Is it possible to convert a passenger A380 to carry cargo? I heard an expert say that one of the A380's weaknesses is that it only comes in a passenger version, not a cargo one like the B747 does. The cargo version of the B747 makes its market wider. One more weakness of very large jumbo jets (A380 and B747) is that, nowadays, most flights fly directly to the destination (point-to-point), not hub-to-hub as in the 1970s. The point-to-point model causes airlines to make more frequent flights with smaller aircraft. In my country, even small airports are now international airports. Two neighbouring cities in neighbouring countries have small propeller aircraft flying directly between them, without having to go through the main airport in one country's capital city. This makes it very clear that very large jumbo jets are no longer required for passenger service.

On the other hand, the demand for air cargo has never been greater. Trade over the globe has to be quicker. This means that large jumbo jets are needed in cargo service, and - to allow easy centralization of cargo-handling equipment and personnel - it's generally preferable for these freighters to operate to and from the largest, most central airports in a country.

The question is, is it possible to convert an A380 for cargo service when there is no longer a market for the passenger version?
Airbus originally planned on a freighter version and UPS and FedEx both ordered some of them. Production delays with the passenger version resulted in the freighter version being delayed and both of them canceling their orders.

The main issue why there is no freighter version is that the internal volume of the aircraft is too large to make it economical for cargo operations. I.e. the density of the cargo generally is too high to take advantage of the full internal volume before hitting the aircraft's maximum take-off weight. As a result, you end up hauling a whole lot of "extra airplane" around. The only time that extra volume is useful is for really low-density cargo loads. 

That extra internal volume is useful if you have to move a large number of passengers into slot-constrained hub airport like London Heathrow or JFK but packages don't care much about personal space...

In words other than mine:


  An A380-F would be too fat to fly at a profit: The plane would hit the maximum payload (a constraint of weight) before its maximum cubic space (a constraint of volume). Its design can’t support the maximum payload required to generate a profit.


Here are some good other more in-depth analyses. 

Something that was surprising to me initially was the amount of modifications that are necessary to convert a passenger aircraft to a cargo aircraft. This paper from a Bedeck Aviation Group (Boeing affiliated passenger to cargo conversion house) provides a good breakdown of the steps required for a 767 conversion. This video from Boeing provides a good visual summary as well (music and hair styles are an added bonus).