The previous flight from Edmonton to Montreal had avoided the error. The fueler at Edmonton knew the density of jet fuel in kg/L, and he calculated the correct number of litres to pump into the tanks. He testified that it was a "regular practice of his" to do such calculations. When fueling was complete, Captains Weir and Johnson checked the figures. The captain knew "from previous experience" the density of jet fuel in kg/L. He also had a working FQIS, which agreed with his calculations.