The incident aircraft was line number 152. All 737s constructed after line number 291 included an additional outer layer of skin or doubler sheet at the lap joint of the fuselage. In the construction of the incident aircraft, this doubler sheet was not used. In the case of production line 292 and after, this doubler sheet gave an additional thickness of at the lap joint. For airplane line number 291 and before, cold bonding had been used, with fasteners used to maintain surface contact in the joint, allowing bonding adhesive to transfer load within the joint. This cold-bonded joint used an epoxy-impregnated woven scrim cloth to join the edges of skin panels. These epoxy cloths were reactive at room temperature, so they were stored at dry ice temperatures until used in manufacture. The bond cured at room temperature after assembly. The cold-bonding process reduced the overall weight and manufacturing cost. Fuselage hoop loads (circumferential loads within the skins due to pressurization of the cabin) were intended to be transferred through the bonded joint, rather than through the rivets, allowing the use of lighter, thinner fuselage skin panels with no degradation in fatigue life.
