AD 73-20-02 ACTIVE

73-20-02 BOEING: Amendment 39-1722 as amended by Amendment 39-2763 is further amended by amendment 39-3393. Applies to all B-17F & G airplanes certificated in all categories.
Compliance required within the next 25 hours' time-in-service or within 6 months after the effective date of this AD, unless already accomplished within the last 25 hours' time-in-service or 6 months, and thereafter at intervals not to exceed 50 hours' time-in-service or 12 months, whichever comes first, (from the last inspection).
To detect cracking in the front spar lower cap center section, P/N 753424-2, accomplish the following:
(a) Remove the most inboard bolt from the 8 bolt pattern attaching the front spar center section lower chord P/N 753424-2 to the terminal plates, P/N 56-3852-500 (aft) and 46-3852 (forward), left and right-hand sides. The bolt is approximately 8 inches inboard of the inboard jack pad. Using eddy current inspection procedures, or borescope methods in conjunction with dyepenetrants, inspect the front spar lower chord center section for cracks around the bolt hole in both the forward and aft wall of the tube. Particular attention should be given to the top and bottom portion of the tube. Removal of the bolt may necessitate installation of an access panel in the wing fillet fairing just forward of the front spar. The access panels may be installed using the procedures of FAA Advisory Circular 43.13-2.
(b) If cracks are found, replace the spar cap with a serviceable part of the same part number, or repair in accordance with Army T.O. No. 01-20E-3 or other method approved by the Chief, Engineering and Manufacturing Branch, FAA Northwest Region.
(c) After repairs per (b) have been accomplished, reinspect in accordance with (a) at intervals not to exceed 150 hours time-in-service or every 12 months, whichever comes first.
(d) The bolt holes described in paragraph (a) above may be reamed .063 inch oversize for a close tolerance oversize bolt, provided no cracks are detected when the chords are inspected in accordance with paragraph (a) above using the eddy current inspection methods. Any holes reworked with the oversize bolts must be reinspected in accordance with (a) above within 1500 flight hours after such rework. Upon accumulation of 1500 flight hours on the reworked holes, the repeat inspection interval reverts to the interval specified in (c) above.
(e) Any new replacement beam chords must be inspected within 2500 flight hours after installation and thereafter at intervals specified in paragraph (c) above.

Amendment 39-1722 became effective upon publication in the Federal Register.
Amendment 39-2763 became effective November 17, 1976.
This amendment 39-3393 becomes effective January 26, 1979.