Summary
This document proposes to revise an existing airworthiness directive (AD), applicable to certain Boeing Model 747 series airplanes, that currently requires repetitive inspections of the access doors to the midspar/spring beam fuse pins on all engine pylons to detect cracks on the external surface; repetitive inspections of each midspar/spring beam fuse pin to detect if it protrudes beyond its mating nut by a specified distance; and repair of any discrepancy found. The actions specified by that AD are intended to prevent migration of this fuse pin, which, if not detected and corrected in a timely manner, could result in failure of the engine pylon and consequent separation of the engine from the wing. This new action would increase the intervals between inspections of the access doors and each midspar/spring beam fuse pin, and consequently decrease the frequency of inspections. This proposal is prompted by new data provided the manufacturer indicating that the reported migration of the fuse pin was apparently the result of an incorrectly installed nut.