This has always been one of those questions that has bugged me as being obvious on the surface, but less clear as it is probed a bit deeper.
I assume it is some sort of blunt trauma to the head or chest which causes internal bleeding or brain swelling, something like that. Not really looking for the gore factor, just wondering what the most common mechanism is. And yes, I realize that all death is ultimately due to hypoxia.
In some cases, it is a little more gruesome than that. I recall a airline crash in Buena Park, CA in which 900 pounds of unidentifiable human tissue was recovered and ultimately disposed of by incineration. In that case, the crash was so violent, the victims bodies disintegrated, which is certain to cause death. I suspect something similar happened to the passengers on the 9/11 plane that nose-dived into the Pennsylvania field.
It really depends on the type of crash.
A high-velocity plane crash (like 2 planes colliding mid-flight, or a plane smashing into the ground or a mountainside at full speed) often causes the fragmentation that Fear Itself mentioned. What happens here is that the impact is so forceful that the plane, and everything in it, just sort of breaks up into pieces, and scatters all over the place. (This is generally the case with explosives in cargo holds as well.) We had a plane crash here in Canberra a few years ago where 2 passengers ended up resulting in over 250 body parts.
A low-velocity plane crash occurs generally during take-off or landing procedures. These are the most common types of plane crashes. Because the plane is not moving as fast, the collision does not produce severe fragmentation. Since the plane is largely in one piece, spillage of fuel from the fuel tanks can easily be ignited by the hot engines. These plane crashes often result in extremely hot fires. There can be some fragmentation of bodies in these crashes, but for the most part the cause of death would be blunt trauma, carbon monoxide poisoning, and fire damage.
I think all of the Sept. 11th plane crashes are high-velocity; the fires in the twin towers probably happened because the plane was partially confined inside the building and couldn’t break up as quickly or spread out as much as a high-velocity crash normally would, which allowed the fuel and the hot engines to mingle.
[This information all taken from lecture notes given during a Forensic Odontology module.]
Which begs the question, are the teeth salvagable for identification in high speed crashes?
A forensic odontologist would tell you, teeth are the best tool for identification in ANY incident! 
Teeth are in fact salvageable in high-speed crash situations, and relatively easy to use because the plane’s manifest tells you who’s on board, you just need to get their dental records. Unfortunately, the high degree of fragmentation means you can only positively identify those bits of the body that have teeth still attached. With the plane crash in Canberra I’ve mentioned, it was relatively easy to work out which of the 2 passengers’ jaw segment was hanging from the pine tree, but there were probably 230 body parts that couldn’t be identified using dental means at all. These would be identified using DNA.
Search is your friend - we did this back in August:
Question about airplane crash fatalities (morbid)
(Granted that in this case search wasn’t super-friendly)
I’ve seen raw footage of a plane crash in Kenya where everyone on board basically burned to death. Bodies were quite recognizable as bodies, and weren’t torn to shreds or anything like that.
That’s pretty typical of a low-velocity crash. Do you recall the cause of the crash?
I think the generic term would be massive trauma.
Or, as I liked to call it back when I was running rescue, Total Body Crunch.
In crash testing of small aircraft, NTSB found an unexpected mechanism of injury for low-speed nose-down crashes that don’t result in fire: A “wrinkle” can form in the top skin of the fuselage, and sort of “rolls” down the length of the fuselage as the aircraft as it slams into the ground. Think of the behavior of a sheet as you snap it out over a bed whilst making it - a ripple travels the length of the sheet.
Well - In small aircraft, this wrinkle or ripple frequently reaches far enough into the cabin to crush or remove the heads of the occupants as it travels down the cabin. They got it on video, and the ripple pretty much destroyed the heads of the crash-test dummies in the aircraft.
A little off topic. But weren’t the space shuttle astronauts literally torn apart piece by piece because of the well designed cockpit? 
Which shuttle - Challenger, Columbia or both?
Columbia. The Challenger was a violent explosion and the bodies were found intact. If I remember correctly there was some spectulation that the some of the Challenger crew may have eventually died by drowning. There again, because the cockpit was design so strong that even though they were critically injured and would have died no matter what it kept them alive long enough for them to drown. Not sure if that was ever fully investagated, maybe just speculated.
Basic law of physics.
When a moving object stops suddenly anything in/on the object keeps moving.
Until impact with a solid surface that’s not moving.
So, the cause of death in a airplane crash is impact with the gound regardless how one reaches that point…
I would have to agree that most of the people that die in a normal plane crash (isn’t that a strange term) die from the impact into the ground. Even takeoff and landing speeds (let alone the altitude drop) are a couple of magnitudes above a car crash.
Heart stopped.
Heart was previously moving 495 MPH and then suddenly it wasn’t.
In medium speed crashes, for cars or airplanes, the G forces from sudden deceleration can rupture major blood vessels like the aorta. Major organs can get ripped away from their connections to the rest of the body.
While some of the crew may have survived the breakup of the orbiter, the impact of the crew module with the ocean’s surface would have killed anyone who was still alive.
The ARFF (Aircraft Rescue FireFighting) industry generally accepts that in a low impact accident (we can’t call them crashes anymore, its not PC), where most of the aircraft remains intact, about 80% to 85% of the passengers and crew will be alive when the plane comes to a stop. The post crash fire is what kills most of them. A post crash fuel fire will burn through the aluminum skin of a commercial aircraft in 60 to 120 seconds, but even if you get out in less than that time, you have to find your way through a 200’ diameter Jet A fire filled with hot, sharp aircraft parts.
High impact crash - everyone’s dead, its just simple physics.
My advice - sit in the last row on the aisle. Planes don’t usually fly in reverse into hills. Plus, the flight data and cockpit voice recorders are kept in the tail. If those are the only parts they expect to survive, be as close to them as you can. But seriously, statistically, those sitting in the very back of the aircraft generally survive most accidents. The front of the aircraft absorbs the impact, and the rear exit doors are not used as much as the front or overwing exits, so you have a better chance of getting out.