The real concern is trans-oceanic long-haul. Supposedly modern engine failure rates have improved to the point that you're less likely to have problematic engine failures than a four-engine 70s-era 747.
I recently rode in a 2-person jet. The pilot said the only reason we might bail out is an engine fire. If the engine fails, the plane's an excellent glider, and we'll glide to an airport/airstrip. Now, this was somewhere with airports all over, not an ocean.
https://en.wikipedia.org/wiki/ETOPS is a good read.
Good to know that the squishy contents of the aluminum can are the least reliable part of the system.
I guess that also limits the size for planes, but probably to 10,000+, so it's not something we're likely to hit.
ETOPS is the general set of rules for how far a plane with two engines can be from an airport. https://en.m.wikipedia.org/wiki/ETOPS
Four engines just means there's 4 things that can catastrophically fail and potentially damage the plane rather than 2.
Say that you need 50% of the engines on the plane to be able to fly safely, then having more engines is actually safer. For example, supposing that engine failures occur 1% of the time, with four engines there are 3.97 three and four engine failures per million flights. For a two engine plane, there are 100 two engine failures per million flights.
There's probably less drag from two engines than four as well.
That's what I meant by "collateral damage".
Doubling the number of engines doubles that risk.
On the other hand, a failure that would take out both engines on a twin jet is likely to take out 4 engines. Failures such as running out of fuel, ingesting a flock of geese, and ingesting hail.
The rational justification is not merely technical but rather empirical, proven by real-world data. Certifications usually because companies can demonstrate e.g. "N years of trouble-free XXX-minutes ETOPS experience". In other words, we've been doing it for this long and it works so there's statistically ever-less reasons to believe the certification is flawed.
So, despite all the stats you may collect about birds, and physical + ethological models to estimate how much chance there is it will happen twice to a twin engine plane on both reactors before it can land, it would only prove reality: it just doesn't happen. Or not in ways that more engines would solve. (Birds are a bigger problem for cockpit windows, though.)
Now, another additional bit of safety: all jetliner planes have a "fixed-wing" design, allowing "deadstick landing"². From wiki:
> All fixed-wing aircraft have some capability to glide with no engine power; that is, they do not sink straight down like a stone, but rather continue to glide moving horizontally while descending. For example, with a glide ratio of 15:1, a Boeing 747-200 can glide for 150 kilometres (93 mi) from a cruising altitude of 10,000 metres (33,000 ft).
More recent planes are usually ever better (the OG 747 is from 1969!) At the very least it allows the pilot to 'land' on sea and wait for evacuation in zodiacs. So even in the extremely unprobable event that ETOPS failed (the statistical outlier where a plane loses both engines), you'd probably just glide to the ground 100-200km away at most to reach some landing spot.
There are reasons why air travel is so safe, the safest of all, and most of those are empirical, real, tested then validated. That's why it was such a shame what Boeing did to the 737 MAX; and I hope the 777X program yields the safest planes we've ever seen from them, that they actually innovate in that respect.
Safe travels!
While I agree with the rest, landing on sea is a terrible idea. That can work on rivers where you have no waves. But gliding onto an ocean with waves will likely be fatal. Luckily, there is no recent case of a double engine failure where landing on sea was necessary (apart from maybe MH370 but we don't know that).
The main argument was really about not dying from engine failure (i.e. from falling) since planes glide — knowing this totally changed my impression of "safety" wrt riding planes, for the better: “planes can't fall!”.