Wow, thanks for all the upvotes,, and comments. BTW, if you do not care to hear a graphic description of what happened then do not read below the line below. The short answer is, the passengers died instantly.
I originally studied this subject to understand what planes hitting the buildings should look like in order to debunk certain conspiracy theories about the planes. The conclusion for that was the planes hitting the sides of those buildings looked as expected. That does not disprove the other conspiracy theories.
Yes, the passengers never even felt the bump of the plane hitting the building before they were dead. They probably experienced the collision as hearing a pop and then nothing. The entire body of a 767 jet traveling at 500 knots passes by a point in space in about a 5th of a second. Your body is moving through space much faster than pain moves through your nervous system (257 m/s vs .61 m/s). The elastic shock wave passing back through the fuselage of the jet caused by the impact with the building does not have time to reach the passengers before they are already dead and sailing through the inside of the building after being ground into little pieces by the steel frame of the building.
Physicists call this an inelastic collision. Everything crumples like the bumper of a car — nothing bounces. The physics of where all the inertial energy of the plane goes is actually very complicated. Destruction of the side of the building uses some of it. Destruction of the interior part of the building uses some of it. Crumpling and destruction of the plane itself. There is noise and heat accounting for some energy. The sudden negative acceleration — the stopping — would cause people in the planes to die instantly.
There are some calculations on g-forces near the bottom of this post. The summary is that a best-case scenario generates a minimum of 120 gs during the crash. 50 gs kills instantly. A car hitting a wall at 85 mph produces 30 gs. In the best case scenario, a body experiences a constant 120 gs during the crash into the building. In this case, the crumpling body of the plane acts as a kind of spring across the interior of the building. Most bodies experienced many 100s if not 1000s of gs. Everyone died instantly. Surprisingly, if a body ejected from the plane does not decelerate until it hits the atmosphere and is headed out the other side of the building, the person dies instantly anyway due to the air drag causing an instantaneous 700 gs of deceleration.
To think about it another way, the airbags in the seats never deployed because the impact never reached them. The impact shock wave cannot propagate faster back through the metal fuselage than the plane is moving into the building. Passenger’s bodies did not begin to decelerate before they were already inside the building in pieces. Passengers didn’t feel a thing before dying instantly. Oddly, most of the passengers would hear a noise just before dying as the ear only needs 25 milliseconds to hear.
Another way to think of these planes is as bullets which make a small hole going in and a big hole coming out.
Update based on comments:
Yes, some airplanes have airbags. Some have them only in business class or first class. Airplane airbags – Wikipedia
Does the propagation of the impact shock wave propagate at the speed of sound? There is an acoustic wave that propagates backward through the fuselage but that is not the wave we are interested in. It is an elastic wave traveling back through the airframe that is important. The two waves travel at different speeds depending on the material. This elastic wave has to do with stiffness, shear of the metal parts and their ability to spring back to their original shape.
Understanding deceleration during a crash. Imagine a car hitting a power pole. The car frame and bumper are not stiff. At 60 miles an hour, the bumper hits the pole and stops but the car behind the bumper is still moving at 60 miles an hour. The pole begins to intrude into the hood and yet most of the mass of mass of the car is in front of the pole and Still moving at 60 miles per hour. We are now 30 milliseconds into the crash. Now as the engine hits the pole, a significant amount of energy is absorbed and the stiffness of the frame is enough to slow down the remaining mass. At 40 milliseconds the remaining car in front of the pole is going to stop. Deformation of the car’s frame absorbs most of the remaining energy of the crash. This deformation travels as a shockwave backward through the car. This shockwave travels backward quickly but not instantaneously.
If the car was moving faster, the pole intrudes further. If the car is traveling fast enough, say 200 MPH, the frame is never able to absorb enough energy to stop the car and the pole splits the entire car into two halves which continue in the direction they were traveling. I actually witnessed such a car crash.
Regarding seeing the collision. Yes, it is possible. There is some disagreement on how fast the human brain can see. Wikipedia claims it takes the human brain 200 milliseconds to see light coming into the eye. This would probably be too slow. People in the back who happen to be looking forward toward the front of the plane might be able to see but perhaps not understand that something was happening.
Regarding what happens to someone if they did feel the deceleration. According to the NTSB, the aircraft were traveling at 510 knots and 430 knots. So this is 257 m/s and 221 m/s respectively. The terminal velocity of a human body through the air is 53 m/s. As a person who has been around a few rock climbing fatalities, I can tell you what 53 m/s does to a person’s body. I was also in Yosemite to see the result of a couple that jumped off the top of Yosemite Falls in a double suicide. There is an old saying among rock climbers: It’s not the fall, it’s the sudden stop that kills you. The sudden deceleration at 53 meters per second splatters a body into many pieces which have to be picked up by the “baggy brigade”. The bodies in the airplanes were traveling 4–5 times faster than that.
We can roughly calculate the g-forces on a body. A human body is known to survive 25 gs over 1.1 seconds with severe injuries. A bad car crash at 85 mph generates 30 gs. over .05 seconds. 50 gs is guaranteed instant death. Let’s assume the best case scenario for a body that stops slowly across the width of the tower without hitting any steel. The plane crumpling and the building crumpling provide a constant deceleration like a spring. Would the g-forces be fatal? Oh yes, We have:
(0 m/s – 275 m/s) / (65 m / 275 m/s) =1200 m/sec^2
gs = 1200/9.8 = 122 gs
So the best case is that a body undergoes constant 120 gs when 50 gs kills instantly.
Let’s suppose the unlikely case where a body flies all the way through the building without hitting anything and out the other side. What is the maximum g’s experienced in that case?
The air drag coefficient on a sitting body is about .75. The body is going to decelerate from 274 m/s to the terminal velocity of the human body which is 53 m/s. What are the maximum g-forces involved in this scenario? The average American weights 80 kg. The drag equation is:
D = Cd A .5 r V^2
Cd = the drag coefficient which we are taking as a seated person at .75. We estimate A the area at 1.7 m^2. The density r of air is about 1.2 kg/m^3 and we have the velocity V=275 m/s.
D=.75 1.7*.5*1.2*275^2=57853 newtons
Since F=ma we can calculate the acceleration for an 80 kg body caused by the Drag which comes to a = 57853/80 which is about 723 gs.
Obviously, the velocity of the jet at 275 ms or 500 knots is squared. That is the real estimate here. The drag decreases very quickly but the person is already dead. The highest g-force ever survived was an instantaneous 48.5 gs followed by 25 gs for 1.1 seconds. The lesson is, don’t jump out of a jet. The air might as well be a concrete wall.