The Boeing 747 aircraft was engaged on pilot and engineer officer training and was making right-hand circuits and landings on runway 13 at Prestwick Airport (PIK). It took off in daylight at 07:12. The weather was fine with ceiling and visibility unlimited and light and variable winds. The handling pilot was a First Officer under instruction whose experience of flying the Boeing 747 was 1 hour on the aircraft and approximately 20 hours on a simulator. He was flying the aircraft from the right hand seat whilst the Training captain (the Commander) was supervising from the left hand seat. All the approaches were being flown manually. Having successfully completed two touch and go landings, it was intended that the third approach should be terminated in a full stop. During this approach, while turning on to the final approach heading at an altitude of about 1,300 feet, the handling pilot asked for the flaps to be selected from 20ø to the landing flap setting of 30ø. At 07:33, shortly after the flap selection was made, a bang was heard and a slight pressure change was sensed by two of the crew members. The Flight Engineer, who was also under instruction, checked the engine instruments but found no evidence of engine malfunction. He noticed, however, that No. 4 door right warning light was illuminated and reported this to the Commander. The supervisory Flight Engineer went to the passenger cabin to inspect the door and found that it had failed inwards and was displaced approximately 2 inches from the door surround. There was debris from the door on adjacent passenger seats and further debris, including an escape chute gas generator, on the cabin floor. After this preliminary inspection he returned to the flight deck and reported the extent of the damage to the Commander. While the aircraft was still turning the handling pilot did not notice any change in handling characteristics. However, after levelling out, it was evident that about 15 ø to 20ø of control wheel displacement to the left was required to maintain the wings level and a small amount of buffet was also felt. The Captain, on confirming that this amount of aileron deflection was necessary, took control of the aircraft at about 600 feet. He aligned the aircraft with the runway centre line, applied sufficient rudder control to offset the lateral out of trim condition, and increased the airspeed slightly to 149 knots, this being the landing reference speed (VREF) + 10 knots at landing weight of 243,000 kg. He was under the impression that more than normal control movements were necessary, both laterally and longitudinally, to achieve the desired flight path but no problem was experienced during the flare-out and landing. Normal drills were carried out after landing on the assumption that a door failure was the only fault. Investigation established that the foreflap section of the right inboard trailing edge flap was missing, and had become detached in flight. Sections of the foreflap were later recovered from the sea, about three miles north-west of the airfield. CAUSE: “The right inboard trailing edge foreflap became detached from the aircraft, during flight, because its outboard sequence carriage attachment fitting failed. The mechanism of the fitting failure was the undetected growth of fatigue cracking in the horizontal flange leading to overload failure of the remainder of this flange. This was followed by fatigue cracking in the vertical channel portion of the fitting leading to its overload failure.”