Aviation dates back to the 18th century, and since then tremendous research has been put into making aircraft safer and more efficient. Though these efforts have resulted in better aircraft, why are they still not strong enough to keep the passengers alive in case of a fiery crash?

In recent times, especially, it's a rare incident that a plane crashed and even some of its passengers survived. So, why has this issue not yet been resolved?

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    more strength means more weight, too much weight and you won't fly today. – ratchet freak Jul 2 '15 at 16:05
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    You have this the wrong way round; its a rare incident when there is a plane crash and there are zero survivors! planecrashinfo.com/cause.htm – Jamiec Jul 2 '15 at 16:19
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    Err, 17th century? The Montgolfier brothers didn't fly the first balloon until the late late 18th (1783). – egid Jul 2 '15 at 16:45
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    ..it's even more rare for passengers to be killed by fire in a crash. The crash itself, that leads to the fire, has already killed them. To build any machine capable of withstanding the forces involved in the kind of crash you are talking about, as others had said, simply would not be able to fly. Why haven't we built cars that will withstand any crash? Or trains, or boats, or pedestrians surrounded with steel cages and impact absorbers? – Simon Jul 2 '15 at 17:00
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    Humans have been walking for a looooooong time, yet they still trip and break limbs. You want 100% protection for them while they're encased in an aluminum tube buzzing along at 400+MPH 5 miles in the sky? – FreeMan Jul 2 '15 at 18:51
up vote 149 down vote accepted

Short answer

The kinetic energy involved in a plane crash is inhumanely high.

Slightly longer answer

We can built bombs which will go through concrete roofs and ceilings of a bunker, counting the number of floors they crash through while descending so they can explode at the level where the bad guys sit and not where the widows and orphans are kept. We could equally well build a plane with this kind of strength, so the fuselage stays intact even when it crashes into a mountainside. That's not the problem.

Limits of the human body

The problem will be that rescuers will only find dead bodies inside. The human body was "designed" to endure things like running into a tree, but not being hurled around at Mach 0.85 and then being stopped almost instantly. Thanks to years of research we now have quite a good idea where the limits are. Martin Eiband collected a lot of data on this, if you want to know more, google for "Eiband diagrams". If you want the full picture, read the Army Aircraft Crash Survival Guide. It comes in five volumes, and volume 1 covers the design criteria. The Eiband diagram below is lifted from this source. Eiband diagram for lying human

Note the time scale: A deceleration with 40 g can only be tolerated for a duration of 0.1 seconds; if the deceleration takes more than 1 second, the limit is only 10 g. Now let's see what deceleration distance is required to stop a human with an average deceleration of 10 g. The energy $E$ of a body of the mass $m$ increases with the square of speed $v$: $$E = m\cdot\frac{v^2}{2} = m\cdot a\cdot s$$ At $a = 98.0665$ m/s², the stopping distance $s$ from an initial speed $v$ is $\frac{v^2}{2\cdot a}$:

  • Car crash at 30 m/s: This needs 4.6 m and good restraints, but is generally survivable.
  • Free fall at 60 m/s terminal velocity: 18.4 m. A few humans have indeed survived this by falling into soft ground like a snow-covered conifer forest. This is also the typical approach speed of airliners, and the 18.4 m is the fuselage ahead of you. That is the reason why a crash is more survivable for the occupants in the rear seat rows.
  • Propeller airplane flies into mountain (120 m/s): 73.42 m. This kind of crumple zone is simply not available, and nobody has survived such an impact.
  • Airliner flies into a mountain (240 m/s): 293.7 m. To make such a crash survivable, every airliner would need to carry a 300 m boom of stiff material around which would be needed as a crumple zone in an accident. Just think what kind of aft fuselage and tail this would require.
  • And to top it: Astronaut on a space walk collides with a satellite in an opposing orbit (16,000 m/s): 1305.23 km. Note that I had to switch units to keep the number manageable.

Please consider lower deceleration limits for elderly and untrained persons; the limit in the Eiband diagram was established using healthy young pilots (and hogs, chimps and corpses for the higher limits. A lot of blood was spilled to arrive at these numbers).

The problem is not the aircraft structure, it is the fact that humans like to go fast but are not built to stop quickly.

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    That last sentence sums it up very nicely. One to remind. – DeltaLima Jul 2 '15 at 20:46
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    @egid: Hard to say, but most likely John Stapp. The short time makes it more an acceleration spike, not continuous acceleration. If this kind of acceleration takes more than a tenths of a second, blood vessels may rupture internally. After suffering 200 g for 0.1 sec, the corpses look outwardly intact, but are a mess inside. – Peter Kämpf Jul 2 '15 at 21:31
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    Basically, to make all crashes survivable to the majority of people we'd have to pass a law forcing all planes to fly no faster than 44mph. Not only would most modern planes not be able to take off (indeed most WW1 biplanes would stall and crash at that speed) but most people would simply take the bus in that case. – slebetman Jul 3 '15 at 4:02
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    @Lohoris: The other way is never to speed up in the first place. Once you accelerate beyond a speed from which a human can be stopped safely, there will always be a nonzero risk - however small - that deceleration is lethal. – Peter Kämpf Jul 3 '15 at 13:25
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    @PeterKämpf a fictional story on that lines from awhile back: Orbit -- I'm not sure at all about its accuracy, but it would give something to work from. – user2896 Jul 3 '15 at 15:39

The principal reason plane crashes are less survivable, which nobody really seems to fully grasp when talking about airliners, is the sheer amount of energy inherent in a commercial airplane. When you watch a plane coming in for approach, especially a big one like a 747 or A380, it usually seems very docile, with the plane very slowly and gently approaching the runway. The other classic image is the plane cruising at high altitude, perhaps leaving a contrail behind it as it slowly tracks across the sky. We compare these images from our experience to images of cars zooming past us along a busy road (or a racetrack). We then watch racecar drivers walk away from spectacular wrecks, while plane crashes kill everyone aboard, and we wonder why planes can't be made as safe as racecars (or even ordinary passenger cars).

That image of the docile aircraft traversing the friendly skies, however, is a forced perspective caused by a much bigger object much further away from us, and belies the fact that dozens or even hundreds of tons of weight is moving up to three times faster than an Indy car has even been clocked.

Basic projectile physics; $E = \frac{1}{2}mv^2$. The car in your driveway, if typical, has a "curb weight" (empty tank but otherwise ready to drive) of about a ton and a half, and cruises at speeds between 30 and 70 mph. Converting mph to fps (multiply by 5280, divide by 3600), the energy, in foot-pounds, of a 3000lb car at a freeway speed of 60mph is about 23 million foot-pounds, plus the additional kinetic energy of driver, passengers and cargo. In a collision, this energy is transferred wherever it will go; the object being collided with, the frame of the car, its occupants, etc. Even at these speeds, a collision can permanently injure or kill someone inside (and a full-speed collision on the highway is more often fatal than not).

A typical airliner, say the B737-700 which is in common use in the U.S. domestic fleet, has an "operating empty mass" (similar to "curb weight" in cars; everything needed to fly except the fuel and flight crew) of about 40 tons. So right there the potential energy of the airliner is 30-40 times the car. It also takes off and lands at roughly 125-150mph, and cruises at up to Mach 0.78, which at 30,000 ft is about 525mph. So, we're also talking about an order of magnitude difference in velocity, and that increases total energy on the square. Doing the math, an airliner at cruising speed, not counting the energy inherent in its cargo or passengers, will have a total kinetic energy somewhere on the order of 50 billion foot-pounds. Even with all other things being equal, such as the distance allowed for deceleration and the distribution of impact forces to the passengers, a passenger in a plane crash would be subjected to more than ten times the forces they would in a car crash.

Now, all these things can be mitigated in both cases. These numbers more or less compare what a passenger in a car vs a plane would go through if the vehicle plowed head-on into an immovable obstruction at full speed. That doesn't happen often in either case; highways are built in part to minimize the chance a driver will ever face a barrier head-on, and drivers can usually hit the brakes to slow the car and steer to hit in an oblique direction, and even if that won't prevent an impact it lessens the severity of it by the square of the change in relative speed between the car and what it's hitting.

Similarly, a CFIT (Controlled Flight Into Terrain) is pretty much the worst case scenario for a plane crash (the only worse one I can think of being a midair collision which is extremely rare especially for airliners), and there are a lot of systems aboard the aircraft to help a pilot realize he's about to do that. A crash landing, such as a belly landing due to hydraulic failure, is usually more survivable because the pilot is doing everything he can to minimize the force of impact and the plane's total kinetic energy, by both slowing the plane's forward velocity and reducing the glide slope. The plane's remaining kinetic energy can then be spent skidding down the runway or over the field instead of being imparted directly into the aircraft's frame and ultimately its passengers.

However, that's still a lot of energy for the plane to get rid of, and even with the inherent weight of an airliner, the ability to fly is favored by designers over keeping the cabin in one piece in a crash. That means that the inherently higher risk to life and limb of flying must be mitigated by keeping the planes well-maintained, and putting well-trained, experienced, healthy flight crews in them. Neither can be said for the average car and driver plucked off the street; only the most severe medical conditions are grounds for revocation of a driver's license, while most cars are driven thousands of miles past scheduled maintenance intervals. Cars, therefore, must be designed and built to keep the occupants alive in a collision, despite the ability or even the intentions of the driver. A plane's safety features are only useful when the pilot is doing his job properly; an oxygen mask or even an escape hatch is useless in a CFIT.

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    Safety in terms of deaths/injuries per passenger mile is actually a bit misleading, because planes cheat the calculation by logging up a lot of passenger miles quickly. The basic reason for safety of air travel versus anything else is the number of people in control as a percentage of the total number of people at risk. An airliner has two people in control of a plane carrying two hundred, and those two trained half their life to do that. In the U.S. there are 800 vehicles for every 1000 people so on average you can expect 1.25 people per car. That's way too many people making dumb choices. – KeithS Jul 2 '15 at 19:19
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    @KeithS While I agree that limiting the chances for people to do stupid stuff is the primary reason that airliner travel is more safe than motor vehicle travel, that still doesn't change the fact that it's true. Of course, there is also a second important reason: for all but the first few minutes and last few minutes of a flight, there's usually nothing nearby that can be crashed in to. While cars operate several inches to a few feet from the nearest obstacles, airliners spend most of their time miles from the nearest obstacle - in all three dimensions. – reirab Jul 3 '15 at 1:41
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    SI units, please. :-) – RoboKaren Jul 3 '15 at 6:27
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    @KeithS - NIce analysis from an energy perspective... How do I know you are American? Because of all foot-pounds and bushels per groat? - no. Because you spelt kerb, curb? - no. It's because you think an average car weighs a ton and a half! – Oscar Bravo Jul 3 '15 at 9:24
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    the ability to fly is favored by designers over keeping the cabin in one piece in a crash - nice risk assessment by the designers. – Mindwin Jul 3 '15 at 18:18

The question is really a cost benefit analysis of assumed risk. You could fly planes with passengers that had full nomex suits on, a parachute, a back up chute, a life jacket, self deploying life rafts full of food and other neat survival gear. The plane could have a full frame chute, a steel roll cage and the best impact protection available. But all of this adds weight to the plane and thus reduces how many people you can fit. In turn you make less money per flight since its counter productive for flights to be impractically expensive no matter how safe. In the end of the day you alone can not move as far or as fast as a commercial plane nor do you have the resources to make an almost perfectly safe plane. So you compromise and assume risk for the reward of moving fast and semi efficiently.

On the contrary it should be noted that some people do have the resources to fly fast and safe. If you had the money to buy a small (or even big) plane you are free to outfit it (with in legal and practical/physical limits) as you like. This could include what ever protection you may desire from what ever emergencies you can think of.

One last note: its usually the emergencies you cant think of that are the real issues...

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    All this is true, and it still won't protect you from all possible contingencies. Flying into terrain at high speed will still cause fatalities for example. – GdD Jul 2 '15 at 16:22
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    In the words of Jeremy Clarkson "Speed does not kill, Sudden Deceleration does" – Dave Jul 2 '15 at 18:22
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    @GdD Not to be too pedantic (like James May), but you could account for that by making the flights go a lot slower. In fact all dangers could probably be nullified if we took extreme measures to do so. But, again, I don't think people would want to take these measures because it nullifies to much of the benefits of air travel. People are okay with the current assumed risk, so they buy tickets... – Jay Carr Jul 3 '15 at 1:40
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    @PieterGeerkens Again, it's trade offs...but I continue to be pedantic. You have to disregard, firstly, what we assume a plane to be. Payne Stewart's plane would have been fine if they'd never gone above 10,000, and AF447 wouldn't have happened if people never flew through storms or at night. But again, these are risks we are willing to take for the benefits they give us. And that's the point being made here. All risks can be mitigated. But achieving that would dilute the benefits so much...it's just not worth it. We trade a small chance of death for a major benefit in mobility. – Jay Carr Jul 4 '15 at 0:02
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    @JayCarr "you could account for that by making the flights go a lot slower." No you couldn't. A car crash at 60mph is already likely to cause fatalities. Airliners have to go twice that fast just to stay in the sky. – David Richerby Jul 5 '15 at 5:46

Though these efforts have resulted in better aircraft, even then why are they not strong enough to keep the passengers alive in case of a fiery crash?

A fiery crash poses many challenges:

Physical

The occupants of the airplane are subject to a high acceleration the moment the airplane makes contact with the terrain. The human body can only sustain a dozen of g-force before sustaining internal damage.

Considering an airplane impacting horizontal terrain with a vertical speed of 1000 ft/min (5 m/s), and an airplane which the cargo space deforms one meter: going from 5 to zero m/s in the distance of 1 meter already results in a acceleration of 12.5 g (5 to zero m/s in 0.4 s) which is barely survivable.

Fire and fumes

An fiery crash would most likely rupture the fuel tanks, spilling the remaining fuel on-board and causing a fire which would release fumes rapidly incapacitating passengers.

Search and rescue deployment

As airplanes fly routes which have no connection with the ground road network, the time required by the search-and-rescue team to locate and reach the crash site is way too long for saving passengers requiring immediate medical help.

In recent times, especially, it's a rare incident that a plane crashed and even some of its passengers survived. So, why has this issue not yet been resolved?

Road accidents, were of the above three factors only the physical one can be considered, can already result in severe injuries and death.

With airplanes, traveling at a speed faster by one order of magnitude, it is easy to imagine that the consequences of a collision with the terrain are far more dramatic.

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    That last sentence is especially true considering that a speed increase of one order of magnitude yields a kinetic energy increase of two orders of magnitude. That's a lot of energy to dissipate very quickly. It's gotta go somewhere. Still, though, fuel ignition isn't all that common (in jets, at least) unless they really are flown straight into terrain. Despite the large fire that eventually burned up the plane after the Asiana SFO crash, for example, IIRC, the fuel never burned (an engine fire started the fire.) – reirab Jul 2 '15 at 19:04
  • If the cargo hold alone deforms one meter, the total stopping distance for the passengers is going to be more like two meters or so (at least), because of the give in the other parts of the aircraft between the ground and the passengers. – Sean Apr 14 at 2:01

Because there is only so much acceleration and temperature a human body can survive.

The other answers provide detailed explanations how immense the energies of a crash can get, and how expensive it can get when you carry much less passengers because of the space needed by all the extra safety features.

However, there is another issue: the rarity of emergencies coupled with the probability of a correct decision regarding the case of an emergency.

Let's assume that money would not be an issue, and that we could install some very powerful systems which can increase the number of survivors in a crash, something like installing ejection seats for the passengers, full-frame parachutes or retro-rockets to slow down the aircraft or other far-fetched solutions like wrapping the whole aircraft explosively into a big bubble of some exotic material. These active countermeasures need to be very quickly deployable, so they would need to be triggered explosively. Even these solutions would not save everyone: for example, with ejection seats in military aircraft, there is an approximately 30% chance of receiving lasting injuries, and 10% chance of not surviving at all. With untrained passengers who are on average much less fit than fighter pilots, the survival rate would be lower.

However, you might say that if these countermeasures could save even just a few people, they are still better than everyone dieing in the crash? Wrong! We have to consider the likelihood of these countermeasures activating accidentally when there is no emergency at all! Not even counting cases where there is an emergency, but trying to ditch the aircraft onto a field or into a river might save more lives than activating the countermeasures.

The odds of being on an airline flight which results in at least one fatality are 1 in 3.4 million, and this counts even cases where the majority of the passengers survived. As the decision of activating the countermeasures has to be made at least once a couple of minutes (or maybe seconds) otherwise it would be too late, and the average airliner flight lasts between 3 to 6 hours, we have additionally at least 2 more orders of magnitude. This means that if you can make a correct decision about activating the emergency countermeasures with an accuracy of less than 99.999999997%, you will have more cases when they activate in a perfectly ordinary flight than in an emergency. Such an accuracy can not be expected from any decision making process, as accidents can have a wide variety of causes and are influenced by many factors from weather to mechanical failures to human psychology. As you can not even get near such an accuracy, such a system would likely kill many thousands times more passengers by activating when it shouldn't, than how many people it could save in actual emergencies.

As it has been said, there is a lot of "cost" and "weight" in the reasons behind this. For small airplanes you have plane-parachute for example but how to make a suck system working for a 200-tons plane flying at 800km/h full of people ? There are real technical challenges behind this question.

Choices have been made to reduce the probability of a crash instead of adding some stuff to be crash-proof : electronics and hydraulics systems are redundant, emergency procedures, collision avoidance system, etc.

You also have to take into account that civil aviation is not fast evolving : adding some new technology requires long time to be tested, validated, and a good reason to add it. The usual flow of event in this case is : crash -> investiguation -> correct what's wrong -> wait for next crash etc...

  • Also people want other stuff like being awake- more air , eating, entertainment- that's priority over safety. – user2617804 Jul 5 '15 at 6:04
  • @user2617804 Given how many people complain about how uncomfortable and boring flying is and how bad the food tastes, compared to how many people complain about being in plane crashes, your statement is rather silly. – David Richerby Jul 5 '15 at 10:05

Fire is lethal, in a crash and otherwise

It's people who keep the passengers alive

To answer your question directly, you survive a "fiery crash" by getting out of the aircraft, which is done by emergency evacuation. For example, a fiery crash in Dubai recently resulted in zero casualties. In contrast, the crash ofSwissair 111 all on board perished; a fire in flight turned from a bad situation into a lethal one. The evacuation is run by the cabin crew, people who are trained on how to get people out of a crashed aircraft.

Fire is a seriously lethal problem to have, be it on a ship at sea, on an aircraft in flight, or after a crash.

For that matter, if you have a fire in your home you'll die if you don't get out, and that's without "a fiery crash." (My wife's best friend lost her mom to a house fire: mom was asleep when fire started ... RIP.)

If a plane crashes, and it catches fire, and the fire can't be extinguished, and if you can't evacuate, you will burn and die.

A great deal of money, time and effort goes into accident prevention, an iterative process since the dawn of commercial aviation. Improvements in the ability to evacuate in the case of accident or malfunction are included.

A great many other accident prevention systems have be put into place over the past century, which has paid off over time with the following goal: don't have the "fiery crash" in the first place.

An ounce of prevention trumps a ton of cure.

As for your answering the question about a 'fiery crash', NASA performed a test in the 1970s using a 720 filled with fuel formulated to reduce the chance of causing a fire. I recall seeing the footage on science programs. Unfortunately the fuel did ignite.

To quote the referenced Wikipedia article "The test resulted in a finding that the antimisting kerosene test fuel was insufficiently beneficial, and that several changes to equipment in the passenger compartment of aircraft were needed"

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