Officials have speculated that the cockpit voice recorder and flight data recorder for Air France 447 may never be recovered, due to the depth and ruggedness of the ocean floor in the area in which the plane is presumed to have crashed. Have there been any crashes, historically, in which the plane was carrying a CVR and FDR that were never successfully recovered? Most of the ocean crashes I can think of (EgyptAir 990, TWA 800, Adam Air 574, Air India 182) occurred much closer to shore, in shallower waters, and the recorders were eventually found. The closest I’ve been able to find was South African Airways 295, in which the CVR was recovered but the FDR was not; have there been any major crashes in which neither one was recovered?
Neither of the boxes for the two planes that hit the WTC were found, and the CVR for the plane that hit the Pentagon was too damaged by fire to be useful.
There may be others, but those were the ones mentioned in the “Why are we still looking for black boxes” thread.