Sitting aboard a 744 from SFO to FRA the other day I looked out and saw two engines and said to myself, even though I have full confidence in two engine planes, "how nice it is to have four engines on this flight." But then the thought occurred to me, has there ever been a time when those two extra engines really made a difference on a commercial airliner in an incident? Has there ever been a crash where there was a belief that had it been a 4 holer, the outcome would have been different? I KNOW two engines is fine and I don't want a bunch or know-it-alls telling me that. I just wonder, in the real world, has this issue ever shown up in a accident report or been speculated about after a crash? Is there some set of circumstances in the allowed flight envelope where you do run a greater risk with just two engines?

[Edited 2014-08-03 17:39:31]

[Edited 2014-08-03 17:40:31]

I did a study on engine caused air crashes some years ago. I tallied all of the engine caused crashes I could find on twin engined jets, and all the engine caused crashes on three and four engined jets. The result? There were exactly the same number of engine caused crashes on twins as on jets with more than two engines. And since there are far more twins flying than jets with more than two engines, that says that you are much less likely to be in an engine caused crash in a twin than in a three or four engined plane. This is counter intuitive, but upon reflection it actually makes sense. The reliability of jet engines has gotten so good that the odds of two of them failing on the same flight for unrelated reasons is very close to zero, which is why we now allow them to go as far as 340 minutes on one engine. But the one mode of failure that is likely to bring a plane down is an uncontained failure, and while that is extremely rare, the odds of it happening are in proportion to the number of engines. (Note that the only crash I could find that was caused by an uncontained failure was UA232, which was a DC-10.) The other anomaly is that there have been two 747's that have crashed because an inboard engine physically fell off the wing, and in doing so struck and caused the outboard engine to also fall off, and in the process damaged the leading edge of the wing to the extent that the plane was rendered uncontrollable. This, of course, could not happen on a twin (there has been at least one and possibly more 737's that have had an engine fall off in flight and landed safely, and at least 1 747 has had an outboard engine fall off and landed safely. Also, at least 1 727.) So while it may feel comforting to look out and see two engines on each side it is actually a false comfort; in reality you are safer seeing only 1. But if that level of increased risk scares you then you had better never get out of bed.  Interestingly, I did not find a single engine caused crash in the first generation of jets, i. e. 707. DC8, 727, DC9, 737Jurassic or CV-880.

But to answer your original question, it is no. Which is why ETOPS was developed in the first place.

[Edited 2014-08-03 18:05:58]

Perhaps SK686 or Lauda 004?

That is an element I hadn't considered and makes sense. You are doubling your risk of a catastrophic engine failure that increases the risk of downing an airplane. Thank you for the thoughtful response.

Not 4 engines, but if the Eastern L-1011 that had all 3 engines flame out on a MIA-NAS flight in 1983 hadn't been able to restart one engine (#2) and make a one-engine landing at MIA, it would have been a much more serious event. With 3 engines they probably had a higher probability that they could restart at least one than if they only had 2 engines.
http://aviation-safety.net/database/record.php?id=19830505-2

[Edited 2014-08-03 18:34:42]

Would the incident where the BA 747 flamed out all 4 engines from volcano ash have ended differently if it was a twin??

I don't remember the specifics of the accident but hasn't a 747 saved a Singapore flight that lost power because of volcanic ash?

Quoting trex8 (Reply 5): Would the incident where the BA 747 flamed out all 4 engines from volcano ash have ended differently if it was a twin??

A KLM 744 had the same thing happen en route AMS-ANC in 1989. They eventually got all 4 engines started but I believe all the engines had to be replaced. The volcanic ash also did a lot of other damage.

Both of those incidents:
http://aviation-safety.net/database/record.php?id=19820624-0
http://aviation-safety.net/database/record.php?id=19891215-1

Quoting trex8 (Reply 5): Would the incident where the BA 747 flamed out all 4 engines from volcano ash have ended differently if it was a twin??

They got the engines restarted, but one stopped again. With that damage, all the other engines faced the same dangers, so easily could have been 2 or 3 engines not working. I believe in this case having 4 engines gave the crew some more confidence.

Imagine being in a twin, both engines stopping. You restart them, and soon after, one of them stops again. There is a real fear that the last engine can go. With 4 engines, and the one stopped, there is at least some more confidence knowing that you can handle one more stopping.

-CXfirst

Quoting SEPilot (Reply 1): There were exactly the same number of engine caused crashes on twins as on jets with more than two engines. And since there are far more twins flying than jets with more than two engines, that says that you are much less likely to be in an engine caused crash in a twin than in a three or four engined plane.

The more relevant question is the hypothetical part.
would this exact incident that happened on a twin have the same result on a triple or quad?

when you think about unrelated engine failures, like random, and one engine fails, the remaining (wether there are 3 on a quad, 2 on a triple or only one on a twin) will bring you down safely.
even given that the chances are twice as high losing an engine on a quad, there will still be half the engines there to bring you down. the chances of losing an engine (randomly) compared to the number of remaining engines stay the same.

related engine failures like BA38 are irrelevant because in such cases all engines are affected.

regarding the quote "less likely to be in an engine caused crash in a twin than in a three or four engined plane" is just caused by the development of the industry. all 4-holers (except for the A340) were designed with 4 engines because there were no other options, wether it was because of the lack of power or lack of reliability. they are outdated. with todays technology 2 engines are more reliable than 4 engines of yesterdays technology. that's all this statistics says.

the only airframe that can fly with either 2 or 4 engines is the A340/A330. I would be interested in statistics comparing in-flight engine shout down between these two types. is it more likely to lose 3 engines on an a340 than losing 2 engines on an a330?

I was gonna say the BA incident and volcanic ash.

There is a problem though with the question. If being a quad saved the aircraft, then,
it didn't crash.. it was merely an incident. So we may not know of things actually being
different if the aircraft had have been a twin. That being said as others have said,
it was probably the BA 747 that closest answers your question. I am not sure that could
happen again however given the changed attitudes towards flying near volcanic ash.

this one sounds close:
http://www.flightglobal.com/news/art...led-dual-engine-failure-on-366656/
as well as this one:
http://blogs.crikey.com.au/planetalk...ider-in-the-absence-of-alan-joyce/

Qantas Flight 32mwould probably have ended very badly if she was a twin with an engine exploding and puncturing/emptying the wing fuel tank like it did.

A testament to the 4 holers over the twins in this case!

Quoting horstroad (Reply 9): The more relevant question is the hypothetical part. would this exact incident that happened on a twin have the same result on a triple or quad?

The one that I can think of is the British Midland 734 where the crew shut down the wrong engine. If it had had more engines it likely would have made the runway. As to the two 747's that flew through ash clouds, it is possible that had they been 777's the outcome might have been different, but by no means certain. Had it been a 777 (or other twin) it is conceivable that only one engine would have continued to run and not produced enough power to enable the plane to make a safe landing. But we do not know.

Quoting RedChili (Reply 2): Perhaps SK686 or Lauda 004?

I don't know why SK686 is even considered. That's a runway incursion.
Lauda 004, if the engines ripping off bled off the hydraulics (which if I remember correctly, happened with this case too), then it's useless... On the lateral acceleration aspect, yes, 4 engines might have helped and prevented the nose running away to one side that caused the vertical stabilizer failure.
The AAL DC10 in ORD(?) was one case where 1 engine ripping off bled off the hydraulics (causing slat retraction on one side).

Quoting trex8 (Reply 5): Would the incident where the BA 747 flamed out all 4 engines from volcano ash have ended differently if it was a twin??

Quoting trex8 (Reply 5): I don't remember the specifics of the accident but hasn't a 747 saved a Singapore flight that lost power because of volcanic ash?

BA008?
If you fly into volcanic ash, you can have 1, 2, 3, 4, or 16 engines and the number of engines wouldn't help.

Quoting CXfirst (Reply 8): Imagine being in a twin, both engines stopping. You restart them, and soon after, one of them stops again. There is a real fear that the last engine can go. With 4 engines, and the one stopped, there is at least some more confidence knowing that you can handle one more stopping.

At 50% of the engines available, the 747 at that weight might not have been able to clear the mountains to cross Java and land in Jakarta Halim. 50% of the engines available on a twin at standard weights using the typical load profiles and gross weight by the time they reached the location, would have been able to clear the mountains to cross Java to land at Jakarta.
At 25% of the engine available, the 747 would have been doomed.

It's a matter of view point.  

Quoting SEPilot (Reply 13): The one that I can think of is the British Midland 734 where the crew shut down the wrong engine.

This is probably one of the only ones I can think of where more than 2 engines might have helped.

Quoting mandala499 (Reply 14): Lauda 004, if the engines ripping off bled off the hydraulics (which if I remember correctly, happened with this case too), then it's useless...

IIRC, that wasn't what actually happened in that accident. If the engine had ripped off the plane, it probably wouldn't have actually crashed. It stayed attached, with its reverser deployed, which caused the drag on that wing that killed its lift and caused the plane to dive to the left. The plane did break apart in mid-air eventually, but only after entering the dive.

I'm not sure what would have happened on that flight with four engines. Would the loss of lift on that wing have been the same, or halved? I'm not an aerodynamic engineer so I'm not sure. Intuitively I would think the increase in drag would have been the same, so the result would have been the same. But I could be wrong.

The interesting thing about thinking about accidents that wouldn't have happened with four engines instead of two is also thinking about accidents that might not have happened with two engines instead of three or four. Having more than two engines increases the chance of failure by either 50% or 100%, and it also means less thrust is available if 50% of the engines are knocked out. Would UA232 have crashed if it was a two engine plane? What about El Al 1862?

I think it's probably a wash overall.

Quoting Viscount724 (Reply 4): Not 4 engines, but if the Eastern L-1011 that had all 3 engines flame out on a MIA-NAS flight in 1983 hadn't been able to restart one engine (#2) and make a one-engine landing at MIA, it would have been a much more serious event. With 3 engines they probably had a higher probability that they could restart at least one than if they only had 2 engines. http://aviation-safety.net/database/record.php?id=19830505-2

Probably true, but I think separated engine maintenance is part of the ETOPS requirements to guard against a common failure mode such as missing O-rings.

In the end, I guess the issue comes down to, all things being equal, would you prefer to board a two or four holer. Sounds like there are inherent advantages and disadvantages of each. Having more engines means more probability of uncontained failures. But having redundancy also allows you, in some situations, greater chances of saving it. There are some cases it would have been better to be on a two engine plane. Others where only a four would have survived. The only (informal) study referenced shows two might be better. But in the end, failures are so few that basically it may be a wash all things considered.

Quoting mrcomet (Reply 17): But in the end, failures are so few that basically it may be a wash all things considered.

That is the thing; the chance of being in an airliner crash of any type is vanishingly small; and engine caused crashes are a very, very small subset of that. So the chance of being involved in one, whether on a two, three, or four engine plane is considerably smaller than the chance of being hit by lightning. It is far more dangerous to walk down the street.

Quoting mrcomet (Reply 17): In the end, I guess the issue comes down to, all things being equal, would you prefer to board a two or four holer. Sounds like there are inherent advantages and disadvantages of each.

Will enough customers *pay* for the 4 holer. It takes a *big* plane to pay for four engines. Greater diameter engines will have better efficiency due to less surface area to flow area and less blade tip leak path area to flowpath area. So they won't be quite equal in costs. Maintenance will also be lower on two engines.

So unless the airframe is too large for two engines, then the economics will dramatically favor a twin.

FWIW, a BWB cannot be a twin due to the lack of a rudder in the engine failure situation (it must be a triple or a quad). So there is an economic solution for a triple or a quad (the BWB), but not on a cigar with wings that isn't larger than what twins could economically perform. In other words, anything smaller than a 779 is going to be a twin.

Lightsaber

Would UA811 from Hawaii be considered a 4 engine blessing? If memory serves me correctly, they ended up landed on 2 left engines.

Quoting flyboy730 (Reply 20): Would UA811 from Hawaii be considered a 4 engine blessing? If memory serves me correctly, they ended up landed on 2 left engines.

It would have been better of as a twin. Both engines failed due to sucking in debris blow out of the cabin and cargo hold. If it was a twin it would have only lost the single engine on that side. A twin with one engine out flies better than a quad with two out.

Quoting CXfirst (Reply 8): They got the engines restarted, but one stopped again. With that damage, all the other engines faced the same dangers, so easily could have been 2 or 3 engines not working. I believe in this case having 4 engines gave the crew some more confidence.

In such a case, you want any engine restarting as soon as possible. Having four engines (as compared to 2 or 3) just raises the chances of having an earlier engine restart. That makes a difference. Whether it saved the flight, it's hard to tell.
Questions like "did this save that" are hard to answer scientifically because anything you remove from a situation can have countless consequences and effects. And you're not talking about just "removing 2 engines" but if it was a 2-engined plane, you'd be talking about a totally different plane model and different procedures.

[Edited 2014-08-04 01:58:00]

Quoting mandala499 (Reply 14): If you fly into volcanic ash, you can have 1, 2, 3, 4, or 16 engines and the number of engines wouldn't help.

I think an argument can be made otherwise...first, as already pointed out:

Quoting CXfirst (Reply 8): They got the engines restarted, but one stopped again. With that damage, all the other engines faced the same dangers, so easily could have been 2 or 3 engines not working. I believe in this case having 4 engines gave the crew some more confidence. Imagine being in a twin, both engines stopping. You restart them, and soon after, one of them stops again. There is a real fear that the last engine can go. With 4 engines, and the one stopped, there is at least some more confidence knowing that you can handle one more stopping.

Second, IIRC BA 9's #4 engine started showing symptoms before the other three did...I could be remembering wrong, but from what I remember, due to the flight crew shutting that engine down relatively early on, the engine didn't sustain as much damage as the other three,..and, because of that, they were able to get #4 restarted first. Hypothetically, had this happened in a twin, they might have been more reluctant to shut the engine down, which could have resulted in that engine sustaining more damage, which could have had an effect on the outcome of the incident.

Also, worth repeating IMHO:

Quoting Lufthansa (Reply 10): There is a problem though with the question. If being a quad saved the aircraft, then, it didn't crash.. it was merely an incident. So we may not know of things actually being different if the aircraft had have been a twin.

This is a matter of personal opinion, but I'm a big fan of redundant systems, especially on something so critical...so, for a flight over water, I'm more than willing to (and have done so before) go out of my way and pay a premium to fly on a four-engine aircraft. Certainly, I would rather be on a 747/A380 with two engines out than on a 777/A350 with two engines out...

After thinking about this a little more... the design philosophy between them can be distilled down as follows ... if you want the max level of safety at the expense of a little extra fuel, go for 4 engines - if you want the best possible fuel economy, go for the twins with an acceptable (close to negligible) reduction in that max level of safety.

Quoting mandala499 (Reply 14): I don't know why SK686 is even considered. That's a runway incursion.

I believe that in the crash, one engine was ripped off the MD80, while the other was damaged and lost thrust. The plane simply didn't have enough thrust to complete take-off. With four engines, it's possible that there would have been more thrust left.

Quoting lightsaber (Reply 19): So unless the airframe is too large for two engines, then the economics will dramatically favor a twin.

Given many think the A380 is too large, doesn't that mean that realistically all future wing and tube airliners will be twins?

I do wonder how powerful they could make an engine. 130,000lbs has already been done on the test bench I think. Could even an A380 sized plane be a twin?

Quoting mrcomet (Thread starter): Has there ever been a crash where there was a belief that had it been a 4 holer, the outcome would have been different?

I seem to remember that there was this two-holer that lost 2 engines and landed on the Hudson........

Quoting part147 (Reply 12): Qantas Flight 32mwould probably have ended very badly if she was a twin with an engine exploding and puncturing/emptying the wing fuel tank like it did. A testament to the 4 holers over the twins in this case!

Unlikely. Twins have suffered engine failures in flight and landed safely.

Why would a twin with a punctured fuel tank be more likely to crash than a quad with the same damage? I don't get your reasoning.

The fact the aircraft had 4 engines had no bearing on the outcome of QF32. Save perhaps for the longer loiter time to troubleshoot.

Quoting part147 (Reply 24): After thinking about this a little more... the design philosophy between them can be distilled down as follows ... if you want the max level of safety at the expense of a little extra fuel, go for 4 engines

Well, no. 4 engines do not give you any better 'level of safety', statistically or anecdotally.

Quoting warden145 (Reply 23): This is a matter of personal opinion

Only from a perception standpoint. It's not a matter of opinion on whether they are truly safer or not.

Jimbo

Quoting spacecadet (Reply 15): I'm not sure what would have happened on that flight with four engines. Would the loss of lift on that wing have been the same, or halved? I'm not an aerodynamic engineer so I'm not sure. Intuitively I would think the increase in drag would have been the same, so the result would have been the same. But I could be wrong.

Well, if the plane have had for engine and on one engine the reverser deployed, there would still be positive trust left on that wing and the result should not have been different from a single engine out on a twin, especially if the inboard reverser was the one kicking in.
I would think as a 4-Engine aircraft Lauda 004 would not have went out of control and broken up midair.

best regards
Thomas

Reminds me of the joke about the 747 LAX-JFK redeye.

Capt: Sorry folks for that jolt it was our no. 4 engine going out. We're fine we'll be getting into JFK about 20 mins. late.
[An hour later]
Capt: Folks are no. 2 just went out. We'll be fine just getting into JFK about an hour late.

Pax to another Pax: If this keeps up we'll be up here all night.  

How at the AF A380 when the engine blew up?

Here is a chart of engine shutdown rate since the start of ETOPS. 4 engines used to be very important because the probability of two engines shutting down on a single airplane on a flight was far higher than the one in a billion requirement.

Quoting SEPilot (Reply 1): But to answer your original question, it is no. Which is why ETOPS was developed in the first place.

Now 2 engines vs 4 engines is not a factor. However, before ETOPS with the engine reliability rates of early jet engines, it was an important factor. I think the JT3s on the 707s started with an inflight shutdown rate of about 0.9 per 1000 flight hours. That gives a probability of 2 shutdowns on any given flight at about 1 in a million flight hours. That is far too high of a rate to be acceptable. Nowadays the GE90s ,001 shutdowns per 1000 flight hours or a probability of 2 shutdowns on any given flight of 1 in a trillion flight hours.

Quoting mrcomet (Thread starter): Is there some set of circumstances in the allowed flight envelope where you do run a greater risk with just two engines?

Typically not because airplanes are designed to account for failures. For example a single engine failure on a twin at rotation requires a bigger rudder to counteract the yaw. A four engined airplane has a rudder big enough to overcome only the outboard engine out.

Quoting kurtverbose (Reply 26): Given many think the A380 is too large, doesn't that mean that realistically all future wing and tube airliners will be twins?

Ironically, I think the A380 is too small. The wing and engines really are meant for a higher MTOW which will only pay for itself with a longer tube (more people).

But with the FAA accepting a few BWB evacuation proposals, I do not see a future wing and tube airframe with more than 2 engines outside of military procurement. One exception with be an A389. Otherwise, it will be all twins.

Airbus, Boeing, and Northrop are all progressing nicely on BWB concepts. I think it might actually happen!    Each concept with very unique responses to solving the BWB 'engine out issue' (due to the lack of a rudder, BWBs are trickier for the engine out scenario). I expect Boeing to partner with Northrop as they did with the 747 (but Northrop will demand more money, in particular with the recent discovery that much of Boeing's standard work is built on stolen Northrop documentation from their cooperation on the 747!)

Lightsaber

Quoting part147 (Reply 24): ...if you want the max level of safety at the expense of a little extra fuel, go for 4 engines

On a long-haul mission, a 777-300ER burns on average around a third less fuel per hour than a 747-400 and the A350-1000 and 777-9 will be closing in on half if they're as good as Airbus and Boeing say they will be.

[Edited 2014-08-04 07:25:20]

Quoting part147 (Reply 24): After thinking about this a little more... the design philosophy between them can be distilled down as follows ... if you want the max level of safety at the expense of a little extra fuel, go for 4 engines - if you want the best possible fuel economy, go for the twins with an acceptable (close to negligible) reduction in that max level of safety.

But if you look at the record, you are getting less safety, not more. Statistically, twins are safer than quads.

Quoting kurtverbose (Reply 26): I do wonder how powerful they could make an engine. 130,000lbs has already been done on the test bench I think. Could even an A380 sized plane be a twin?

I have read that the limit of current technology is about 150,000 lbs. But that was quite a few years ago; I am sure it is higher now. It is a moving target, and I expect that the next VLA will be a twin, even if it is larger than the A380. We will not see another quad airliner, period. And I suspect that if a BWB requires more than two engines the advantage will not be enough to overcome the cost of more engines. There is a simple solution, however; put a rudder on the BWB.

Quoting PhilBy (Reply 27): I seem to remember that there was this two-holer that lost 2 engines and landed on the Hudson.....

Yes, and if it had been a quad we likely would have had a four-engine plane landing in the Hudson. There have been two airliners that crashed with fatalities resulting due to birds, and both of them were quads.

Quoting bond007 (Reply 29): Only from a perception standpoint. It's not a matter of opinion on whether they are truly safer or not.

Yes. The record clearly shows that twins are safer.

Quoting tommy1808 (Reply 30): Well, if the plane have had for engine and on one engine the reverser deployed, there would still be positive trust left on that wing and the result should not have been different from a single engine out on a twin, especially if the inboard reverser was the one kicking in. I would think as a 4-Engine aircraft Lauda 004 would not have went out of control and broken up midair.

This is a valid point; but we cannot know the answer for sure. I am sure that nobody wants to try it.

Quoting Roseflyer (Reply 33): Now 2 engines vs 4 engines is not a factor. However, before ETOPS with the engine reliability rates of early jet engines, it was an important factor.

Actually, I have been unable to find any case of two unrelated engine failures on the same flight on any jet plane, going right back to the B-47 and B-52. While the statistics do not support ETOPS operations on early jets, as you say, I cannot find ANY engine caused crashes of any of the first generation jets.

Slightly off topic, but I was impressed with this video of a Swiss crew and the process to shut down an engine on a flight from Zurich to Shanghai. Appears they weren't too far away from ZRH. Very skilled and professional crew.

https://www.youtube.com/watch?v=rEf35NtlBLg

Quoting SEPilot (Reply 36): Actually, I have been unable to find any case of two unrelated engine failures on the same flight on any jet plane, going right back to the B-47 and B-52. While the statistics do not support ETOPS operations on early jets, as you say, I cannot find ANY engine caused crashes of any of the first generation jets.

You are right. People were used to Props and their reliability back then. The reason why ETOPS wasn't introduced earlier was not that Turbo Jet Engines were that much less reliable then the ones of the 80ies and 90ies, it was because people didn't realize just how reliable they were.

Quoting SEPilot (Reply 36): I have read that the limit of current technology is about 150,000 lbs. But that was quite a few years ago

I don't know how much the limit is today, but from a technilogical standpoint it will increase in the future once GTFs are introduced into large engines for example.

Quoting SEPilot (Reply 36): Yes, and if it had been a quad we likely would have had a four-engine plane landing in the Hudson. There have been two airliners that crashed with fatalities resulting due to birds, and both of them were quads.

I think you're a bit quick to dismiss the Hudson accident as likely inevitable with any aircraft. Sully's bird strike happened at 2700'. Odds that if he had a 4 engine airplane, the geese would have taken out more than 2 is, my guess, pretty low. While it's just fantasy to consider a domestic airliner that has 3 or more engines in this day and age, I'll let my imagination run rampart and say that if he had been flying say a BAe 146, then maybe the wet landing would have been avoidable with his expertise.

As I understand it-Air Force One is required to be a 4 engine aircraft. I guess there must be an argument for 4 holers over twins. But maybe the USAF will change their minds? Much has happened since the 1980's.

As some have stated, engine reliability has increased. This also means that it has increased on quads. No matter what, quads will always be safer - technically. But airliners are so safe now that their advantage is statistically negligible.

I can't help but believe that four engines reliability for safety goes back to piston days. Without having data, I'm guessing that an engine out during flight was more common with pistons than with turbines.

Once there are written expectations....those are hard to break. I took a lone time for regulators and aircraft designers to fully grasp the dependability of turbofans. Shoot, it took a long time for turbine engines to become acceptable for flight.

Drawing from what essentially my intuition, this safety of twin turbine v. more the two turbine aircraft for safety is a result of the long evaluation of jet aircraft rather than a direct comparison of incident data.

Quoting tjcab (Reply 41): No matter what, quads will always be safer - technically

Actually, a lot matters. Engine(s) out performance, chance of a failure * no. of engines, chance of bird strike affecting >1 engine etc. etc.

Might be worth reading some of the earlier posts  

Jimbo

Quoting RetiredWeasel (Reply 39): Sully's bird strike happened at 2700'. Odds that if he had a 4 engine airplane, the geese would have taken out more than 2 is, my guess, pretty low.

But if you take out two engines on a four engine plane, it's coming down. It no longer has the thrust to climb.

however:

Quoting tjcab (Reply 41): As some have stated, engine reliability has increased. This also means that it has increased on quads

this is also true.

Quoting tjcab (Reply 41): No matter what, quads will always be safer - technically

(though this part is not true, based on the part above. If you lose 50% of your engines on a twin, you still have a plane that can climb, and fly for as much as 5 hours now. If you lose 50% of your engines on a quad, you will be coming down immediately.

Quoting PanAm1971 (Reply 40): As I understand it-Air Force One is required to be a 4 engine aircraft. I guess there must be an argument for 4 holers over twins.

My understanding is that this is because AF1 requires a huge electrical draw which two engines cannot produce.

Quoting SEPilot (Reply 36): Actually, I have been unable to find any case of two unrelated engine failures on the same flight on any jet plane, going right back to the B-47 and B-52. While the statistics do not support ETOPS operations on early jets, as you say, I cannot find ANY engine caused crashes of any of the first generation jets.

I think such a statistic would be hard to find. I do not know where to look for an incident report from a 707 or DC-8 vintage airplane that had a dual engine failure. Most likely the flight would have continued to a diversion airport and not be reported. The whole concept is 4 engines preventing a crash, so I do not know where you would find information about a crash not happening.

I would consider the Kalitta 747 crash in Columbia in 2008 to be two unrelated engine failures.

Quoting d l x (Reply 44): My understanding is that this is because AF1 requires a huge electrical draw

More than a 787?

Quoting hivue (Reply 46): Quoting d l x (Reply 44): My understanding is that this is because AF1 requires a huge electrical draw More than a 787?

That is what I read. No idea where I read it though. Maybe in the MilOps forum?

Quoting d l x (Reply 44): But if you take out two engines on a four engine plane, it's coming down. It no longer has the thrust to climb.

It may not be able to climb, but a 747 can certainly maintain altitude with 2 engines if not too heavy and down at lower altitudes (less than 20,000). Captains practice two engine flight and landings twice yearly in the simulator.

Quoting mrcomet (Thread starter): Sitting aboard a 744 from SFO to FRA the other day I looked out and saw two engines and said to myself, even though I have full confidence in two engine planes,

I doubt a SAS 747 coming in on two Engines to ARN once would have survived