For those who don't know, I'm a pilot for American Airlines, and also a C-5 Galaxy pilot in the Air Force Reserves. Coincidentally, I'm also a trained aviation accident investigator, although fortunately I have never had to put that training to use.
There are very, very strict, written guidelines for securing all types of cargo in the C-5, and I would presume similar guidelines exist for 747 freighters. Our floor is strong enough to allow wheeled vehicles to drive directly on it, whereas the KC-10 tanker has to have pallets loaded on their cargo box floor, then vehicles can drive onto the pallets. I suspect the 747 freighter uses a similar system.
Based partially on conjecture, and partially on the statement that the crew radioed that the cargo had broken loose, I would suggest that it was secured improperly. The guidelines assume failure of individual tiedowns is possible, and thus there is lots of redundancy built in. I don't know what percentage of tiedowns would have to fail before wheeled cargo would break loose, but I suspect it would be rather a lot.
However, nothing can prevent a disaster if the tiedowns are installed incorrectly, or in insufficient numbers. I hesitate to cast aspersions unfairly, but it seems that a combination of crew loadmaster and/or ground loading crew error is likely the cause of this tragedy.
Having said that, does anybody remember when the same thing happened outside of Sacramento, CA some years ago? At the time, the general assumption was that the cargo shifted on takeoff, but the NTSB investigation later determined that the accident was caused by poor maintenance of the airplane; a critical bolt had failed in the elevator assembly, rendering the airplane uncontrollable in the pitch axis:
Poor maintenance blamed for Sacramento air crash - SFGate
Besides the fact that the crew died due to this mistake, it was the last straw for Emery Worldwide, and under heavy scrutiny from the FAA, management decided to pull the plug and shut down the whole aviation branch of the company (leaving one of my friends suddenly unemployed) rather than continue in the face of an increasingly (and justifiably) hostile government investigation into their rather shoddy practices.
So, given this example, the reasonable thing is to wait until the investigation is complete, and the true cause of the crash can be determined. At the end of the day, ultimately it's human error, but whether it was caused by the crew, ground handlers, or maintenance personnel remains to be seen.
