Twinjet

This article is about a type of aircraft. For the airline, seeTwin Jet.

Boeing 737Twinjet

TheVought F7U Cutlasswas one of the first modern twinjet fighters.

Atwinjetortwin-engine jetis ajet aircraftpowered by twoengines.A twinjet is able to fly well enough to land with a single working engine, making it safer than a single-engine aircraft in the event of failure of an engine.Fuel efficiencyof a twinjet is better than that of aircraft with more engines.These considerations have led to the widespread use of aircraft of all types with twin engines, includingairliners,fixed-wingmilitary aircraft,and others.

Aircraft configurations

There are three common configurations of twinjet aircraft. The first, common on large aircraft such as airliners, has apodded engineusually mounted beneath, or occasionally above or within, each wing. Most notable examples of such a configuration are theBoeing 737andAirbus A320.

The second has one engine mounted on each side of the rear fuselage, close to itsempennage,used by manybusiness jets,although some airliners like theFokker 70,Douglas DC-9andCOMAC ARJ21utilise such a design as well.

In the third configuration both engines are within the fuselage, side-by-side, used by mostfighterssince the 1960s. Later fighters using this configuration include theSu-27'Flanker', theF-15 Eagle,and theF-22 Raptor.

History

The first twinjet to fly was the German fighter prototypeHeinkel He 280,flying in April 1941 with a pair ofnacelledHeinkel HeS 8axial-flowturbojets.

The twinjet configuration was used for short-range narrow-bodied aircraft such as theMcDonnell Douglas DC-9andBoeing 737.TheAirbus A300was initially not successful when first produced as a short-range widebody, as airlines operating the A300 on short-haul routes had to reduce frequencies to try and fill the high-capacity aircraft, and lost passengers to airlines operating more frequent narrow-body flights. However, thanks to the introduction of ETOPS rules that allowed twin-engine jets to fly long-distance routes that were previously off-limits to them, Airbus was able to further develop the A300 as a medium- to long-range airliner to increased sales; Boeing launched its widebody twinjet, theBoeing 767,in response.

In the 1980s theBoeing 727was discontinued, as its central engine bay would require a prohibitively expensive redesign to accommodate quieter high-bypass turbofans, and it was soon supplanted by twinjets for the narrow-body market; Airbus with theA320,and Boeing with the757and updated "classic" variants of the737.During that decade onlyMcDonnell Douglascontinued development of the trijet design with an update to theDC-10,theMD-11,which initially had a range advantage over its closest medium wide-body competitors which were twinjets, the in-production Boeing 767 and Airbus A300/A310. In contrast to McDonnell Douglas sticking with their existing trijet configuration, Airbus (which never produced a trijet aircraft) and Boeing worked on new widebody twinjet designs that would become theAirbus A330andBoeing 777,respectively. The MD-11's long range advantage was brief as it was soon nullified by the Airbus A330-300 and the extended-range Boeing 767-300ER and Boeing 777-200ER.

TheAirbus A320twinjet stands out as the most produced jet airliner. TheBoeing 777Xis the world's largest twinjet, and the 777-200LR variant has the world's second longest aircraft range (behind Airbus A350-900 ULR). Other Boeing twinjets include the767,757(With the latter having stopped production, but still in commercial service) and787.Competitor Airbus produces theA320 family,theA330,and theA350.

Some modern commercial airplanes still use four engines (quad-jets) like theAirbus A380andBoeing 747-8,which are classified as very large aircraft (over 400 seats in mixed-class configurations). Four engines are still used on the largest cargo aircraft capable of transportingoutsize cargo,includingstrategic airlifters.

Efficiency

Twin-jets tend to be more fuel-efficient thantrijet(three engine) and quad-jet (four engine) aircraft. As fuel efficiency in airliners is a high priority, many airlines have been increasingly retiring trijet and quad-jet designs in favor of twinjets in the twenty-first century. The trijet designs were phased out first, in particular due to the more complicated design and maintenance issues of the middle engine mounted on the stabilizer. Early twinjets were not permitted byETOPSrestrictions to fly long-haul trans-oceanic routes, as it was thought that they were unsafe in the event of failure of one engine, soquad-jetswere used.Quad-jetsalso had higher carrying capacity than comparable earlier twinjets. However, later twinjets such as the Boeing 777, Boeing 787 and Airbus A350 have matched or surpassed older quad-jet designs such as the Boeing 747 and Airbus A340 in these aspects, and twinjets have been more successful in terms of sales thanquad-jets.

In 2012, Airbus studied a 470-seat twinjet competitor for theB747-8with lower operating costs expected between 2023 and 2030, revived after Boeing launched the777Xin November 2013, while then-CEOFabrice Brégierpreferred to focus on product improvement rather than all-new concepts for 10 years. It would have a10-abreasteconomy like the 777; its 565 m²(6,081 sq ft) wing, slightly more than the 747-8, would have an 80 m (262 ft) span, as wide as theA380,for a 892,900 lb (405 t)MTOWcompared to 775,000 lb (352 t) for the 777X, with a composite structure for an operating empty weight of 467,400 lb (212 t), and a 8,150 nmi (15,090 km) range at Mach 0.85.

ETOPS

When flying far fromdiversionary airports(so calledETOPS/LROPSflights), the aircraft must be able to reach an alternate on the remaining engine within a specified time in case of one engine failure. When aircraft are certified according to ETOPS standards, thrust is not an issue, as one of the engines is more than powerful enough to keep the aircraft aloft (see below). Mostly, ETOPS certification involves maintenance and design requirements ensuring that a failure of one engine cannot make the other one fail also. The engines and related systems need to be independent and (in essence) independently maintained. ETOPS/LROPS is often incorrectly thought to apply only to long overwater flights, but it applies to any flight more than a specified distance from an available diversion airport. Overwater flights near diversion airports need not be ETOPS/LROPS-compliant.

Introduction to transoceanic flights

Since the 1990s, airlines have increasingly turned from four-engine or three-engine airliners to twin-engine airliners to operatetransatlanticandtranspacific flightroutes. On a nonstop flight from America to Asia or Europe, the long-range aircraft usually follows agreat circleroute. Hence, in case of an engine failure in a twinjet (likeBoeing 777), the twin-jet could make emergency landings in fields inCanada,Alaska,easternRussia,Greenland,Iceland,or theBritish Isles.The Boeing 777 has also been approved by theFederal Aviation Administrationfor flights betweenNorth AmericaandHawaii,which is the world's longestregular airline route with no diversion airports along the way.

Other advantages

On large passenger jets, the cost of the engines makes up a significant proportion of the plane's final cost. Each engine also requires separate service, paperwork, and certificates. Having two larger engines as opposed to three or four smaller engines will typically significantly reduce both the purchase and maintenance costs of a plane.

Regulations governing the required thrust levels for transport aircraft are typically based upon the requirement that an aircraft be able tocontinue a takeoff if an engine failsafter thetakeoff decision speedis reached.Thus, with all engines operating,trijetsmust be able to produce at least 150% of the minimum thrust required to climb andquad-jets133%. Conversely, since a twinjet will lose half of its total thrust if an engine fails, they are required to produce 200% of the minimum thrust required to climb when both engines are operating. Because of this, twinjets typically have higherthrust-to-weight ratiosthan aircraft with more engines, and are thus able to accelerate and climb faster.

See also

• Wide-body aircraft

External links

• The dictionary definition oftwinjetat Wiktionary