A translated letter from a TransAtlantic pilot:
"My friends....I have a theory re possible causes of the Airbus Air France Crash, and it is based on my own experiences.
The Met phenomenon which happened to me was in the general area of the Air France crash, in May 2001, when I was returning to Spain from Buenos Aires in a B743. From overhead Rio, we followed the exact same route as the Air France Airbus, and passing the area of the accident crossing the Intertropical Front at F370, we found moderate to severe turbulence. For around 1 to 2 minutes of the flight we then experienced a sudden increase of outside air temperature, it went from -48C to -19C.
As a result of this temperature discrepancy, we went from flying with a margin of 10,000Kgs to 15,000kgs outside the flight envelope for that flight level, and the aeroplane started an immediate pitch down, with very strong oscillations. I disconnected the Autopilot and we descended, losing 4,000Ft...we were well in the "Coffin Corner", and I am certain had we not disconnected the Autopilot and regained control of the descent, we might be at the bottom of the Atlantic ourselves, as the Autopilot would have tried to maintain altitude and would not have been possible. I have since been flying an A340 in those routes and have not found the same conditions since, which in my 40 years flying I had never ever considered possible. I would describe it as a massive funnel of 40NM in diameter of incredibly warm air with an embedded CB rising at extremely fast rate...after 5 minutes of flying by the seat of our pants, everything started to get back to normal, temp went back to 048C and I was able to regain climb back to F370.
Airbus advises not to disconnect the Autopilot when entering Turbulence, but a situation like the one I described has not been documented before that I am aware of."