Summary
The FAA proposes to adopt a new airworthiness directive (AD) for certain Boeing Model 747-400 and 747-400D series airplanes. This proposed AD would require a general visual inspection of the power feeder wire bundle of the auxiliary power unit (APU) where it crosses the hydraulic system 4 return tube to determine if parts are installed to provide separation between the wire bundle and hydraulic tube. This proposed AD would also require related investigative and corrective actions if necessary. This proposed AD results from a report that the power feeder wire bundle of the APU was found touching the hydraulic system return tube during inspection of an airplane. We are proposing this AD to prevent insufficient clearance between the wire bundle and hydraulic tube that could lead to chafing of the wire bundle, which could cause arcing and a consequent hydraulic fluid fire in an area outside of the smoke detection and fire extinguishing zone; this condition could result in an uncontrolled fire on the airplane.