I've came across on youtube a new design for stacking passengers in an airplane cabin, that looks at first sight interesting.
What would be the advantages and drawbacks of such a design? Why this is not common in today's airplanes?
I see several challenges with this design, both technological and not:
The problems I can see with that design:
Increased structural weight: all things remaining the same, you will have less payload availability for luggage/cargo.
Cabin servicing you either add an elevated kitchen (more weight) or you have to go up those stairs with the carts every time you want to service the cabin.
Possible claustrophobia those modules look tiny, and the center rows are quite different from a car where you can open the window.
Possibly longer evacuation times you are increasing the passengers, making them lay down more (more time to get up/out of those compartments), and maintaining the same volume. And people will try to get their belongings from under those seats. You will have to demonstrate that this does not adversely affect the prescribed evacuation times.
Now that you have added screenshots, and that I have more time to look at them, I would add that
The video shows the seat-pitch at 42". Current seat pitch is typically around 30-33". I'm pretty skeptical that you can increase seat-pitch by 10" and still increase seating, no matter how much seats are stacked on top of each other.
Also, the video shows seating in a 2-2-2-2 configuration, with 3 aisles. Typical wide-body seating is 2-4-2, 2-5-2, 3-3-3, or 3-4-3 with 2 aisles. This implies the seats would be even narrower than current seats. And current seats are already pretty narrow for most average sized people.
Typical wide-bodies have overhead bins inboard and outboard (4 overhead bins per row). This diagram shows the outboard overhead bins, as normal, but no inboard overhead bins. There is some small space above the upper-deck seating, but none for the lower-deck seating.
In my estimation, they're cramming more passengers into narrower seats, with less luggage space.
It is an interesting video, but I want to see Numbers and Math to back up the pictures.
The depicted seating configuration wouldn't fit in a typical wide body aircraft. It would require a jumbo platform, such as the 747 or 380. A wide body aircraft can only do 2+4+2 seating, whereas this configuration requires another 15" aisle. Further, this configuration appears to suggest additional walls and structural components that would increase the width.
If you move to a jumbo platform, you find they're already stacking passengers in two levels, so there's no advantage there.
Further, there isn't enough height for the elevated aisleway without significantly impinging on the aircraft backbone. You may be able to convince the aircraft company to build a widebody with two separate backbones, but that would increase weight, assembly costs, and would limit the ability to route cables, hoses, ducts, and other necessary items through the area above the cabin.
Lastly, you'd have to increase the amount of wiring, lighting, ducting, and emergency equipment to support the additional passengers. This may involve more than simply space considerations.
This doesn't even begin to approach considerations for claustrophobia, evacuation, loading, and so forth.
I could think of 2 reasons not yet mentioned by the other answers.
1) The "passenger leg room and capacity increased by 6.4%" would turn into "passenger leg room unchanged and capacity increased by 9.3%" or some such thing. If an airline can stuff you in a bit tighter, they will probably try it. So really not much would improve for the passengers.
2) Airplanes are incredibly expensive to build. Many airlines are still using airplanes that are fairly old. This would likely only be possible if the entire airplane was redesigned to fit this which would mean it would only be seen 10 years in the future at the earliest.