
787-8 and 787-9 ETOPS Lowered (RR Engine Flaws) - hodgesrm
https://www.pprune.org/tech-log/607899-787-8-787-9-etops-cancelled-rr-engine-flaws.html
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ocdtrekkie
Out of curiosity, is the benefits (I assume cost/efficiency) of having only
two engines significant? Or with the significant conversation about engine
failure of late, it make sense to lean back towards the four engine design of
a 747, which presumably, doesn't tax the remaining engines as hard during a
failure?

(ETOPS, apparently, stands for "Extended-range Twin-engine Operational
Performance Standards", and refers to how far you allow a plane with only two
engines to fly from an airport it can land at, in case one fails and a single
engine has to do all of the work.)

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hodgesrm
2 engine aircraft are cheaper to operate per seat due among other things to
greater fuel efficiency which can be up to 50% over older 4-engine models. [0]
The current four-engine models are at the bottom of the heap these days; the
787 is at the top.

Also smaller cabins are easier to fill which means you can fly point-to-point
from regional airports like Oakland CA (OAK) and still operate at max
capacity.

[0]
[https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft](https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft)

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AnimalMuppet
> 2 engine aircraft are cheaper to operate per seat due among other things to
> greater fuel efficiency which can be up to 50% over older 4-engine models.

Why is this true? Is it because of the "older" part - because we've gotten
better and designing fuel efficiency? Is it because the engines also increase
drag? Is it because the four-engine planes typically don't have to run the
engines as close to full throttle, and the engines are more efficient if run
at higher power? Or something else?

~~~
hodgesrm
I'm not an aeronautical engineer but my understand is that a lot of the
increase in efficiency is due to introduction of turbofan engines with higher
bypass ratios. These economize on fuel since the fuel increasingly runs the
turbofan to compress air that enters the bypass duct rather than generating
thrust directly. Thrust instead comes from decompression of the air as it
flows out behind the engine.

B787, B777, AB380, and AB350 all have GE or RR turbofans with 8:1 or greater
bypass according to numbers in Wikipedia. [0]

This is not the only design improvement but it's a big one.

[0]
[https://en.wikipedia.org/wiki/Bypass_ratio](https://en.wikipedia.org/wiki/Bypass_ratio)

~~~
ocdtrekkie
Wouldn't these design improvements equally apply to a new hypothetical four
engine plane?

~~~
hodgesrm
That's a good question. Other big factors in operating expense are load factor
(i.e., percent of seats filled) and weight. The A380 is in trouble because it
measures poorly on the preceding and in addition can only fly out of hubs with
specially modified gates. The 747-8 seems to be largely a freighter with
production down to about 1 a month.

At this point it seems unlikely we'll see another big aircraft design program
any time soon unless a country like China decides to do one for status
reasons.

