Just to clarify about the airworthiness certificate: Every airplane has one as it is one of the required documents to legally fly an aircraft. So long as the owner (Baltia) renews the registration on their airplane they will continue to have an airworthiness certificate. The actual "airworthiness" of the aircraft is ultimately determined by the pilot that flies the aircraft.

This is separate from the certification process, as the FAA's airworthiness certificate basically says "This is a B742 and we have approved this design." The conformity evaluation is the FAA saying "We approve this aircraft to fly passengers or cargo in scheduled service operations." Every aircraft in Part 135 and Part 121 service undergoes a conformity evaluation. When Delta adds a new type of plane to their fleet, you guessed it, C-Check. In summation, the airworthiness certificate only allows the plane to be flown, whereas the conformity check allows the plane to be flown for commercial purposes by a commercial operator.

Hope I made sense there!

And a reference that will make it as clear as mud:

http://fsims.faa.gov/PICDetail.aspx?docId=890...,Ch16,Sec1