|Quoting flyingturtle (Reply 4):|
For what reason are 4-engined A/C not equipped with larger-diameter #2 and #3 engines? Ground clearance would be the same. So the reason must be commonality, so that you can save the trouble of training mechanics for another engine type, and having to store additional replacement parts?
1. It costs billions to develop each engine size. So no one is going to develop multiple engines for one airlines. They will spend the money instead on a more competitive engine. We're talking an added cost of about $500 a takeoff here or $1.25 (US dollars) per flight per passenger.
2. As you noted commonality. Every size has different parts. The training isn't an issue, it is all the certification of all the different part sizes. This is mostly in that cost in point #1.
3. You do not gain much. The engines are optimized for the best cruise thrust versus nacelle drag for the mission. Savings of about $0.03 in fuel per flight.
4. Engine out: The airframe must work with any one engine failing. Having different engine sizes complicates rudder certification. I am unable to estimate the cost of this quickly.
5. It takes a large number of one engine out there to debug an engine. The CF-6 on the A330 has different issues than the CF6 on the 767. This means added fuel burn of about $0.02 later in the engine life due to fewer PIPs.
6. Engine rotation. In general, the oldest engine on the airframe is placed on an inside position as that is the safest place for an engine out and older engines are the most likely to have an issue. So if the inner and outer engines are of different sizes, that messes up maintenance schemes and would add a cost of about 50 US cents (half a dollar) to every passenger for every flight if that flexibility were to be removed. Why pay that for a negligible fuel burn savings? The added maintenance costs would outweigh any propulsion advantage.
So net, the added cost per flight is about $1.74 per passenger. So cost/benefit says keep to one engine type.
Heck, its common for different airframes to share engines (or their cores) due to high development costs. Since widebody engines sell so few engines, we saw the same engines on the 767, DC-10/MD-11, 744, A300, and if one stretches commonality L1011. It is likely any future quad will be lucky to even gain a custom fan diameter. Quads are tough business cases to start with.
Besides, the next quad is likely to be a BWB. With the changed engine packaging, it will necessitate a quad have four identical engines and if built as a 3-holer, the engines must be of same size for the engine out condition (rudders on a BWB must be drag rudders and that constrains the design).