flight distance increases. However, fuel use per passenger-kilometre increases for very long-haul flights because of the large quantity of fuel that has to be carried during the early stages of the flight. The most fuel efficient flight distance is around 2,300 nautical miles.

A new design concept, the blended wing- body, shows promise with lower drag than the conventional wing-fuselage airframe design. Its fuel usage could perhaps be reduced by as much as 30 per cent, with the attendant advantages. In the near term, the greatest improvements can clearly come from better use of the existing stock of aircraft. When comparison is made with other transport modes, in particular, freight is much more damaging environmentally that it must be regarded as reserved for very high value and usually perishable goods.

Policy and action

In the conclusions and recommendations in its 22nd Report, the Commission has expressed concerns about the environmental consequences of the growth in air transport. Short-haul passenger flights make a disproportionately large contribution to the global environmental impacts of air transport. Air freight is much more damaging than other modes of transport.

A wide range of recommendations are made in the Report. These are: the imposition of climate protection charges for aircraft taking off and landing within the EU; restriction of airport development; encourage a modal shift to more benign methods of transport for short-haul flights; support technological development; and include international aviation in the emissions trading scheme envisaged in the Kyoto implementing mechanisms.