A major reason for the flammability of the fuel/air vapor in the CWT of the 747 was the large amount of heat generated and transferred to the CWT by air conditioning packs located directly below the tank; with the CWT temperature raised to a sufficient level, a single ignition source could cause an explosion.
Examination of the heat flow around the CWT revealed that the airplane’s air conditioning packs, located underneath the CWT, may have contributed to heating of the fuel/air vapor while operating for 2 1/2 hours at the gate at JFK.