Two cracks were discovered in a deck plate of an aircraft during overhaul and repair after 659 h of service. The cracks were on opposite sides of the deck plate in the flange joggles. The plate had been formed from 7178-T6 aluminum alloy sheet. Analysis (visual inspection, 0.2x/2x/2.3x electron microscope fractographs, hardness testing, and electrical conductivity testing) supported the conclusions that the failure was caused by fatigue cracks originating on the inside curved surface of the flanges. The cracks had initiated in surface defects caused by either corrosion pitting or forming notches, acting in combination with lateral forces evidenced by the moderate distortion of the fastener holes. Recommendations included eliminating the surface defects by revised cleaning and/or forming procedures. Revised design and installation should also alleviate the lateral forces.

The floors (fabricated from aluminum alloy 7178-T6 sheet, with portions of the sheet chemically milled to reduce thickness) of the fuel tanks in two aircraft failed almost identically after 1076 and 1323 h of service, respectively. Failure in both tanks occurred in the rear chemically milled section of the floor. An alkaline etch-type cleaner was used on the panels before chemical milling and before painting. Various tests and measurements indicated that the aluminum alloy used for the fuel-tank floors conformed to the specifications for 7178-T6. Low power magnification, fractographs taken with a scanning electron, and optical microscopic examination of the milled sections revealed extensive pitting on both sides of the floors. Evidence found supports the conclusions that the floors failed by fatigue cracking that initiated near the center of the fuel-tank floor and ultimately propagated as rapid ductile-overload fractures. The fatigue cracks originated in pits on the fuel-cell side of the tank floors. The pits were attributed to attack caused by the alkaline-etch cleaning process. Recommendations included monitoring of the alkaline-etch cleaning to avoid the formation of pits and careful inspection following alkaline-etch cleaning, to be scheduled before release of the floor panels for painting.