Originally posted by robbie carrobie
why does everything get pulverised when an aeroplane crashes? Is it a transference of energy when the aeroplane meets something more dense than itself, like a mountain or a tall building? Can anyone explain it in really clear and simple terms? Also why can aircraft flight not be made more safe as in providing parachutes for passengers or protective airbags that go off on impact?
It depends on how the crash occurs.
Planes can crash and remain almost completely intact.
When you have a plane impacting a mountain at full flight speed and descending
at over 3000 feet per minute it's a different story.
Without the details from the flight recorders, which have not yet been released,
we can't know for certain how fast this plane was travelling.
However with the info we do have we would expect that the plane was travelling
between 400 and 550 mph on impact. [180 to 246m/s] [other aircraft can get over 600mph]
An A320 is ~ 40m in length, and so this plane was going 3.5 to 6.15 times it's own length
per second when it hit the ground.
An A320 will have a mass somewhere between 40 and 70 tons with the likely weight
at impact here being 60~65 tons.
60,000kg at 180m/s has a kinetic energy of 972 Million Jules.
And at 270m/s it has a kinetic energy of 1,815 million Jules.
That's the equivalent of 230 to 435 kg TNT [500 to 960 lbs TNT]
See this BBC picture of a 1000lb* WWII bomb being detonated for reference
*That's 1000lb total weight including casing, explosives probably ~60 to 70% of that
Rocks don't bend much, so pretty much all that energy goes into the airframe.
Aircraft are made to be incredibly light, this plane has an airframe weight of only 39.5 tons.
It achieves this by vary cleverly spreading the load across a large expanse of very thin
sheets of materiel [commonly aluminium alloy/glass fibre composite, or carbon-fibre composite]
where the very shape of the materiel gives it it strength.
When you smack into rock at 400~550mph that materiel is bent out of shape.
The thin sheets of materiel are torn apart at the seams. And again much of the interior is
made from lightweight plastics and thin sheets of insulation materiel.
When you sit in a plane and look at the apparently thick hull, what you are seeing is plastic cladding
over layers of thermal and acoustic insulation materiel and then a few mm of actual structural
airframe. Much of it is light enough to blow on the wind.
Also, as the front of the plane smashes into the ground it will come to a near instantaneous stop,
but the back of the plane will still be going at full speed, so the plane will compress causing a
pressure build-up inside the plane that will blow it apart...
And that's before you thrown in the possibility of the fuel exploding.
Also, if you have any object disintegrating on impact, you will get bit's spread over a large area
as the energy is not uniformly distributed through the debris.
You can see this if you throw a composite object like a muffin at the ground.
You will see most of the debris closely centred on the impact site, but other bits will have been
scattered far and wide.
Also why can aircraft flight not be made more safe as in providing parachutes for passengers
or protective airbags that go off on impact?
Aircraft are very safe, you are much more likely to die driving to the airport than you are to die on
your flight. In fact the excess deaths from people driving instead of flying in the months after 9:11
were considerably larger than the number of people actually killed in the attacks.
The A320 has a fatal accident rate of ~ 0.14 per million take-offs** which is pretty damn safe.
However, the air industry achieves numbers like that by constantly driving to make things safer.
Air bags can help for certain seats, in certain kinds of accidents.
They would have done nothing in this instance however.
Having a 1 foot thick bag of air to slow you down from 4~6 hundred miles an hour to zero is not going to work.
As for parachutes... it takes skill to safely parachute down to the ground, and most of the time on occasions
where you might want to jump out of the plane, you can't or don't have time.
One of the leading suspects for this crash is cabin decompression causing people to pass out... you can't
bail out if you're unconscious.
The recommendations for making flying safer in the wake of this crash are going to depend on what caused it.
But if it was decompression then better systems for detecting decompression and possibly having the autopilot
auto evade obstacles [like mountains] in the event of pilot incapacitation. Also, possible safety recommendations
to avoid decompressions in the first place depending on what caused the decompression. As well as investigating
why the pilots didn't put their oxygen masks on, or why they didn't work if they did...
But it all depends on what actually caused the crash... Which we probably wont know for months.