The crew reported for duty at 06:25 UTC at London-Stansted Airport (STN) and were scheduled to position the aircraft, without passengers, to Chamb‚ry Airport (CMF) in France. They were then scheduled to return, with passengers, to London-Gatwick Airport (LGW). An onboard hand-held Electronic Flight Bag (EFB) HP iPAQ computer was used to calculate the takeoff performance data. Once this was completed, the EFB was placed in the standby mode. The flight to Chambery was uneventful and the aircraft landed at 09:15, 15 minutes behind schedule. The return flight to Gatwick was scheduled to depart at 10:00. The pre-flight preparations proceeded normally and the pilots reported no distractions other than those normally experienced during a turnaround. The co-pilot, who was the non-handling pilot for this flight, completed the manual load sheet whilst the commander programmed the route into the Flight Management Computer (FMC). The commander reported that he then cross-checked the load sheet information and, having ensured it was correct, entered the Zero Fuel Weight (ZFW) into the FMC. The FMC then calculated the Takeoff Weight (TOW) using the fuel weight automatically retained in the system. The commander read back the FMC calculated TOW to the co-pilot, who cross-checked it against the load sheet data and confirmed that they agreed. Having loaded the ZFW into the FMC and cross-checked the TOW, the commander used the EFB computer to calculate the takeoff performance data. This involved waking up the EFB from the standby mode, then entering the airfield, weather and aircraft data, which included the flap position for takeoff. A specific flap setting could be entered, or the computer could be used to provide an optimum setting. The commander stated that he normally entered Flap 5 for the takeoff performance calculation. However, at airports where performance was considered to be more critical, he would set the performance computer to select the optimum setting. As the runway at Chambery is relatively short, the commander chose the optimum flap position calculated by the performance computer. This gave a selection of Flap 1 which, under the circumstances, he did not consider unusual. Similarly, the computed takeoff speeds did not seem unusual to the commander, particularly as they were predicated on a flap setting he did not normally use. The computed figures, based on the data entered on the EFB, allowed for a reduced thrust takeoff. The calculated assumed temperature to be set to achieve this did not seem unreasonable at the time, although the commander stated that, in hindsight, he considered it to be too high for the airfield and the prevailing conditions. Both pilots stated that they would normally cross-check the performance figures once they had been calculated on the EFB. However, on this occasion, and for reasons the pilots could not recall, this was not done. The commander wrote the speeds he had computed using the EFB on the flight paperwork and then entered them into the FMC, overwriting the FMC generated speeds. The commander stated that this was standard practice and on this occasion he did not take note of any difference between the two sets of speeds. The rest of the pre-flight preparation was completed and the aircraft took off from runway 36 at Chambery at 10:08 UTC (12:08 local time), on schedule, with 131 passengers onboard. The pilots reported the weather at the time was good, with a light wind from the east, good visibility and dry conditions. The ATIS reported a temperature of 8øC and a QNH of 999 hPa. On takeoff both pilots felt a slight judder, which they considered was due to turbulence from the preceding aircraft. Early in the climb they received a call on the intercom from the rear cabin station informing them that the cabin crew to the rear of the aircraft had also felt a judder. This call was intended for the cabin purser at the front of the cabin, but was mistakenly made to the flight deck. The pilots reassured the cabin crew member, still believing the judder was due to turbulence. Their opinion was reinforced by the absence of any abnormal flight deck indications or calls from ATC at Chambery to the contrary. The remainder of the flight went without incident and the aircraft landed at Gatwick at 11:30. The aircraft was taxied to stand and, after shutting down, the pilots were informed by ground personnel that the underside of the rear fuselage had sustained damage consistent with a tailstrike. This prompted the crew to reconsider the cause of the judder felt at takeoff and they reviewed the takeoff performance data. This revealed that the commander had omitted to enter the aircraft’s TOW into the EFB computer at Chambery, with the result that the computer had reverted to the previous TOW data retained from Stansted to calculate the takeoff performance figures.