52-02-01 Convair: Applies to All Model BT-13, -13A, -13B and BT-15 Aircraft With Bronze Rear Spar to Center Section Attachment Fittings. \n\n\tCompliance required as indicated. \n\n\tWhen it has been determined that the rear spar to center section attachment fittings are bronze castings, compliance with this Airworthiness Directive should be effected every 24 hours flying time or every 6 months nonflying time, whichever occurs first. \n\n\t(1)\tInspect visually the rear spar to center section attachment fittings on both sides of the rear spar for evidence of cracks. All cracked fittings (see Figure 1) should be replaced with new fittings of equivalent or greater strength. \n\n\n\n\nFIGURE 1 \n\n\n\t(2)\tA magnet may be used to determine if the fitting is a steel casting. \n\n\t(3)\tFittings that have been made from aluminum alloy forgings can be readily differentiated from those made from bronze castings by visual inspection.