Many of the first purpose-built jet airliners had four engines, among which stands the De Havilland Comet, the world's first commercial jetliner.
[4] The supersonic airliner Concorde had its engines mounted in rectangular pods conformal to the underside of the wing, without any pylons.
This design reduces both drag and the risk of ingesting foreign objects, but increases the difficulty of maintenance and complicates the wing structure.
The aircraft was initially designed as trijet, but the Trident 3B added a fourth engine as additional power was required by the stretched fuselage, increased wing chord and raised gross weight.
[15] This allows them to satisfy demand on extremely busy routes, and when filled with passengers to distribute the cost, they can be very profitable.
[16] With jet fuel a large part of total costs, this makes quadjets less attractive to airlines and many are shifting their attention towards more efficient aircraft types.
[19] Conversely, the point-to-point model transports passengers directly from origin to destination, spreading them out across different routes and requiring fewer seats on the servicing aircraft.
appearance of low-cost carriers which operate many point-to-point flights, it is more difficult to fill the seats of the largest airliners.
On the other hand, the large four-engined types were less suited for frequent short-haul services, which demanded multiple take-offs and landings daily, costing more fuel while also typically carrying fewer passengers per flight.
The FAA's 60-Minute Rule also prevented them from flying farther than 60 minutes away from diversion airports due to their lower engine redundancy.
[29] Its capacity and performance were unmatched, even after the launch of wide-body trijet competition in the form of the McDonnell Douglas DC-10 and Lockheed L-1011 TriStar.
However, in large part due to high operating costs and noise issues, Concorde never achieved the predicted level of success.
This prompted the introduction of ETOPS ratings for twinjets, allowing them to circumvent the 60-Minute Rule and fly on transoceanic routes previously serviced by four-engined types.
All but the largest four-engined types, such as the Boeing 747, became uneconomical and this led to the retirement of the ageing 707 and DC-8 fleets for passenger service.
Nonetheless, Boeing rolled out the 747-400 in 1989, which enjoyed high capacities (over 300 passengers) and long range, a combination still unmatched by twinjets at the time, making this the most commercially successful 747 variant.
[38] The subsequent development of the 777-300ER pushed the passenger capacity to just under 400, approaching the 747 and superseding the A340, while being more efficient and incurring lower engine maintenance costs.
In the years following the September 11 attacks, the increase in fuel prices and decline in the aviation industry heightened the need to minimise operating costs and expenditures.
Production of the 747-400 passenger variant ceased by 2005 and deliveries of the A340 dropped to 11 per year, as they faced competition from more efficient and comparably capable twinjets.
[42] This can be attributed to a modern trend towards point-to-point travel using smaller but highly efficient twinjets such as the Airbus A350 and Boeing 787, as opposed to a spoke-hub model which favours massive aircraft such as the A380.
[12] The largest operator of the A380, Emirates, profits from its fleet because its primary hub is situated at Dubai International Airport, where many long-haul routes have their stopovers.