747 Nose Bolts
Having been in the bolt business at the time, I heard this about the 747 nose. After each of the early test flights, bold heads were found in the nose section - from the bolts that hold the nose on.
The manufacturer was accused of making bolts not up to spec, but proved randomly selected bolts from the spares stock met spec.
Further investigation found the cause. The nose was mounted on a trolly and rolled up to the fuselage. The bolts were put in place and torqued up to spec (a high percentage of the bolts tensile strength). Then the trolly was removed, adding the weight of the nose to the stress on the bolts, taking them beyond design tensile stress. Add to this the stress and vibration of flight, and the bolts started to break, starting with the uppermost ones.
On another subject, the company I worked for was a major manufacturer of the "fuse pins" that hold the engines on jet aircraft. They are made to very strict specifications, and we had to break between 5% and 10% of production to prove consistency. The ones for the 747 are about 3" in diameter, hollow, with a contoured inside. All are hard chrome plated on the outside so eliminate wear from vibration that would change their break point.
