Airworthiness Directives; The Boeing Company Airplanes

We propose to adopt a new airworthiness directive (AD) for certain The Boeing Company Model 707-100 long body, -200, -100B long body, and -100B short body series airplanes; Model 707-300, -300B, - 300C, and -400 series airplanes; and Model 720 and 720B series airplanes. For certain airplanes, this proposed AD would require using redefined flight cycle counts, determining the type of material of the horizontal stabilizer, rear spar, upper chords, 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 modification of chord segments made from 7079 aluminum, if necessary. For all airplanes, this proposed AD would require inspecting certain structurally significant items, and repairing discrepancies if necessary. This proposed AD was prompted by reports of stress corrosion cracking in the chord segments made from 7079 aluminum in the horizontal stabilizer rear spar, and fatigue cracking in the chord segments made from 7075 aluminum. We are proposing this AD to detect and correct stress corrosion and/or fatigue cracking in the horizontal stabilizer, which could compromise the structural integrity of the stabilizer.