Optimal number of engines

[helmutkohl]

There's been a lot of claims about the function/need of how many engines are needed in an aircraft
much of it on single engined vs twin-engined.

some of the common statements are
twin-engined is preferable for maritime aircraft, thats why the US Navy went for it
single engined is more fuel efficient
twin-engined provides much more power and carry for a little more weight
twin-engined is safer to fly, etc

I am wondering how true these statements are, or how relevant it is during different eras.
For example I've also heard the claim that twin-engined were preferred due to higher failure rates in the past, but that much of it has reduced allowing for single engined aircraft
as well as statements such as.. the only reason why some designers went for twin-engined was simply because there wasnt a single engine powerful enough for that aircraft on its own.

For slower larger aircraft,
there's been a trend in seeing 4 engined aircraft being reduced to two larger engines..
and I think even a proposal to turn a large 747 to a three engined design..
yet there are some exceptions like the Japanese P-1 which is 737 sized but went with 4.

what are your thoughts

 

[overscan (PaulMM)]

twin-engined is preferable for maritime aircraft, thats why the US Navy went for it

There are less places to land in an emergency over sea than land. You might be able to land with no engine on an airbase but I doubt it'd be easy on a carrier, so you have to ditch the aircraft.

single engined is more fuel efficient

Fuel efficiency comes from SFC of the engine primarily. Range is related to fuel fraction and efficiency. Number of engines is not relevant. A large high bypass ratio engine is more fuel efficient than two smaller low bypass ratio engines because of the relative bypass ratios, not the number of engines.

twin-engined provides much more power and carry for a little more weight

Don't see how this makes any sense at all. A large single engine can produce double the thrust of two smaller ones.

Adding a second F135 to an F-35 would add a load of weight directly in engine, structures, airframe, and would double the rate of fuel consumption, so you'd have to add tonnes more fuel, adding more structure, then the wings would need enlarging - now you have something significantly bigger than the F-22.

twin-engined is safer to fly, etc

The chance of having an in-flight engine problem is doubled, but the effect of an in-flight engine problem is greatly reduced - instead of losing all power you only lose half. You will find it easier to get back to a safe landing on one engine than none.

In the earlier Navy jet days power was so marginal that even half thrust might not make much of a difference to survival, however from say F-4 onwards this was less true.

Single engine fighters will generally have a higher rate of crashes than twin engine ones when the reliability of the engines is constant. The more reliable engines get, the less chance of it mattering much. Two highly unreliable engines would be worse than one highly reliable one.

 

[red admiral]

The optimum number is zero as this avoids many reliability and cost issues.

"There's no such thing as a good aircraft engine" - Camm

 

[Zoo Tycoon]

One observation on improving engine reliability over the years.

In the late nineteen sixties/early seventies an average airline pilot could expect one in flight engine shut down per year. Now a first officer joining the industry at the age of 21 and retiring as a captain at the age of 50 has a better than even chance of never having an in flight engine shut down .

If this reliability trajectory continues, a single engine commercial aircraft is entirely possible,(maybe backed up with a much smaller engine, normally not running, one shot use so highly tuned thrust, and sized for cruise only to get you to a landing strip). However the problems with such an arrangement are not so much technical and has a lot in common with the issues of introducing single pilot operation.

Remember also this excellent reliability is a function of a relatively benign operating environment so may not transfer if the engine or aircraft is radically repurposed in its role. This is what’s behind the P1’s four engines in that the intend was to retain the low level P7 mission profiles. The Boeing 737 derivative P8 is intended to have much higher altitude mission profiles so is more akin to its airliner heritage. This has an impact on the sensors carried, such as no Magnetic Anomaly Detector which is a low level detector (except the Indian aircraft) and the need for accurate sonar bouy placement drops.

 

[red admiral]

More seriously on the reliability / airworthiness front:

Its now possible to produce a single engined fighter design with equivalent safety targets to a multi-engined design through mitigating various failure modes. But there are some risks that are more difficult to mitigate - e.g.large single birdstrike that multi-engined designs can be more resilient to because of their inherent redundancy.

Whereas for airliners with higher safety targets than fighters you're still after that increased redudancy to single point failures. Its difficult to ETOPS certify a single engined aircraft...

 

[overscan (PaulMM)]

Its difficult to imagine a sensible single engine commercial airliner design. It would have to be in the tail, presumably, and would need to be relatively large and high in thrust.

 

[Zoo Tycoon]

More seriously on the reliability / airworthiness front:

Whereas for airliners with higher safety targets than fighters you're still after that increased redudancy to single point failures. Its difficult to ETOPS certify a single engined aircraft...

If you care to read the certificates standards for both military and civil (call them safety targets) you’ll find they’re the same. Also FAR/CS25 1301 forbids single point failures leading to loss by design as does Def Stan 970 Chapter 1.1301;- Note the common requirement nomenclature to further aid the integration.

Back in the early nineties many in the industry said it would be difficult to certify a twin to a commercially useful level ETOPS (say 180) especially upon first delivery of a new design;- Engines Turning Or Passengers Swimming was the phrase I remember from the time. We’ve moved on since then.

 

[Zoo Tycoon]

Its difficult to imagine a sensible single engine commercial airliner design. It would have to be in the tail, presumably, and would need to be relatively large and high in thrust.

Most likely a rear fuselage installation with a boundary layer intake then the fin can be much smaller;- it’s currently sized by the yaw caused by a failure of a wing mounted engine . The much smaller emergency use engine could sit above it say a little bit like the Trident 3B RB162 central engine location. A bit like this;-

Anyway the industries is not currently moving in this direction as their focus is elsewhere ie environment.We’ll see if single pilot operation makes it first.

 

[stealthflanker]

Aside from redudancy/reliability one might look at how much thrust required and the availability of technology and to some extent electrical power generation requirement from the aircraft itself.

Another reason to adopt twin engine layout over single engine is that it's an "easy" way to get T/W ratio of 1 without resorting to larger or potentially heavier engine.

 

[red admiral]

If you care to read the certificates standards for both military and civil (call them safety targets) you’ll find they’re the same.

Hmm loss rates / flying hour targets are very different for airliners and fighters. Like 2 orders of magnitude.

Still a lot safer than general aviation though. Or helicopters

 

[Zoo Tycoon]

And that’s the operational environment loss model. If you constantly operated a military aircraft in the same flight regimen as a civil you would get a similar (much better) rate of loss until the flight hours/cycles ran out. And vis-versa except probably the civil aircraft might do quite a bit worse in the military environment.

The basic design targets (certification standards) are both the same but meeting the targets is function of the environment they’re used in.

Of course there are also some scaling effects;- a Hawks small intake is more likely to get stuffed full of dead birds in the event of a multi
bird strike and you can’t make the small compressor face as robust.

 

[helmutkohl]

Its difficult to imagine a sensible single engine commercial airliner design. It would have to be in the tail, presumably, and would need to be relatively large and high in thrust.

im wondering if say, a single 767 engine could work on a smaller airliner like an A220 (I believe the single engine on any of the 767 has similar or more thrust than the two on the A220). The engine would probably have to be located in the middle like those single engined business jet that either use an L1011 arrangement with an S duct, or a DC-10 style thats simpler

 

[overscan (PaulMM)]

Its difficult to imagine a sensible single engine commercial airliner design. It would have to be in the tail, presumably, and would need to be relatively large and high in thrust.

im wondering if say, a single 767 engine could work on a smaller airliner like an A220 (I believe the single engine on any of the 767 has similar or more thrust than the two on the A220). The engine would probably have to be located in the middle like those single engined business jet that either use an L1011 arrangement with an S duct, or a DC-10 style thats simpler

It could, but what is the advantage of doing so?

Advantages of underwing mount:

• Engines mounted in pods underneath the wing reduce the upward bending moment of lift creation, and therefore allows for lighter construction.
• If the pods stick out forward of the wing, the backwards aerodynamic twisting moment is also reduced: lighter construction.
• The engines can be easily reached for maintenance.
• All of the fuselage can be used for useful load - tail mounted engines have structural reinforcements in the fuselage between the engines.

Advantages of fuselage mount:

• The yawing moment of a single engine fail is smaller, allowing for a smaller vertical tail.
• Fuselage can be lower to the ground and allow for low door sill height.
• Landing gear can therefore be shorter.
• Clean wing configuration.
• Lower trim change due to thrust.
• Less noise in most of the cabin.
• In smaller planes, the wing bending & twisting moment is a relatively smaller constructive issue than in larger planes. And indeed, being closer to the ground allows for easier boarding via air stairs.

Why do most airliners have underwing engines, while business jets have rear-mounted engines?

Why have modern airliners converged on a configuration with two or occasionally four underwing engines, while modern business jets have converged on a configuration with two or occasionally three r...

aviation.stackexchange.com

 

[Zoo Tycoon]

The advantage is cost.As simple as that and a single massive step reduction.

The engines represent a big percentage of the aircraft non recurring cost and is the single largest direct maintenance cost.

As pointed out before it’s similar to single pilot operation, again a cost reduction driven scheme.

 

[overscan (PaulMM)]

How does engine cost scale with size? I believe it was claimed that 2 x F404s was the same as 1 x F100.

 

[overscan (PaulMM)]

So.

Here's an example how complex this is.

From Jay Miller's Aerofax on the F-16

ENGINE SELECTION—The combination of engine cycle (bypass and pressure ratios) and number of engines was a very significant parameter in defining the gross weight (mission weight) of the YF-16. During the initial conceptual phase of the lightweight fighter program, General Dynamics evaluated two engines with different bypass ratios (BPR). These were the Pratt & Whitney F100 with a BPR of 0.72 and the General Electric YF101 with a BPR of 0.20. The former was examined for use in single engine designs, and the latter for use in twin engine.

The evaluation of the two powerplants and their respective configurations was made by comparing the combined engine and fuel weights - including cruise, combat, and reserves - required to accomplish the design missions. The YF-16 design mission included a 500 n. mile high-subsonic cruise leg, a maximum afterburner acceleration, maximum afterburner subsonic and supersonic sustained turns, and a 20-minute sea level reserve.

For this mission, the combined engine and fuel weight of the single F100-PW-100 turbofan engine was 7,882 pounds; for the twin YJ101 engines, it was 10,234 pounds. The twin YJ101 engines, including installation, weighed 1,024 pounds [more] and required 1,328 pounds more fuel. For missions requiring longer ranges, lower altitudes, or higher speeds (at sea level or at high altitude), the differences would be even greater.

At cruise speed, the F100 was estimated to have a 7% higher thrust-to-weight ratio with a 25% lower fuel flow; at Mach 2.0 and 30,000 feet, it had a 41% higher thrust-to-weight ratio with a 6.5 percent lower fuel flow. In addition, it was found that, although two YJ101 engines were rated at 5,200 pounds more thrust at sea level static conditions, the single F100 delivered 2,850 pounds more thrust at Mach 2.0 and 30,000 feet; and 7,500 pounds more thrust at Mach 1.2 and sea level.

The merits of one versus two engines, which had been (and still are being . .) argued for years, were also taken into consideration. Many evaluations of accumulated accident rate data were made with varying conclusions. The number of variables involved in these evaluations made it impossible to arrive at a specific conclusion.

Hillaker and his design team eventually concluded that a single F100 turbofan engine with its higher bypass ratio provided the best balance of combat capability and mission radius for the lowest weight. A basic configuration meeting the stated performance/mission goals could be achieved at a mission weight of 17,050 pounds, whereas twin GE YJ101 engines resulted in a mission weight of 21,470 pounds. A single GE YJ101 configuration was also examined to see how small an aircraft could be defined, however, the configuration could not meet performance/mission goals.

 

[Zoo Tycoon]

One is always not enough and two is always one too many.

 

[F119Doctor]

Its difficult to imagine a sensible single engine commercial airliner design. It would have to be in the tail, presumably, and would need to be relatively large and high in thrust.

im wondering if say, a single 767 engine could work on a smaller airliner like an A220 (I believe the single engine on any of the 767 has similar or more thrust than the two on the A220). The engine would probably have to be located in the middle like those single engined business jet that either use an L1011 arrangement with an S duct, or a DC-10 style thats simpler

It could, but what is the advantage of doing so?

Advantages of underwing mount:

• Engines mounted in pods underneath the wing reduce the upward bending moment of lift creation, and therefore allows for lighter construction.
• If the pods stick out forward of the wing, the backwards aerodynamic twisting moment is also reduced: lighter construction.
• The engines can be easily reached for maintenance.
• All of the fuselage can be used for useful load - tail mounted engines have structural reinforcements in the fuselage between the engines.

Advantages of fuselage mount:

• The yawing moment of a single engine fail is smaller, allowing for a smaller vertical tail.
• Fuselage can be lower to the ground and allow for low door sill height.
• Landing gear can therefore be shorter.
• Clean wing configuration.
• Lower trim change due to thrust.
• Less noise in most of the cabin.
• In smaller planes, the wing bending & twisting moment is a relatively smaller constructive issue than in larger planes. And indeed, being closer to the ground allows for easier boarding via air stairs.

Why do most airliners have underwing engines, while business jets have rear-mounted engines?

Why have modern airliners converged on a configuration with two or occasionally four underwing engines, while modern business jets have converged on a configuration with two or occasionally three r...

aviation.stackexchange.com

One other advantage of the pod mount of the engines is the separation of the engine from the structure of the wing or fuselage. In the event of a engine fire or catastrophic seizure, the pod strut is designed to fail first, allowing the engine to depart the airframe, which might be able to flying with the other engine(s). This was more of a consideration in early jets where engines were much less reliable than today.

 

[starviking]

One other advantage of the pod mount of the engines is the separation of the engine from the structure of the wing or fuselage. In the event of a engine fire or catastrophic seizure, the pod strut is designed to fail first, allowing the engine to depart the airframe, which might be able to flying with the other engine(s). This was more of a consideration in early jets where engines were much less reliable than today.

This doesn’t sound feasible. I would think it would need advanced computer simulations to design the strut so that it would fail in a beneficial way during a fire, but not in situations where it is overstressed - in effect we are getting rid of the “fudge factor” in engineering design, and then some.

 

[Yellow Palace]

One other advantage of the pod mount of the engines is the separation of the engine from the structure of the wing or fuselage. In the event of a engine fire or catastrophic seizure, the pod strut is designed to fail first, allowing the engine to depart the airframe, which might be able to flying with the other engine(s). This was more of a consideration in early jets where engines were much less reliable than today.

This doesn’t sound feasible. I would think it would need advanced computer simulations to design the strut so that it would fail in a beneficial way during a fire, but not in situations where it is overstressed - in effect we are getting rid of the “fudge factor” in engineering design, and then some.

Boeing used fuse pins. The design intent wasn't so much breaking away in case of a fire, as breaking away cleanly during a wheels-up landing to prevent them being ripped off and damaging the fuel tanks. After a couple of crashes attributed to fatigue failure of the fuse pins, and presumably more than a few non-fatal failures, they've moved away from that design concept.

Airbus never used them, relying on a different structural approach to prevent the engines from rupturing fuel tanks.

 

[P-STICKNEY]

More seriously on the reliability / airworthiness front:

Its now possible to produce a single engined fighter design with equivalent safety targets to a multi-engined design through mitigating various failure modes. But there are some risks that are more difficult to mitigate - e.g.large single birdstrike that multi-engined designs can be more resilient to because of their inherent redundancy.

Whereas for airliners with higher safety targets than fighters you're still after that increased redudancy to single point failures. Its difficult to ETOPS certify a single engined aircraft...

I would suggest that "Impossible" be substituted for "Difficult", since the actual translation for ETOPS is Engine Turns Or People Swim.

 

[red admiral]

I would suggest that "Impossible" be substituted for "Difficult"

Quite

 

[shin_getter]

It appears to me that:

A good baseline for fighters is putting the largest high speed/attitude engine into it. Twin engine improves payload and range but not as much in head to head combat performance. This make twin engined aircraft favored for offensive and use in very large battle areas (ocean, very large landmasses).

Twin engine passenger aircraft has been economical, the main problem was engines weren't powerful enough for some routes as larger aircraft itself is economical if the route can support it.

 

[MadRat]

Efficiency is a product of power factor. One engine running at 100% power factor is impractical if not impossible. If starting and stopping engines was practical then you could run 100% power on exactly enough engines to run optimally. (Yes, I am ignoring drag of idle engines here.) While it is true that smaller engines generally cost less, they tend to have relatively more maintenance and engine swaps within the same timeframe, negating unit cost advantages. The larger engine can therefore be cheaper in the life of the product. Being able to automate the engine work could reverse this trend in the future.

 

[helmutkohl]

Efficiency is a product of power factor. One engine running at 100% power factor is impractical if not impossible. If starting and stopping engines was practical then you could run 100% power on exactly enough engines to run optimally. (Yes, I am ignoring drag of idle engines here.) While it is true that smaller engines generally cost less, they tend to have relatively more maintenance and engine swaps within the same timeframe, negating unit cost advantages. The larger engine can therefore be cheaper in the life of the product. Being able to automate the engine work could reverse this trend in the future.

what would you consider a small and large engine examples

say 2 F404s vs 1 F110? or something like that?

 

[MadRat]

Small engine would be like a J85.

Large engine would be like an F135.

 

[1635yankee]

Single/twin safety is complicated by a) pilot training (this is probably why GA hull loss insurance rates are higher than twins than for singles) and b) single points of failure that cause both engines to stop making noise and c) where one engine fails and causes the other to fail and d) operational considerations that result in the loss of a plane regardless of the second engine still functioning (iirc, the USN won't let an aircraft with one engine inoperative land on a carrier). For helicopters, it's even messier as the data showed that a twin-turbine helicopter was more likely to have a transmission failure, forcing a landing right now than a single-turbine helicopter was to have an engine or transmission failure forcing autorotation.

One does wonder about the advantages of twin-engines in fighters, especially when at least one F-18 was lost when one engine failed and fragments from it fodded the other and when an F-18, flying from a land base (I think somewhere near San Diego), lost one engine shortly after takeoff, but there was no attempt made to land the aircraft, instead the pilot ejected (probably was directed to do so).

 

[Avimimus]

Leaving out maintenance, I personally see some appeal in three engine designs:

Upon engine-out you retain 2/3rds power - and that power is remains either symmetrical or at least half of it is still coming from the centre-line. If one can get by with less than 2/3rds power upon engine-out, then one can reduce the size of the wing-mounted engines relative to the centre-line engine... further enhancing these effects.

This means you can get by upon engine-out with a smaller rudder than typical twin leading to a slight increase in cruise efficiency. There is also the possibility of idling or turning off the centre-line engine during cruise.

Of course maintenance is king... which makes a single-engine ideal.

 

[Avimimus]

There's been a lot of claims about the function/need of how many engines are needed in an aircraft
much of it on single engined vs twin-engined.

some of the common statements are
twin-engined is preferable for maritime aircraft, thats why the US Navy went for it
single engined is more fuel efficient
twin-engined provides much more power and carry for a little more weight
twin-engined is safer to fly, etc

I am wondering how true these statements are, or how relevant it is during different eras.
For example I've also heard the claim that twin-engined were preferred due to higher failure rates in the past, but that much of it has reduced allowing for single engined aircraft
as well as statements such as.. the only reason why some designers went for twin-engined was simply because there wasnt a single engine powerful enough for that aircraft on its own.

For slower larger aircraft,
there's been a trend in seeing 4 engined aircraft being reduced to two larger engines..
and I think even a proposal to turn a large 747 to a three engined design..
yet there are some exceptions like the Japanese P-1 which is 737 sized but went with 4.

what are your thoughts

It is a widespread belief in Canada that twin-engined aircraft are superior for military uses - to the point where some of the opposition to the F-35 was related to its single-engined design. This is based on perceptions of the F-86 & F-104 loss rates relative to the CF-100, F-101, F-5 in Canadian service. So not necessarily the newest data.

The Canadian North often has less SAR assets than a CVBG, so it is more like the situation of long ranged maritime patrol aircraft. It is worth noting that the Soviet Union and Russia moved to twin-engined designs in the 1980s and haven't gone back. I believe their rationale is survivability, including combat survivability (e.g. widely spaced engines). The PVO (more than the VVS) also faces a similar patrol environment to what Canadians face (i.e. the high arctic, Siberia, and remote coastal/ocean regions).

 

[helmutkohl]

There's been a lot of claims about the function/need of how many engines are needed in an aircraft
much of it on single engined vs twin-engined.

some of the common statements are
twin-engined is preferable for maritime aircraft, thats why the US Navy went for it
single engined is more fuel efficient
twin-engined provides much more power and carry for a little more weight
twin-engined is safer to fly, etc

I am wondering how true these statements are, or how relevant it is during different eras.
For example I've also heard the claim that twin-engined were preferred due to higher failure rates in the past, but that much of it has reduced allowing for single engined aircraft
as well as statements such as.. the only reason why some designers went for twin-engined was simply because there wasnt a single engine powerful enough for that aircraft on its own.

For slower larger aircraft,
there's been a trend in seeing 4 engined aircraft being reduced to two larger engines..
and I think even a proposal to turn a large 747 to a three engined design..
yet there are some exceptions like the Japanese P-1 which is 737 sized but went with 4.

what are your thoughts

It is a widespread belief in Canada that twin-engined aircraft are superior for military uses - to the point where some of the opposition to the F-35 was related to its single-engined design. This is based on perceptions of the F-86 & F-104 loss rates relative to the CF-100, F-101, F-5 in Canadian service. So not necessarily the newest data.

The Canadian North often has less SAR assets than a CVBG, so it is more like the situation of long ranged maritime patrol aircraft. It is worth noting that the Soviet Union and Russia moved to twin-engined designs in the 1980s and haven't gone back. I believe their rationale is survivability, including combat survivability (e.g. widely spaced engines). The PVO (more than the VVS) also faces a similar patrol environment to what Canadians face (i.e. the high arctic, Siberia, and remote coastal/ocean regions).

thats the same arguments here in Japan too. twin-engines are safer to operate over a vast maritime environment. if one engine flames out or something, the aircraft can make it back with a remaining engine.

but proponents of single engine say, those loss rates of the single engined fighters of the 60s and 70s came at a time when engines were unreliable. that's changed alot and single engined aircraft (when possible), are more fuel efficient.

 

[GTX]

Let's not start the 1 engine vs 2 argument again please.

 

[Avimimus]

Let's not start the 1 engine vs 2 argument again please.

Well I tried to bring in a "vs 3 engine" argument in order to make things more interesting

 

[kaiserd]

No disrespect to contributors but this is an essentially empty discussion without being very specific about very specific contexts.

Jet engines are an order of magnitude more reliable than they were 30-40 years plus ago so reliability based arguments for more versus fewer are now, putting it charitably, far more marginal.

Otherwise a lot will come down to availability of engines of certain sizes and their relative merits and wider design considerations of the specific aircraft.

 

[Dave_sic]

The number of engines may also be related to the intended purpose of the aircraft.

Twin engine aircraft can cancel gyroscopic effects from the propellers allowing for higher stability in flight.
for example the HS129 groundattack aircraft had 2 propellers (and motors) turning in opposite directions to stabilize the plane.

two engines on the wings create a higher moment of inertia reducing the agility of the plane.
a fighter plane that has to be as agile as possible is thus more likely to have a single engine to concentrate all its mass close to the rotational axis and reduce the moment of inertia.

 

[Scott Kenny]

ETOPS, and the greatly increased reliability of engines, have really killed the 4-engine airliners. Two big fans have better fuel economy than 3-4 smaller ones, generally speaking, and fewer engines saves maintenance costs.

CFM56s are so reliable that their current inflight shutdown rate is something like once in every 500,000 hours. No, that's not a typo, I mean five hundred thousand hours between inflight shutdowns.

But in a military setting it's different. Lower bypass ratios except on transports, so the fuel economy difference is much closer between two small engines (F-18) and one larger engine (F-16). What matters there tends to be thrust, engine growth margins, and the likelihood of losing one engine.

 

[BlackBat242]

Its difficult to ETOPS certify a single engined aircraft...

As I am an old person, my automatic reaction was "Of course its difficult - single is not twin!*

But then I see the definition has changed.

*

ETOPS used to stand for Extended Twin-Engine Operations, and now is Extended Operations. Originally, it was a certification that permitted twin engine aircraft to fly routes which may, at the time, be greater than 60 minutes flying time from the nearest airport that is suitable for an emergency landing.

 

[Scott Kenny]

Its difficult to imagine a sensible single engine commercial airliner design. It would have to be in the tail, presumably, and would need to be relatively large and high in thrust.

Think small, like Cessna Caravan, or Turbo Beaver/Otters, particularly on floats.

Those work well, and there's even an electric version that has ~60-90min flight capabilites. Seattle to Vancouver/Victoria BC plus 30 minutes reserve.

 

[Nik]

I'm reminded of niche circumstances, such as that double bird-strike putting a modern twin-engine passenger jet into Hudson.
Sadly, I reckon no three or four jet design hitting that flock would have fared much better...

A three-engine design may avoid a bird-strike on the tail engine, or strike 'disassembly' may fatally compromise the tail surfaces...

However you reckon it, Dire Lord Murphy will so find a way to align those 'Swiss Cheese Slices' and bring you down...

Twin layouts: I seem to remember a 'remarkably chunky' UK ASW design with two stacked fuselage turbo-props, one being routinely shut-down to fly low, slow and lean...

And, yes, the two stacked fuselage turbo-jets of the EE Lightning. Of course, thus packed, there wasn't much room inboard for anything else. So, Radar in the big nose intake cone, missiles on rails under wing, fuel-- Not a lot !!

(OT: In mitigation, IIRC, the familiar 'Lightning' was considered a sub-scale 'experimental' design, and the RAF would get a much bigger beast, more akin to B-58. When that plan was killed, EE hastily 'militarised' what they had, despite its innate shortcomings...)

 

[Scott Kenny]

I'm reminded of niche circumstances, such as that double bird-strike putting a modern twin-engine passenger jet into Hudson.
Sadly, I reckon no three or four jet design hitting that flock would have fared much better...

Since that particular bird strike was enough to take down two widely spaced engines, I'm willing to bet that a 4 engine plane would have probably lost all 4.

The interesting question would have been a 3 engine plane, as goose formations are largely flat. I think that would have saved the #2 engine from eating a couple of geese, but as to whether the plane could have landed safely that soon after takeoff would be an interesting discussion. For values of "interesting" equal to "simulator nightmare scenario"