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Comment by sajomojo | Hacker News Reader

It just seems like a very bad idea that twin-engine planes can fly so far from an air strip. If the #1 engine had failed as well it would have been game over. There's no possibility of gliding a 787 for 2+ hours and hundreds of miles.

If the FAA wanted to prioritize safety, it would still require the use of four-engined planes, as it used to for most long ocean routes. The desire to fly twin-engine planes over the ocean is entirely driven by a desire to maximize airline profits.

Admittedly, it does appear that modern twin-engine planes are reliable enough that it's not entirely crazy. But it would only take a couple of fatal engine failures to make it seem really dumb in retrospect.

People are weirdly dogmatic around this ETOPs rule change by the FAA. There is almost no one questioning it and yet the entire motivation is purely monetary. The FAA and airlines continue to push the limits further and further.

https://en.wikipedia.org/wiki/ETOPS

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ETOPS seems to have done pretty well. As far as I know, no ETOPS-certified twinjets have been lost due to both engines failing. There are a couple cases of the jet running out of fuel (the famous Gimli glider and Air Transat Flight 236), but that is going to be bad no matter how many engines you have. There are cases of airplanes flying into volcanic ash clouds and losing all engines; but more engines doesn't help you if the air around the plane kills engines. There are some poorly-engineered planes with 3 engines that have had a single-engine failure bring down the entire plane (United Airlines Flight 232).

ETOPS is not just some rubber stamp for a particular model of airplane that manufacturers self-certify (hello, 737 MAX debacle), but rather a process applied to (airframe, engine, airline). You have to service particular engines to particular standards to be allowed to fly ETOPS, and it depends on the airline. I could go to the store, buy a 777 and hire 4 ATPs and fly a 777... but I wouldn't be allowed to fly it ETOPS, for example.

All in all, the program seems massively successful to me. It saves fuel, which means cheaper flights and less CO2 being generated. That's a good thing. The main downside seems to be medical emergencies; I remember some stories about the aircraft that flew EWR-SIN... it has some special corpse compartments to stash away the inevitable deaths that occurred 9 hours away from an airport. (It had 4 engines, but engines don't perform heart surgery on passengers.)

The reality is that engines are pretty reliable. 2 is the right number.

Thanks for the thoughtful replies, all. I still don't buy the idea that this change was made with public safety as the top priority. I do get that it might be a reasonable trade off of safety/profits. I really do hope the ETOPS rule changes turn out to be a good choice over the long term.

ETOPS is not new. The 767-200ER entered service in 1985. We live in the future you are alluding to. Your concerns have not been validated by reality.

The safest way to avoid aviation incidents is to not have planes at all. From there, there is a continuum of how much risk you want to take on.

If we add many safety bells and whistles to planes, it would mean nobody could afford flying. If people couldn't afford it, they'd use other transportation methods. Those methods might be more dangerous than flying. So it wouldn't actually increase the safety of travellers. Oceanliners and cars are far from 100% safe, for example.

ETOPS-like rules have been in use since late 70's. It is an evolving standard with different craft types being approved for different ETOPS. While some current frames are rated ETOPS 330 or 370, this type of operation began officially with planes allowed to do only 90-minute ETOPS.

If flying commercial over oceans, it is either ETOPS, A380, or a current type of 747 (747-8).

Volcanic ash is a completely different beast from normal ops: you could have four engines and it still wouldn't be enough, prompting what was perhaps the most British announcement in the history of aviation, as narrated here: https://everything2.com/title/The+most+engine+failures+in+on...

Contrary to your assertion the two engined craft are just as safe (or safer!) as the four engined craft because of changes in procedures. Fuel management, weather forecasting, general engine reliability, lubricant reserves are all different and much more stringent for a twin engined jet over water on a straightline route. The effect is the aircraft is less long in the air anyway which has a big effect on the risk calculation.

The main motivation is money, that's true, but the secondary motivation, the environment is also important and the math really does work out to a negligible safety difference (advantage twins) that to date has not led to the observation of numbers that would indicate a deviation from the predicted reliability figures.

Finally, the chances of something going wrong on a twin engined aircraft vs a quad are higher for the quad because it is a much more complex system. So yes, if an engine fails a four engined plane would be able to deal with it better but the chances of something going wrong are twice as high.

In the 60's, when the 747 was designed the reliability of jet engines was such that four engines was more or less a must but we are now a good 50 years away from those days and our ability to deal with complex reliability issues has gone up substantially (recent Boeing issues notwithstanding). The A380 has four engines because it is simply too heavy to take off with only two engines.

Engine shutdown is so rare now that pilots can go a whole career without encountering it even once and typically it is the monitoring system that has a fault rather than the engine itself but you would shut down the engine anyway in those situations as a precaution.

Doesn't the fact that the twin-engine planes have to submit to more stringent ETOPS requirements mean the FAA believes they're less reliable than four engine planes?

2 engines of the type that they put on 4 engine planes are less reliable than 2 ETOPS-certified engines. ETOPS-certified engines (actually engine, airframe, airline combination) are going to be more expensive than engines where you have three extras next to them.

I could build a jet engine in my garage and the FAA wouldn't let me fly 270 minutes away from an airport with paying passengers on it just because I made four of them. Certification is always going to be important.

That’s not a reasonable conclusion. The 787-8 and 747-8 are both ETOPS-330 certified.

I'm genuinely baffled at how you're going back and forth between authoritatively talking about how ETOPS is definitely scary, unsafe and new, but you also clearly know nothing about it.

Why not research a thing BEFORE deciding you have a strong opinion about it?

> If the FAA wanted to prioritize safety, it would still require the use of four-engined planes, as it used to for most long ocean routes. The desire to fly twin-engine planes over the ocean is entirely driven by a desire to maximize airline profits.

That intuitively makes sense, but it's incorrect. The FAA certified twin engines over water because it's safer. Half the number of engines mean half the failures, and half the risk of cascaded failures from the failing engines.

Statistics bear this out. Twin engines are safer.

Can you point to the statistics? I'm sure modern twin-engine planes are more reliable than very old four engine planes but what about modern vs modern?

No. I was simply told this by the other engineers at Boeing.

The ICAO has accident statistics available here: https://www.icao.int/safety/iStars/Pages/Accident-Statistics...

You'll probably have to do some processing, but the answer should be in there!

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?

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

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.

You're right, and that's why 787s with Rolls Royce engines had their ETOPS certification reduced from 330 minutes to 140 or 180 depending on which version of the engine the frame has.

I dunno, man; I think it is deeply weird how we put so much effort in to airline safety, but let anyone drive essentially anything on the road, without even requiring more than homeopathic liability insurance, and only the most cursory of skill tests. (In California, it's totally legal to drive with $35K in liability insurance; and not all of that needs to cover personal injury!)

I personally think that any additional safety effort ought to be put into making cars safer, and not just safer for the occupants, but safer for the poor SOBs who they run into. I mean, sure, I'm not saying we should decrease airplane regulation (except maybe in the security theatre bits) but I think there's a lot more to be gained, safety wise, by regulating cars more than by regulating airplanes more.

If we are concerned about loss of life resulting from air travel, the main issue is co2 emissions. We should drastically refuce especially long distance air travel.

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