When a plane is finally considered to not be airworthy, what happens to them? Do they take them apart bit by bit? Do they sink them offshore to make them into reefs? Are parts stripped and sold as spares? Are there legal procedures the airlines have to go through? What happens?
Generally the planes are scrapped by a recycling company. The parts will either be removed and sold as spares, or chopped up and recycled or disposed of. So a lot will depend on how easy it is to remove the part, and how much of a market there is for it. Airplanes are disassembled to a large extent during heavy checks, which can provide an opportunity for substituting in spares.
Engines: Probably the most valuable part of the airplane. These will be removed and either used as whole spares or for spare parts, depending on the condition and how many of the type are still in service.
Cockpit instruments: These will be removed and used as spares as well, if there is enough of a market (everyone wants those fancy glass cockpits these days).
Interiors: These may be removed as well, they can be used as spares.
Control surfaces: These can be removed and used as spares as well.
Windows: These can be removed and reused.
Systems: Pumps, electronics, lights, actuators, etc.: Most of these can be removed and sold as spares.
Landing gear: This can also be removed and reused, depending on the number of cycles it already has.
Tires: The tires may be reused if not worn out. The tubes may not be reused.
Airframe: Although some parts may be able to be used, generally the effort and the fact that there are probably many cycles on the part will prevent much from being used as-is. The metal will be chopped up as scrap. They are already working on recycling major composite structure as well, since manufacturing scrap is already being created. According to Boeing, with modern recycling processes, the metal, plastic, wires/electronics, and carbon fiber composites are used for:
- 15% High-grade industrial
- 50% Low-grade industrial
- 35% Landfill
As it often is with business charts, it's not clear if these are by weight, volume, or what. This company claims to recycle 90% of the weight of an aircraft.
They send it to a professional demo team that drains the fluids, grabs the spares and cuts up the hull for the materials and send it all to be recycled as possible.
There are plenty of youtube videos detailing the process like https://www.youtube.com/watch?v=xDxJAO__6t8
FedEx has lately been donating their 727 retired fleet to flight schools and training departments.