As most crashes are on takeoff and landing, and fire is the real killer, most often. Why not have breakaway wings? That's where the gas is. Let's assume a plane has to make a hard landing..upon touchdown/contact jettison the wings. They serve no purpose at that point and they definitely kill people. Breakaway parts/modules are employed in many arenas, so why not wings?
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2$\begingroup$ Fuel is carried in more places than just the wings for one, and think of all the equipment that would be required to jettison a wing and the fail safes involved in such a system. $\endgroup$– Ron BeyerJan 7, 2017 at 2:35
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16$\begingroup$ In all other situations, "breakaway" doesn't sound like a desirable adjective to apply to wings. $\endgroup$– DronzJan 7, 2017 at 4:23
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3$\begingroup$ What happens if your wing jettison switch is accidentally activated at FL300? $\endgroup$– jamesqfJan 7, 2017 at 6:51
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5$\begingroup$ You seem to use assumptions as if they were obvious facts, e.g. "They serve no purpose at that point" (wings help prevent the fuselage to start rolling -- how to assess if this has a positive effect?). By doing this you encourage answers using the same kind of bias/shortcut. So at this stage you are asking for opinions and I'm VTC for this reason, but you can improve the question to remove biases. Ben's answer shows where they are. $\endgroup$– minsJan 7, 2017 at 12:11
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3$\begingroup$ Before I VtC, I would ask the OP: will you accept any answer that answers your question, as asked: "what reasons exist for..."? Or are you really so biased towards your belief that breakaway wings are the panacea you seem to believe they are that no answer will suffice? $\endgroup$– CGCampbellJan 7, 2017 at 15:27
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4 Answers
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I can't think of an accident scenario where this might help. If you're hitting the ground hard enough to crash but have survivors, you're looking at an accident like TK 1951, where not only was the wreckage all in pretty much the same area, but it didn't even catch on fire. Aviation fuel is not as volatile as movies make it seem.
Maybe you are thinking of something like AF 358 which overran the end of the runway and burst into flames? Despite that fire everyone survived, but what if they realised it was going to overrun and jettisoned the wings whilst still on the runway?
Well the biggest problem to me is how to slow the aircraft down without wings. At higher speeds, the wheel brakes are ineffective without the use of ground spoilers. Without the reverse thrust from the engines you are guaranteeing that the aircraft will be going faster when it leaves the runway, leading to a much higher chance of casualties.
No piece of engineering is perfect either - what happens when it fails and the wings accidentally detach when everything is going well? What if they are flying in severe turbulence and the sensors detect high G forces and confuses that for a hard landing? Or if it a manual jettison, what if the pilots accidentally pull it, or a suicidal pilot does it while the other is looking the other way?
There are many other problems too, such as how some aircraft are designed with the gear support in the wings, that some have large fuel tanks in the fuselage, and the extra weight and cost of getting such a system certified. Ultimately this suggestion falls into the same bucket as every other drastic safety idea - in trying to fix a problem that barely exists, it creates a lot more in its place.
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$\begingroup$ You are right. Crashes are rare. Fixing a non problem makes no sense. However, there are dozens, maybe a hundred crashes where the plane gets down and bursts into fire due to fuel from the wings. I'm not suggesting it's a perfect solution, I'm stating that wings=fuel=fire=death .....wings break off, more people live. As to the mechanics, fixed wing aircraft come in many configurations, some could be modified some not. Future aircraft could adopt this idea. $\endgroup$ Jan 7, 2017 at 8:58
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5$\begingroup$ @Thatchweave the answer highlights the quantities of problems in regards to the (almost null) benefit. Making the fuel tank more resistant (antideflagration materials, separations inside tanks,...) is quite more effective and rise fewer problems (almost only extra weight and extra pumps). $\endgroup$– Manu HJan 7, 2017 at 11:07
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1$\begingroup$ Best most concise answer. I understood this all along ..I wanted to see feedback. $\endgroup$ Jan 8, 2017 at 2:59
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$\begingroup$ Nitpick: the main reason the wheelbrakes are ineffective at high airspeeds without the use of ground spoilers is because the wings produce lift which takes the aircraft's weight off the landing gear and limits its ability to slow the aircraft. Without wings, that particular issue would be no more (although many others would arise). $\endgroup$– VikkiMar 26, 2021 at 23:12
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"It's not complicated."
As with most things in aerospace engineering, it is actually a lot more complicated than someone not familiar with aircraft design might think. There are a number of reasons why this isn't done:
Wing Spars
Wings have to be really strong. They literally bear (almost) the entire weight of the aircraft in normal 1 G flight and multiples of the weight of the aircraft when maneuvering. For example, in a 60 degree bank while maintaining altitude, the load on the wings is twice the weight of the aircraft. Of course, airliners don't normally do 60 degree banks, but they're designed to be able to withstand loads much higher than what they would normally experience on a routine flight.
In order to keep the wings from separating from the aircraft under such loads, they're designed with a single, very strong structure called a wing spar that runs most or all of the wing span. This is not one spar per wing, but rather a single spar that runs from one wingtip, through the fuselage, to the other wingtip. This spar is responsible for holding up the weight of the fuselage during flight and the weight of the wings (and, depending on where the gear is located, possibly also the fuselage) while on the ground. Using a single structure running the entire span for the spar is the safest and most efficient way to bear the needed loads, as it removes any connections that would otherwise be possible points of failure.
By design, the spar is really hard to break. Separating the wings from the aircraft would require breaking the spar. This would require enormous amounts of force and/or heat. The systems to supply that force and/or heat would be heavy and also, by design, rather dangerous. Neither of these is a good thing for an airplane.
Main spar of a de Havilland DH.60 Moth Source: Wikipedia
Landing Roll
It turns out that the wings (especially of an airliner) are actually more useful than you might think during the landing roll.
During a landing, the primary methods of slowing down are wheel brakes, engine reverse thrust, and aerodynamic drag. All three of these, at least to some extent, depend on the wings being attached to the aircraft. The engines, flaps, and spoilers are all usually mounted to the wings. The wing flaps and spoilers aid in providing large amounts of aerodynamic drag during the landing roll. The engines also require fuel flow and, as you've noted, that usually comes primarily from the wing. The wheel brakes might have some effect without the wings, but it would be greatly diminished. Braking action requires weight on the wheels. A lot of this weight comes from the weight of the wing structures and more comes from the downward aerodynamic forces provided by the spoilers.
Attitude Control
During the landing roll, the wings are still aiding quite a bit in controlling the attitude of the aircraft. In particular, the ailerons are still effective during the high-speed parts of the landing roll and are used during that time to keep the aircraft level in the roll axis. Additionally, a tall and very long tricycle, by itself, isn't a terribly stable vehicle. Especially at high speeds, it would be more prone to tipping over without the wings attached.
Landing Gear
Take a look at the main landing gear on these aircraft:
Boeing 777 Source: Wikipedia
Boeing 757 Source: Wikipedia
Boeing 737 Source: Wikipedia
Airbus A330 Source: Wikipedia
As you will notice in all of these pictures, the main landing gear are attached to the wings.
It's very common in aircraft design for the main landing gear to be attached to the wings. This makes sense as the wing root is structurally a good place from which to support the weight of the aircraft and also happens to provide a nice, wide wheel base to keep the aircraft more stable on the ground.
If the wing separates from the aircraft at touchdown, the main landing gear goes with the wings. As you can imagine, this creates lots of problems for a landing roll. In addition to the loss of wheel brakes, you are now skidding down the runway on the bottom of the fuselage instead of rolling down it on wheels. Hopefully I don't need to go into the reasons why this is not considered to be desirable.
Risk of Failure
Any system can fail. If one of these systems accidentally activated (either by accidental command from the flight deck or by failure of the system,) losing one's wings while in flight is a rather bad outcome. Given that flights needing wings are extremely common and flights crash landing in such a way that jettisoning the fuel tanks would be desirable are extremely uncommon, having such a system around is probably more likely to kill someone than save someone.
Burning Fuel
While one might think that jet fuel is easy to burn, it actually turns out that it isn't. In fact, it's rather difficult to ignite it. This is intentional in order to minimize the chance of the fuel igniting during an accident. Even in the rather violent crash of Asiana 214 and the large post-crash fire, the fuel never ignited (see first paragraph of page 37.) So, the risk being mitigated here might not actually be as large as one might think.
Note, however, that avgas (used commonly in light, piston-engine aircraft) is quite different from jet fuel. Avgas is similar to normal car gasoline and will burn rather easily.
Cost/Benefit Ratio
As previously mentioned, a system capable of quickly breaking the main wing spar would be heavy. It would be expensive to design to be safe. It would be expensive to build it. It would also be expensive to carry all of that extra weight around on millions upon millions of flights per year. All of this cost and added safety risk is not deemed to be worthwhile to provide a system that is rather unlikely to ever be useful.
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$\begingroup$ Re Attitude Control, anyone ever landed in a good crosswind? $\endgroup$– jamesqfJan 7, 2017 at 19:14
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Just wanted to add that if wings were to be 'disengaged' when needed/emergency-case, the aeroplane-fuselage or a 'metal Tube' now will fall faster than a human would free fall, there will be nothing to slow it down (assuming pilot/autoPilot can still control wing-control-surfaces or whatever its called).
If the aeroplane is very close to ground or on ground and moving, discarding the wings now means throwing them away with undercarriage, the wings would most likely tumble and continue to move with the plane and may be even into it.
But if the aeroplane is on ground and not moving and then the wings are disconnected, means the Plane still has wings next to it, but now the fuselage is below it. The fuselage could roll and make it difficult to evacuate.
Also, lot of aviation fuel (30% or more) is in Central tank + ACT. i.e. in fuselage.
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$\begingroup$ Why would it fall faster than a human would fall? It should actually be slower because of the air resistance working on a larger body. Neglecting air resistance two bodies will fall at exactly the same rate (9.8m/s/s or so depending where on the planet you are), be it a bowling ball or a feather. Terminal velocity happens when the upward force of the air resistance equals the downward force of gravity resulting in zero additional acceleration. $\endgroup$ Aug 30, 2017 at 23:16
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1$\begingroup$ @RonBeyer Because aircraft fuselage is designed to have air resistance as small as possible. Unlike human body. $\endgroup$– Agent_LAug 31, 2017 at 10:41
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The early 310s with "tuna tanks" were designed with exactly this scenario in mind. The idea was that the tanks would keep the fuel as far from the cabin as possible and shear off in a crash. Cessna abandoned this idea some years later when they incorporated aux tanks into the wings (310L) and then later into the nacelles (310N).
I do not know if data exists to quantify the effectiveness of this design feature.
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1$\begingroup$ I suppose I was referring or intending to refer to commercial airliners. None the less, the concept doesn't seem that far fetched. $\endgroup$ Jan 7, 2017 at 2:57
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$\begingroup$ The tip tank placement addressed a problem which was not related to the biggest killer in twins - engine failure and asymmetric thrust loading at low speeds causing departure from controlled flight. $\endgroup$ Jan 7, 2017 at 6:52
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1$\begingroup$ I don't think this actually answers the question. Although I'm not going to D/V it, also no U/V. $\endgroup$ Jan 7, 2017 at 15:26