CM seems to be under the misapprehension that black boxes are indestructible and will continue working under all possble environmental stresses.
This is not so.
FDRs are constructed with the forces of a plane crash in mind. This is the kind of forces that the AA77 and UA93 recorders were subjected to, and these were all found, and three of the four were readable.
In addition to these design loads, the recorders of AA11 and UA175 experienced the massive forces of building collapses, and quite possibly days, weeks or months of underground heat and fires.
Flight recorders are painted bright yellow or orange and have a beacon, both to aid recovery. However, neither of these features can survive prolonged exposure to heat
[1] above certain temperatures: The color will get charred, the electronics cease to work. As all four recorders were certainly burried at unknown depths into the debris piles, they likely endured unknown temperatures and unknown pressure for times that exceed specifications by large factors.
[1] Current specifications for FDR survivability, EUROCAE document ED-112 which applies to US and European airliners since 2003, are listed on page 14 of this presentation:
http://capacg.com/2009 GA-FDM Conference/03Schmutz_L3_ERAU_GAFDM_Conference_8_18_09.pdf
ED-112 superceded ED-55, which was current in 2001, but for which I haven't found a good link yet. We may suppose though that requirements have not been eased. So here is what an FDR must be able to endure:
- Impact: 3400 Gs, 6.5 ms
- Pin Penetration: 500 lb., 10 ft. 1/4 in. Pin
- Static Crush: 5000 lb., 5 min., All Axes
- Low Temp Fire: 260° C, 10 Hrs.
- High Temp Fire: 1,100° C, 1 Hr.
The WTC black boxes are likely to have exceeded both their Low Temp Fire duration and the Pin Penetration specifications during the plane crashes and/or building collapses (perhaps some engineer here can explain what this latter requirement means and work out some estimate of what can be expected during the building collaps?). Also, a static crush of 5000 lb (2.25 metric tons) doesn't appear outlandish in the pile, if the recorder found itself between a rock and a hard place under the debris.