A heat exchanger is a vessel that allows the transfer of heat from a hot stream to a cold stream, and so does not operate at a single temperature, but experiences a range of temperatures throughout the vessel. Temperatures throughout GP905 normally ranged from 60 °C to 230 °C (140 °F to 446 °F). Investigators estimated that, due to the failure of the lean oil pump, parts of GP905 experienced temperatures as low as . Ice had formed on the unit, and it was decided to resume pumping heated lean oil in to thaw it. When the lean oil pump resumed operation, it pumped oil into the heat exchanger at —the temperature differential caused a brittle fracture in the exchanger (GP905) at 12.26pm.