The plane was operating in what are described as unusually low temperatures, and one theory is that this affected the fuel, though recorded temperatures were within safe limits.
AAIB words
The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’
Actually I think the Beeb is doing the English language a favour in this section and correcting the double tautology in the AAIB statement ...
=> Unusual implies "not average"
=> Unusual is not the same as "exceptional"
Duncan