Aircraft engines are generally turbojets where the air is sent from the air intake, in the first compression phase, to a compressor (made up of various stages), in the second compression phase. From here the air is sent to the combustion chamber, where it is mixed with the atomized fuel by the injectors and ignited by a "spark plug". 
Continuous combustion causes a significant increase in air temperature which, as it cannot expand, is sent to the turbine where its energy is extracted. Turbojets respond, from a thermodynamic point of view, to the Brayton Cycle so, as thermal machines, they become increasingly efficient the higher their compression ratio and the maximum temperature of the cycle.
Gas turbines reach a rotation speed of around 13,800 rpm: this is definitely the part of the turbojet that uses the most advanced technologies. Its blades are subjected to very high thermal and mechanical stresses and, because of the rotation, the ends can reach a peripheral speed of 400 m/s and be hit by incandescent gases at temperatures above 1,300°C and at speeds of around 600 m/s.


As you might guess, engines built in the 1970s and 80s did not have the technology to withstand this type of stress due to a lack of suitable alloys and materials. Therefore, since the thrusts produced were quite low, more engines were needed to achieve the total thrust necessary. It should be emphasized that today, in the engines used in the Alitalia fleet, for example, technological advances have allowed us to have engines with not only much higher thrust than in the past, but also with a much higher level of reliability in terms of safety and operation. However, the thrust required is not the only parameter to take into consideration when choosing engines (type and number). Even today we have modern aircraft fitted with four engines, like the A380. In this case, the choice was due only to the technology available and the size of the engines in relation to the aircraft architecture. Engine size is also limited by the total aerodynamics of the aircraft, the height of the wings above the ground and other engineering criteria that go beyond engines. I'll give another example: in the case of aircraft of the same "size" (understood as load capacity), like the smallest Airbus 340 (with four engines) and the larger version of the Airbus 330 (with two engines), both aircraft have similar thrust overall but it is distributed differently: two large engines for the A330 and four "small" engines for the A340 (the same engines as used in the A320, an aircraft used for medium-haul flights). This shows clearly that the choice made for the A340 was not linked to issues with the engines but the general aerodynamic architecture of the aircraft, which meant that small engines were preferred over the other solutions available. In the event of two engines on the A340 failing, the residual thrust is the same as in the event of one A330 engine failing. As you will have understood, the number of engines has no impact on residual thrust. To conclude, I would like to emphasize that engine failure, even if it is very rare, is not a surprise for our pilots as the training programs used by the Company include a specific part dedicated to this in all simulator sessions.

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