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?
The kinetic energy involved in a plane crash is inhumanely high.
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.
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.
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 $\frac{v^2}{2\cdot a}$:
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.
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.
the ability to fly is favored by designers over keeping the cabin in one piece in a crash - nice risk assessment by the designers.
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Commented
Jul 3, 2015 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...
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.
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...
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"
