Jet aircraft powered by two engines
This article is about a type of aircraft. For the airline, see
Twin Jet
.
A
twinjet
or
twin-engine jet
is a
jet aircraft
powered by two
engines
. 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.
[1]
Fuel efficiency
of a twinjet is better than that of aircraft with more engines.
[2]
These considerations have led to the widespread use of aircraft of all types with twin engines, including
airliners
, fixed-wing
military aircraft
, and others.
Aircraft configurations
[
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]
There are three common configurations of twinjet aircraft. The first, common on large aircraft such as airliners, has a
podded engine
usually mounted beneath, or occasionally above or within, each wing.
The second has one engine mounted on each side of the rear fuselage, close to its
empennage
, used by many
business jets
.
In the third configuration both engines are within the fuselage, side-by-side, used by most
fighters
since the 1960s. Later fighters using this configuration include the
Su-27
'Flanker', the
F-15 Eagle
, and the
F-22 Raptor
.
History
[
edit
]
The first twinjet to fly was the German fighter prototype
Heinkel He 280
, flying in April 1941 with a pair of
nacelled
Heinkel HeS 8
axial-flow
turbojets.
The twinjet configuration was used for short-range narrow-bodied aircraft such as the
McDonnell Douglas DC-9
and
Boeing 737
. The
Airbus A300
was 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, the
Boeing 767
, in response.
In the 1980s the
Boeing 727
was 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 the
A320
, and Boeing with the
757
and updated "classic" variants of the
737
. During that decade only
McDonnell Douglas
continued development of the trijet design with an update to the
DC-10
, the
MD-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 the
Airbus A330
and
Boeing 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.
The
Airbus A320
twinjet stands out as the most produced jet airliner. The
Boeing 777X
is 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 the
767
,
757
(With the latter having stopped production, but still in commercial service) and
787
. Competitor Airbus produces the
A320 family
, the
A330
, and the
A350
.
Some modern commercial airplanes still use four engines (
quad-jets
) like the
Airbus A380
and
Boeing 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 transporting
outsize cargo
, including
strategic airlifters
.
Efficiency
[
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]
Twin-jets tend to be more fuel-efficient than
trijet
(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 by
ETOPS
restrictions to fly long-haul trans-oceanic routes, as it was thought that they were unsafe in the event of failure of one engine, so
quad-jets
were used.
Quad-jets
also 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 than
quad-jets
.
In 2012, Airbus studied a 470-seat twinjet competitor for the
B747-8
with lower operating costs expected between 2023 and 2030, revived after Boeing launched the
777X
in November 2013, while then-CEO
Fabrice Bregier
preferred to focus on product improvement rather than all-new concepts for 10 years.
It would have a
10-abreast
economy like the 777; its 565 m
2
(6,081 sq ft) wing, slightly more than the 747-8, would have an 80 m (262 ft) span, as wide as the
A380
, for a 892,900 lb (405 t)
MTOW
compared 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.
[3]
ETOPS
[
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]
When flying far from
diversionary airports
(so called
ETOPS/LROPS
flights), 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
[
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]
Since the 1990s, airlines have increasingly turned from four-engine or three-engine airliners to twin-engine airliners to operate
transatlantic
and
transpacific flight
routes. On a nonstop flight from America to Asia or Europe, the long-range aircraft usually follows a
great circle
route. Hence, in case of an engine failure in a twinjet (like
Boeing 777
), the twin-jet could make emergency landings in fields in
Canada
,
Alaska
, eastern
Russia
,
Greenland
,
Iceland
, or the
British Isles
. The Boeing 777 has also been approved by the
Federal Aviation Administration
for flights between
North America
and
Hawaii
, which is the world's longest
[
citation needed
]
regular airline route with no diversion airports along the way.
Other advantages
[
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]
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 to
continue a takeoff if an engine fails
after the
takeoff decision speed
is reached.
[4]
Thus, with all engines operating,
trijets
must be able to produce at least 150% of the minimum thrust required to climb and
quad-jets
133%. 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 higher
thrust-to-weight ratios
than aircraft with more engines, and are thus able to accelerate and climb faster.
See also
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]
References
[
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]
External links
[
edit
]
- The dictionary definition of
twinjet
at Wiktionary