Today's jet engines are radically different from those fitted to early jet airliners. In modern high by-pass turbofan engines, the huge fan at the front of the engine acts more and more like an old-fashioned propeller, driven by the smaller jet engine in the middle. The rotation of the fan generates much more thrust than the expulsion of hot gases which have passed through the combustor in the core of the engine. In the case of the GE90, only 11 per cent of air sucked in at the engine's entrance is used in combustion, giving a by-pass ratio of 9 to 1, the highest yet for a commercial engine.
Generating most of the thrust in this way has several advantages. Fuel consumption, exhaust emissions, and engine wear and tear are all reduced. But the most dramatic impact has come in noise reduction.
Noise in a jet engine is very complex, with different sounds radiating from different parts of the engine. The large-diameter fan at the front of turbofan engines tends to create deeper toned, buzz-saw noises. The high-pitched whine that people often find particularly annoying comes from other rotating parts, deeper within the engine. Designers counter the buzz-saw noise by reducing the speed of the fan tips, and employing highly efficient sound absorption materials in the outer casing of the engine.
However, it is the small percentage of hot exhaust gases that make the most noise. The compressed, high-velocity gases shear through the surrounding air with a violent turbulent roar. Modern engines envelope this stream of hot gas with a cloak of slower, colder air from the turbofan, which helps to reduce the noise. In addition, the hot exhaust gases from the latest generation of aircraft engines are expelled with a lower velocity than in previous designs. The two factors combine to reduce noise dramatically.
There will never be a totally silent engine, but General Electric claims that the GE90 gets close to a practical limit in terms of minimising noise. Speaking at Heathrow last week, James Barrett, deputy chairman of GE Aircraft Engine Services, said: "We are now getting to the point where noise generated by the airframe - the `wind-rush' if you like - is of the same magnitude as that caused by the engines themselves." The results certainly appear impressive. Mr Barrettsuggests that the take-off of one of BA's new Boeing 777s, fitted with two GE90s, will barely be noticed by those living outside Heathrow's perimeter fence.
The new generation of jet engines are also much cleaner than before. The control of emissions has become increasingly important because European countries will soon adopt tough new standards governing the pollution emitted when an aircraft takes off. These regulations were first recommended nearly 10 years ago by the International Civil Aviation Organisation (ICAO). Sweden has already gone one step further and now levies a tax on emissions from commercial jets in all phases of flight.
According to General Electric, jet engine emissions can be classified into four categories: soot, unburned hydrocarbons, carbon monoxide, and various oxides of nitrogen (NOx). Over the past two decades, all emissions other than NOx have been reduced to relatively low levels. But NOx emissions are of particular concern since they tend to react readily with other pollutants in the lower atmosphere to create a chemical "soup" that, when exposed to sunlight, forms ozone and smog.
The GE90 incorporates a new design of combustor whose NOx emissions are 22 per cent below the proposed ICAO standard. In the past, increased power typically meant hotter engines and hence greater NOx pollution. Designers at General Electric believe that they have found a way of mediating this problem by employing a combustor that burns the fuel at lower temperature in two concentric chambers, instead of the single high temperature stage found in current engines.
The "Treble Seven" is the first Boeing to incorporate "fly by wire" all- computer control systems. Although the European Airbus has had computer control for several years, some pilots have expressed concern over the role computers play in the control of such aircraft. On board computers monitor every aspect of the aircraft to detect any abnormalities, and can be programmed to take over from the pilot should the computer calculate that the plane is in any danger. Pilots can feel nervous when using this system, and in response Boeing has ensured the pilot can if need over- ride the computer's instructions.
British Airways will pay out £2bn for the first 15 "Treble Sevens", although the price includes options to buy 15 more. The company hopes to use them to increase capacity on busy routes to the Gulf and the United States. Quiet computer controlled jets will be dominating our skies within a very few years.