Currently I am investigating the feasibility of transmitting a limited amount of FDR data via radio link for domestic flights within continental USA. I am working on building a prototype system in which I need to connect the FDR with a computer. USB interface is the most appealing one. I am using a small Flight data recorder (FDR) which has a USB interface. However, it is for hobby planes. Is there a USB interface for FDR's in the passenger planes? Or something that converts between ARINC and USB?
Certified FDR's for commercial aircraft don't have USB ports. They have interface ports because you obviously need to be able to extract data, but they are usually proprietary and each one uses a specific cable for the FDR you want to talk to. To do what you want to do you'd have to design a device that talks to FDR units to download and decompress the data. There are commercial test set units out there that can do this. They are not cheap. As was mentioned in a comment, to do this while in flight will require an STC from the FAA. Also, converting between ARINC and USB doesn't make sense. ARINC is a company, USB is a computer interface specification.
That said, I don't think connecting to the FDR is the best way to do this. As was mentioned, ACARS may already do part or all of what you are interested in doing.