How would the plane know it's weight? You could measure the forces on the landing gear, but that would be subject to huge errors (inflation of the tires, stress

How would the plane know it's weight? You could measure the forces on the landing gear, but that would be subject to huge errors (inflation of the tires, stresses from the hydraulics, etc.), much higher than the 1% error required to notice this discrepancy. They do have checks in place, which is how they noticed the issue, but it still comes from keeping track of what goes on the plane. > As part of the prestart procedure, the flight crew reviewed the flight plan, which gave an expected takeoff weight (TOW) of 66,495 kg (Figure 1), and the load sheet, which gave a TOW of 64,889 kg (Figure 2). They noticed that there was a discrepancy, with the load sheet showing 1,606 kg less than the flight plan. They noted that the number of children shown on the load sheet was higher than expected, at 65, compared to the 29 which were expected on the flight plan. The commander recalled thinking that the number was high but plausible; he had experienced changing loads on the run-up to the temporary grounding as passengers cancelled and altered trips at short notice.