Photo © Derek Pedley
I have had dozens of emails on this picture to the possible cause of this 5th contrail on this AF 744 most saying the APU. But if you look at where the contrail starts I can't see it. I am no expert but would the APU be on anyway with an hour or so to go before landing?
One other email from a AC engineer says:
The AFT drain mast (there are 3 drain masts on the B747-400 (FWD, MID & AFT) which expulse the water from the lavatory and galley sinks (when you wash your hands in this airplane, the water is actually thrown directly outside!!)) These drain masts are heated at very high temp to prevent ice formation. The outside air temperature, coupled with the temperature of the drain mast must have been enough to cause the 5th contrail.
Be interesting to hear what you all think.