Even with automatic fuel heat in a RR powered B747-200, encountering a very cold air mass could still mean that the temperature of the fuel in the tanks could fall to its limiting temperature.
The night of 19 February 1993 was an extremely cold night over the North Atlantic. Whilst cruising on the NA track system, operating MCO to LGW, in a RR powered B747-236, we encountered a SAT of -72°C.
Speeding up had no visible effect on the ever decreasing tank temperature, and with no lower level available from ATC, we eventually had to declare an emergency, turn off track and establish a parallel offset track and then descend to FL 320 in search of warmer air.
By the time we left oceanic airspace that night we had heard several other aircraft also offset from the track system in order to descend in to warmer air.