Summary
The FAA proposes to adopt a new airworthiness directive (AD) for certain Boeing Model 747 airplanes identified above. This proposed AD would require modifying the outboard flap track and transmission attachments. This proposed AD results from a joint Boeing and FAA multi-model study (following in-service trailing edge flap structure and drive system events) on the hazards posed by skewing and failed flaps. This study identified the safety concerns regarding the transmission attachment design and the potential loss of an outboard trailing edge flap. We are proposing this AD to prevent certain discrepancies associated with this design (for example, a flap skew or lateral control asymmetry that can cause collateral damage to adjacent hydraulic tubing and subsequent loss of a hydraulic system), which could result in the asymmetric flight control limits being exceeded, and could adversely affect the airplane's continued safe flight and landing.