AD FAA-2015-2965

We are superseding Airworthiness Directive (AD) 2012-17-13, which applied 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. This new AD adds a requirement to replace all chord segments made of 7079 aluminum with new, improved chord segments made of 7075 aluminum. This AD was prompted by a determination that all chord segments made of 7079 aluminum must be replaced with new, improved chord segments made of 7075 aluminum. We are issuing 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.