Action
Notice of proposed rulemaking (NPRM).
Summary
We propose to supersede Airworthiness Directive (AD) 2012-17- 13, which applies to certain The Boeing Company Model 707 airplanes, and Model 720 and 720B series airplanes. For certain airplanes, AD 2012-17-13 required using redefined flight cycle counts; determining the type of material of the horizontal stabilizer, rear spar, and upper and lower chords on the inboard and outboard ends of the rear spar; repetitively inspecting for cracking of the horizontal stabilizer components; and repairing or replacing the chord, or modifying chord segments made of 7079 aluminum, if necessary. For all airplanes, AD 2012-17-13 required inspecting certain structurally significant items, and repairing discrepancies if necessary. Since we issued AD 2012-17- 13, we have determined that all chord segments made of 7079 aluminum must be replaced with new, improved chord segments made of 7075 aluminum. This proposed AD would add a requirement to replace all chord segments made of 7079 aluminum with new, improved chord segments made of 7075 aluminum. We are proposing this AD to detect and correct stress corrosion and potential early fatigue cracking in the horizontal stabilizer, which could result in reduced structural integrity of the horizontal stabilizer.