Are there any regulations preventing one from converting the cargo version of the Airbus A380 for passenger use?
Converting a freighter to an airliner would require lots of work:
- you have to install an HVAC system capable of supporting hundreds of passengers, which means installing air ducts from the engines to the cabin, etc. Major surgery to the wing and fuselage.
- you have to install a sufficient number of doors and emergency exits (with associated systems). Major surgery to the fuselage.
- big changes to the electrical system to support e.g. entertainment for the passengers.
So the conversion would require taking most of the aircraft apart.
There is no cargo version of the A380, but for other types:
There are no regulations prohibiting converting a cargo aircraft to a passenger one. It will just need to meet all of the passenger certification requirements, such as evacuation times, seat and structure crashworthiness, oxygen systems, and so on.
I don't think this has ever been done because it is extraordinarily expensive. It is cheaper to convert a passenger aircraft to cargo because for the most part you are removing items instead of installing them. Passengers also prefer new aircraft, so after going through the cost of converting the aircraft you'll struggle to fill it with people.
Can cameras replace windows? From a legal standpoint I'm not sure, I think yes as I recall a patent for it, but that would be an interesting question to ask here.
Normally the retrofitting path is from passenger service to freight service. In my experience, many freight operations already have the necessary airpacs, as certain cargo (zoo animals, livestock, biologicals and chemicals) may require environmental control.
The cost comes in outfitting the aircraft for people, with the accouterments of modern air travel.
There are options for convertible interiors which will handle freight and can be rapidly converted to passenger use, and these are particularly popular with operators in under served areas which have lower volumes. The convertible interiors can be switched out in very short times (less than 30 minutes) and are suited for operations where people are moved by day, and freight by night.
One larger effort will be in electrical needs for the cabin and associated lighting and electronics.
Without checking the US regulations, it is not clear to me that windows are a requirement, except for certain stations. Some configurations, such as medvac, may use few windows. Some prison transports are done with windowless (or nearly so) passenger aircraft.
Different freight has different risks. For example one carrier found that giraffes tended to munch on cables and wiring in the ceiling, requiring protective coverings on subsequent carriage of those animals. Those freight mitigations could increase some costs.
Generally, cargo aircraft have adequate doors, as well as larger doors to the upper decks. The bulk of the retrofit is seating, bins, environmentals (lighting, O2, air distribution) and seat rails. It will depend upon the aircraft, but the retrofits do not usually involve major airframe work, although the cost may be high.
To answer the specifics on an A380 conversion, one would have to study the type certification and the delivery configuration of a given aircraft, and then analyze the requirements to move to dedicated passenger service.