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Comment by mattbrewsbytes | Hacker News Reader

That is entirely where I'm coming from - plane designs from the past had longer wingspans and supported 4 engines. Engines have gotten more efficient and powerful so cross-ocean routes don't need 4 engines anymore.

So I was wondering what if current plane designs had 4 smaller but equally as efficient engines instead of 2. The vast majority of airplane accidents happen at takeoff or landings, if some of those can be avoided by having 4 engines for commercial aircraft, its a worthwhile idea to explore.

Also I would venture that 99% of comments on public forums like this are from people without expertise. My expertise in this space is a curiosity about planes for a few decades, taking some actual flying lessons, and being generally interested in aviation to go to airshows, watch youtube content from pilots, etc. I probably have about the same aviation knowledge as an average HN person.

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> was wondering what if current plane designs had 4 smaller but equally as efficient engines instead of 2

Impossible to answer until we know the cause. If it was independent powerplant failures, then yes. If it was e.g. fuel contamination, pilots improperly shutting down the engine, some other crap failing, then no.

Speaking as someone with aerospace engineering and GA pilot experience.

>if this was a bird strike incident on both engines that maybe having 4 engines [...]

It's pretty clear what the guy is inquiring about.

Speaking as someone without aerospace engineering or GA pilot experience. My only relevant experience here is being able to read.

> My only relevant experience here is being able to read

Double bird strikes are “independent powerplant failures.”

Current speculation around the raising of flaps suggests that independent engine failures weren't the cause. Proper investigation with access to better data than grainy video will tell us if that is the case and why.

"4 smaller but equally efficient engines" feels like a unicorn though (we'll probably get to the point in future where four large engines are superior in efficiency to two of today's engines, but two large engines to that latest design will still be more efficient than four smaller ones...)

> "4 smaller but equally efficient engines" feels like a unicorn

With turbines, yes, for fundamental reasons. With electric motors, on the other hand, perhaps not, though not particularly relevant to a long-haul route like AMD-LON.

True. Also the electric/turboprop propulsion compromises like Heart Aerospace's that trade off the limitations of the respective powerplants by fitting pairs of both. But that's a different use case...

(I get to write about arrayed space thrusters in the day job too, but again, fundamentally different physics and goals...)

It's not only that engines have gotten more efficient and powerful, its that they have also increased the reliability. The other issue is that 4 engines share almost all of the failure modes that 2 engines do. If you have 2 engines that fail that are on opposite sides of the aircraft, having 2 others isn't going to help as its likely a system failure in the aircraft or a fault in that particular model of the engine that could affect all of them. For example, if you run of fuel or have a failure in the fuel delivery system, it's not going to matter if you have 2, 4 or more engines. The mistake is thinking the probability of a 4 engine failure is significantly less than 2 engine failure for all types of failures.

>4 smaller but equally as efficient engines

Larger jet engines are more fuel efficient than smaller ones, because larger diameters allow for more bypass air and therefore more fuel efficiency [1]. It is a function of size and a lot of the engineering goes into materials and designs to be able to increase size and maintain strength. So you simply can't make 4 engines that are as efficient as 2 large ones, and that is compounded by the significant additional weight (and drag) of the duplicated engine parts and mounting structures.

>if some of those can be avoided by having 4 engines for commercial aircraft, its a worthwhile idea to explore

Everything comes with tradeoffs. Adding more engines mean more complexity, more maintenance, more chance of single engine failures, etc. You don't want to introduce more failure modes than what you are trying to fix. The move to two engines for large aircraft and the evolution of ETOPS (Extended Twin-engine Operational Performance Standards) involved a lot of people considering a lot of scenarios. I can guarantee the "why not 4 engines" question has been studied extensively.

[1] https://simpleflying.com/why-do-jet-engines-keep-getting-lar...

>taking some actual flying lessons

Unironically, you're probably way more qualified than most of the people here throwing rubbish at you.

A classic on places like this.

I am happy to acknowledge I had a completely wrong knee-jerk reaction to your phrasing, and incorrectly assumed your "One has to wonder..." was meant as a suggestion of a solution, instead of instance of thinking out loud!

Apologies!

> So I was wondering what if current plane designs had 4 smaller but equally as efficient engines instead of 2.

Smaller turbofan engines are less efficient than larger ones. This is because they have a lower bypass ratio - thrust generated by turning the big fan over the thrust generated by combustion.

So expect even bigger engines.

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