4 ms·

Doesn't that make a plane less engine-fault tolerant? Completely noob here, but I would feel better if I flew in a 4-engine plane than 2-engine. In a 4-engine p

Doesn't that make a plane less engine-fault tolerant? Completely noob here, but I would feel better if I flew in a 4-engine plane than 2-engine. In a 4-engine plane, even if 2 engines faults (like, bird strike), the other 2 will still make the plain airbone.

foryourpleasure 7y ago
Robust testing should make the dual engine failure scenario quite unlikely. That’s why the miracle on the Hudson was so remarkable. 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 https://en.m.wikipedia.org/wiki/ETOPS
marcinzm 7y ago
A bird strike (keep in mind birds fly in flocks) I'm guessing would either take out 1 engine (which is fine on a new 2 engine plane even during takeoff) so no benefit or take out all engines (or all engines on one side which again means just 1 engine on a 2 engine plane). A flock big enough to take out both engines on a 2 engine would be big enough I presume to take out all 4 on a 4 engine. Four engines just means there's 4 things that can catastrophically fail and potentially damage the plane rather than 2.
dehrmann 7y ago
You don't necessarily lose engines instantly to bird strikes, they're most likely to happen at takeoff and landing, and it hasn't been an issue for the massive fleet of twin-engine short-haul jets. 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.
- dx034 7y ago
  Which is shown by statistics. There are thousands of daily flights over Atlantic & Pacific. Engine failures over the ocean are incredibly rare (I couldn't find any recent ones). Most reasons why airliners choose to divert at this stage are medical issues nowadays. And for the plane to crash not one but both engines would have to fail.
  - dehrmann 7y ago
    > for the plane to crash not one but both engines would have to fail 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 https://en.wikipedia.org/wiki/ETOPS is a good read.
  - dehrmann 7y ago
    > medical issues 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.
WalterBright 7y ago
Statistically, 2 engines are safer than 4. 4 engines will have twice the failure rate, and when engines fail the risk is collateral damage.
- jfim 7y ago
  That really depends on the minimal number of engines needed for safe operation. 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.
  - apta 7y ago
    There's correlation involved with other factors however. If I'm not mistaken, things like maintenance are relatively easier (and cheaper) on a 2 engine aircraft compared to a 4 engine aircraft. So we cannot consider failure rates as an abstract measure.
    - jfim 7y ago
      I agree, and it's also very unlikely that engine failures are independent, since they're the same model, coming from the same manufacturer, and the same maintenance procedures. There's probably less drag from two engines than four as well.
      - WalterBright 7y ago
        Even if two engines have the same defect, it's highly unlikely they'd both fail on the same flight.
  - WalterBright 7y ago
    Engine failure can be more than just quitting. It can catch fire and burn the wing, it can throw turbine blades around, it can shake itself violently. These can cause other damage to the airplane. It can even tear itself off, surge forward, fall back and hit the wing. These things have all happened. I remember a DC-10 that crashed losing everyone in it when one engine came off and took out the flight controls with it. Then there's the Sioux City crash where the compressor disintegrated and took out all three hydraulic systems. 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.
ryder9 7y ago
A very good video on this subject matter of 4 Engines vs 2 Engines https://www.youtube.com/watch?v=HSxSgbNQi-g https://www.youtube.com/watch?v=HSxSgbNQi-g
K0SM0S 7y ago
The associated concept in regulation is a certification (by plane model, e.g. for all Airbus A321) called ETOPS¹ (Extended Operations). The cert applies to twin engines and basically tells you that a plane can fly up to XXX minutes on 1 functional engine only (e.g. ETOPS = 60, then you have ETOPS 120, ETOPS 180 and currently ETOPS 240 is permitted on a case-by-base basis). Note that "ETOPS-180 and ETOPS-207 cover about 95% of the Earth" (how much you can reach while staying 180-207 minutes from an airport at normal altitude/speed). 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! [1]: https://en.wikipedia.org/wiki/ETOPS https://en.wikipedia.org/wiki/ETOPS [2]: https://en.wikipedia.org/wiki/Deadstick_landing https://en.wikipedia.org/wiki/Deadstick_landing
- dx034 7y ago
  > 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. 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).

foryourpleasure 7y ago

Robust testing should make the dual engine failure scenario quite unlikely. That’s why the miracle on the Hudson was so remarkable. 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 https://en.m.wikipedia.org/wiki/ETOPS

marcinzm 7y ago

A bird strike (keep in mind birds fly in flocks) I'm guessing would either take out 1 engine (which is fine on a new 2 engine plane even during takeoff) so no benefit or take out all engines (or all engines on one side which again means just 1 engine on a 2 engine plane). A flock big enough to take out both engines on a 2 engine would be big enough I presume to take out all 4 on a 4 engine. Four engines just means there's 4 things that can catastrophically fail and potentially damage the plane rather than 2.

dehrmann 7y ago

You don't necessarily lose engines instantly to bird strikes, they're most likely to happen at takeoff and landing, and it hasn't been an issue for the massive fleet of twin-engine short-haul jets. 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.

dx034 7y ago
Which is shown by statistics. There are thousands of daily flights over Atlantic & Pacific. Engine failures over the ocean are incredibly rare (I couldn't find any recent ones). Most reasons why airliners choose to divert at this stage are medical issues nowadays. And for the plane to crash not one but both engines would have to fail.
- dehrmann 7y ago
  > for the plane to crash not one but both engines would have to fail 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 https://en.wikipedia.org/wiki/ETOPS is a good read.
- dehrmann 7y ago
  > medical issues 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.

WalterBright 7y ago

Statistically, 2 engines are safer than 4. 4 engines will have twice the failure rate, and when engines fail the risk is collateral damage.

jfim 7y ago
That really depends on the minimal number of engines needed for safe operation. 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.
- apta 7y ago
  There's correlation involved with other factors however. If I'm not mistaken, things like maintenance are relatively easier (and cheaper) on a 2 engine aircraft compared to a 4 engine aircraft. So we cannot consider failure rates as an abstract measure.
  - jfim 7y ago
    I agree, and it's also very unlikely that engine failures are independent, since they're the same model, coming from the same manufacturer, and the same maintenance procedures. There's probably less drag from two engines than four as well.
    - WalterBright 7y ago
      Even if two engines have the same defect, it's highly unlikely they'd both fail on the same flight.
- WalterBright 7y ago
  Engine failure can be more than just quitting. It can catch fire and burn the wing, it can throw turbine blades around, it can shake itself violently. These can cause other damage to the airplane. It can even tear itself off, surge forward, fall back and hit the wing. These things have all happened. I remember a DC-10 that crashed losing everyone in it when one engine came off and took out the flight controls with it. Then there's the Sioux City crash where the compressor disintegrated and took out all three hydraulic systems. 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.

ryder9 7y ago

A very good video on this subject matter of 4 Engines vs 2 Engines https://www.youtube.com/watch?v=HSxSgbNQi-g https://www.youtube.com/watch?v=HSxSgbNQi-g

K0SM0S 7y ago

The associated concept in regulation is a certification (by plane model, e.g. for all Airbus A321) called ETOPS¹ (Extended Operations). The cert applies to twin engines and basically tells you that a plane can fly up to XXX minutes on 1 functional engine only (e.g. ETOPS = 60, then you have ETOPS 120, ETOPS 180 and currently ETOPS 240 is permitted on a case-by-base basis). Note that "ETOPS-180 and ETOPS-207 cover about 95% of the Earth" (how much you can reach while staying 180-207 minutes from an airport at normal altitude/speed). 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! [1]: https://en.wikipedia.org/wiki/ETOPS https://en.wikipedia.org/wiki/ETOPS [2]: https://en.wikipedia.org/wiki/Deadstick_landing https://en.wikipedia.org/wiki/Deadstick_landing

dx034 7y ago
> 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. 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).