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World Wide Web (WWW)

or Web

Leading information-exchange service of the Internet. It was created by Tim Berners-Lee and his colleagues at CERN and introduced to the world in 1991. The Web gives users access to a vast array of documents that are connected to each other by means of hypertext or hyperlinks. A hypertext document with its corresponding text and hyperlinks is written in HTML and is assigned an on-line address, or URL. The Web operates within the Internet's basic client-server architecture. Individual HTML files with unique electronic addresses are called Web pages, and a collection of Web pages and related files (such as graphics files, scripted programs, and other resources) sharing a set of similar addresses (see domain name) is called a Web site. The main or introductory page of a Web site is usually called the site's home page. Users may access any page by typing in the appropriate address, search for pages related to a topic of interest by using a search engine, or move quickly between pages by clicking on hyperlinks incorporated into them. Though introduced in 1991, the Web did not become truly popular until the introduction of Mosaic, a browser with a graphical interface, in 1993. Subsequently, browsers produced by Netscape and Microsoft have become predominant.

Learn more about World Wide Web (WWW) with a free trial on Britannica.com.

A wide-body aircraft is a large airliner with twin aisles and typically a fuselage diameter of 5 to 6 metres (16 to 20 ft.). Passengers usually are seated 7 to 10 abreast. For comparison, a traditional narrow-body airliner has a diameter of 3 to 4 metres (10 to 13 ft.), with a single aisle and seats arranged 2 to 6 abreast. Typical wide-body aircraft can accommodate between 200 and 600 passengers, while the largest narrow-body airliners typically hold about 280 passengers. Commercial freight or cargo wide-body aircraft also are in operation.

History

Following the success of the Boeing 707 (with fuselage) and Douglas DC-8 (with fuselage) in the late 1950s, airlines began seeking larger aircraft to meet the rising global demand for air travel. Engineers were faced with many challenges as airlines demanded more passenger seats per aircraft, longer fuel ranges and lower operating costs.

Early jet aircraft such as the 707 and DC-8 seated passengers along either side of a single aisle, with no more than six seats per row. Larger aircraft would have to be longer, higher (i.e. double deck) or wider in order to accommodate the greater number of passenger seats. Engineers also realized that lengthening the fuselage would have resulted in aircraft that would be too long to be handled by airports, while having two decks created difficulties in meeting emergency evacuation regulations, which were extremely challenging provided the technology available at the time. These parameters left a wider fuselage as the best option: by adding a second aisle, the wider aircraft could accommodate as many as 10 seats across.

The first true wide-body airliner was the four-engine Boeing 747, which made its debut in 1969 and remained the largest aircraft in regular passenger service until October 25, 2007, when the Airbus A380 entered commercial service. The main deck of the 747 features twin aisles and seats 10 abreast, while the upper-deck "hump" of the aircraft seats six abreast along a single aisle. The 747 fuselage is in diameter.

Other wide-body aircraft soon followed, including the McDonnell Douglas DC-10 and the Lockheed L-1011 Tristar with a diameter fuselage. In 1974, Airbus introduced the Airbus A300, the first twin-engine wide-body jet. Subsequent commercial wide-body jets include (by greatest outside diameter):

In addition, two new wide-body designs are currently in development or test mode:

Design considerations

Although a wide-body aircraft has a larger frontal area (and thus greater form drag) than a narrow-body aircraft of similar capacity, it has several advantages over its narrow-body counterpart:

  • Lower ratio of surface area to volume, and thus (for equal volume) lower frictional drag
  • Twin aisles that accelerate loading and unloading/evacuation relative to a single aisle
  • Wider fuselage that reduces the overall length of the aircraft, improving ground maneuverability and reducing the risk of tail strikes
  • Greater under-floor freight capacity
  • Better structural efficiency for larger aircraft than would be possible with a narrow-body design

British and the Russian designers have proposed aircraft similar in configuration to the Vickers VC-10 and Boeing 717, but with a wide-body fuselage. The British Three-Eleven project never left the drawing board, while the Russian Il-86 wide-body proposal eventually gave way to a more conventional wing-mounted engine design, most likely due to the inefficiencies of mounting such a large engine on the aft fuselage.

Operating considerations

Aircraft are categorised by ICAO according to the wake turbulence they produce. Because wake turbulence is generally related to the weight of an aircraft, these categories are based on weight—aircraft with a maximum certificated take-off weight of or more are classed as Heavy; those between and are classed as Medium; and those below 15,500 lb (7,000 kg) are classed as Light. Due to their weight, all current wide-body aircraft are categorised as Heavy. The lightest widebody aircraft ever built was the Airbus A300B1 with a maximum take-off weight of .

The wake-turbulence category also is used to guide the separation of aircraft—Heavy-category aircraft require greater separation behind them than do those in the Medium category, which in turn require more separation than aircraft in the Light category. In some countries, such as the United States, it is a requirement to suffix a heavy aircraft's call sign with the word "heavy" when communicating with ATC in the Terminal Radar Area. If the aircraft is operating on an Air Route Traffic Control Center (ARTCC) frequency, it is not required to append "heavy" at the end of its call sign. For example, flight UAL342 operated by United Airlines using a wide-body aircraft would use the call sign "United 342 Heavy" while operating in the United States.

Solely wide-bodied fleets

Very few airlines have been economically successful operating a fleet consisting only of dual-aisle, wide-body aircraft. Four notable airlines with this fleet type (Singapore Airlines, Emirates Airline, Cathay Pacific Airways, and Air Tahiti Nui) are based in small countries or territories. Passenger airlines that operate an all wide-body fleet, and which are operated and owned as a single company (that is, without regional or low-cost subsidiaries that also operate narrow-body aircraft), include:

References

External links

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