Apologies if this has been covered elsewhere or folks are sick and tired of 9/11 related questions…
The 9/11 Commission Report states that only flight 93’s Cockpit Voice Recorder (CVR) was recovered:
Note 76:
"In accordance with FAA regulations, United 93’s cockpit voice recorder recorded the last 31 minutes of sounds from the cockpit via microphones in the pilots’ headsets, as well as in the overhead panel of the flight deck. This is the only recorder from the four hijacked airplanes to survive the impact and ensuing fire. The CVRs and FDRs from American 11 and United 175 were not found, and the CVR from American Flight 77 was badly burned and not recoverable. See FBI report, "CVR from UA Flight #93,“Dec. 4, 2003; see also FAA regulations, 14 C.F.R. §§ 25.1457, 91.609, 91.1045, 121.359; Flight 93 CVR data. A transcript of the CVR recording was prepared by the NTSB and the FBI.”
However, in this recent interview on WBUR, James Meigs editor-in-chief of Popular Mechanics says the “black boxes have been studied” (minute 37:09) in response to a caller who says they haven’t been recovered or found.
What’s the SD? Why is such basic, factual info unclear?
IIRC, part of the FDR from flight 93 was played at Zacarias Moussaoui’s trial, so I’d say that it was found. It was in pretty bad condition though, again from memory, so it was probably incomplete. The things aren’t indestructible, as some folks think, just pretty tough and placed in parts of the plane where they are least likely to be crushed or burned.
ETA: Yeah, here is a cite that shows a picture of it all mashed up.
Flight 93’s black boxes (flight data recorder and cockpit voice recorder) were recovered and, as has already been noted, still contained valid data. Flight 77’s black boxes were recovered but they were too badly damaged to get any data from them.
The real discrepancy comes from the other two planes. According to the official reports the other black boxes were not recovered. However, workers from the WTC site claim that they found three of the four missing boxes.
Why the discrepancy? I don’t know. Maybe the boxes were recovered but they were too badly damaged to yield any data. Maybe the workers at the WTC site were mistaken about what they found. Who knows. The woo crowd has latched on to this as one more piece of proof of the vast 9/11 conspiracy though.
Not true - Flight 77’s (the Pentagon plane) flight data recorder was damaged but we got the data off of it. As a matter of fact, amateurs have gotten their hands on the data and successfully decoded it.
I think the confusion in the OP is based on the difference in the flight data recorders, which log the aircraft data like position, speed, and engine performance, and the cockpit voice recorders, which of course record the cockpit audio.
We have both the FDR and CVR data from Flight 93 (the Pennsylvania plane), and we have the FDR data from Flight 77.
The boxes from Flights 175 and 11 were never recovered.