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Comment by DuskStar | Hacker News Reader

The only thing that a 3rd+ engine helps with in this context is uncorrelated engine failures. (Correlated ones - like "oops we used the wrong fuel units" or "that volcanic ash cloud sure is pretty" - are the same for two engines as they are for 4)

These failures are really, really rare. A very conservative approximation would be one in 100,000 flights. (That would be once every other day in the US, for reference. Like I said, conservative) That means you'll have both engines fail randomly every 100,000,000,000 flights, which frankly no one cares about.

Carrying around an extra engine for the 1 in 100+ Billion situation it'll be useful isn't a good idea. It'll probably kill more people than it saves, for that matter - how many extra compressor disks will be thrown into cabins because of that?

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If you must allow for the failure of one engine, adding more engines reduces weight.

2 engines: You need to install and carry 200% of the engine power needed to fly the plane.

3 engines: You need to install and carry 150% of the engine power needed to fly the plane.

4 engines: You need to install and carry 133% of the engine power needed to fly the plane.

It is really quite interesting that twin-engine planes are cheaper to operate than planes with more engines. The engines in twin-engine planes should be larger and thus more expensive in every way. It seems that larger engines must be discounted somehow, either by the manufacturer or by the maintenance crews, or that there is a scaling factor that hugely effects efficiency of large passenger jet engines.

For example, are maintenance costs dominated by paperwork? That could be enough to overcome the extra expense of needing powerful equipment to lift and otherwise manipulate the parts of a huge engine.

Flight with inoperative engine(s) is altitude constrained. The aircraft is going to be able to fly with one engine operative, but not at its usual altitude, so the 200/150/133 idea doesn’t really hold.

Further, larger engines are more efficient than smaller ones. You pretty much want to install the largest engines you can fit. It’s the underlying reason for the 737 Max debacle.

Engines are sized for failure at take-off. At that time they are at maximum power, the aircraft is heavy with fuel, and the ground is mighty close.

I see no evidence that larger engines are more efficient than smaller ones. Larger total frontal area (counting all engines together) is more efficient, as is a higher bypass ratio. Both of those tend to lead toward larger engines, but you could leave those unchanged while modifying the number of engines.

For example, switching from 2 engines to 8 engines, you could maintain the same frontal area by changing the diameter from 10 feet to 5 feet. The bypass ratio, which is the portion of air that bypasses the combustion, could be 12-to-1 or 10-to-1 or whatever, and need not change with engine diameter.

Except this ignores that the motivation was not increased safety but money. It does seem true that twin-engine planes can be quite safe but if Boeing/Airbus were motivated to create the safest possible plane, it would probably have four engines.

I doubt the US President will be flying on a twin-engine plane any time soon. They'll choose the safest (American made) option. Probably an updated 747 model, no?

And my point was that motivation is irrelevant, and I would find it quite plausible that a two engined airliner would be safer than a four engined jet. Two fewer things to explode, after all.

The president flies a 747 for size and prestige more than safety. Safest option would be to recommission an Iowa and use it as an armed ocean liner or something and skip planes entirely. Or just to stay home.

If we're talking about a command center, "stay at home" is not safe at all. Even Google Maps knows the coordinates and elevation for 2600 Pennsylvania Avenue (or Cheyenne Mountain, or Area 51, or whatever). Hence, Air Force One.

Pedant note: 1600 Pensylvania Avenue. 2600 Pennsylvania Avenue appears to be an apartment building, or maybe Hacker Haven.

Oh. Right. Silly me.

Presidents often fly on a 757 when the destination airport cannot accommodate the larger 747 [1].

They also fly on Marine One (helicopter) which is probably less safe overall than any modern airliner.

1. https://en.wikipedia.org/wiki/Boeing_C-32

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