occasional,
There seems to be an assumption that the effective temperature within the wings is reasonably uniform and similar to or above the TAT.
From the latest Interim report:
On long flights the temperature of the fuel in the main wing tanks will tend towards the temperature of the boundary layer around the wing, which can be up to 3°C lower than TAT
One of the difficult to find and, I think, generically interesting pieces of information to emerge from this investigation.