Just over 20 minutes into the flight, air traffic controllers in Dubai received word from officials in the nearby Gulf nation of Bahrain that the plane was on its way back after reporting smoke in the cockpit. The jet was "unable to maintain altitude," the report said.This lines up with previous reports about smoke in the cockpit and even witnesses seeing flames from the aircraft itself. UPS is not denying this at present.
Aviation observers will no doubt point to Swissair 111, which crashed off the coast of Newfoundland back in the 90s due to an onboard fire. Dubai officials are saying the CVR was recovered but they are still looking for the FDR, which figures to be much more critical in determining cause on this one. Based on the location and the fact aviation has been a 40+ year target of extremists, 'sabatage' can't be ruled out, and UAE officials did not immediately dismiss it unlike many other recent crashes.
That said, many factors are in play. If the AP story is correct and the crew "couldn't hold altitude" (and they came in very high) it might suggest damage to the hull or control surfaces, or simply too much fire damage to cockpit instrumentation. However, they were able to communicate with ATC, so some things were functioning. Witnesses said the engines were spooling and it's clear the aircraft made a bank on approach and again after it missed, meaning the pilots were able to execute turns (although turns can performed using differential thrust). And obviously being a cargo plane they will have to look at the cargo for clues.
This crash will be felt hard in Louisville, just as the Fed Ex crash in Narita Tokyo was felt hard in Memphis last year. As soon as the FDR is found and analyzed the NTSB experts and other officials will get a good sense of what happened, so let's hope they find it soon.