HEN IT involves an airplane, even the seemingly insignificant things can be most important. The tiny little dots and nicks can be the start of corrosive actions which can result in dire consequences. One such case was recently passed on to SPORT AVIATION by the Bureau of Aviation Safety of the National Transportation Safety Board, with a recommendation that it be published in order to bring such a situation to the attention of the flying public in the interest of accident prevention. The manufacturer of the propeller involved also concurred in publishing the report. This information is also presented here not only for its own value, but to alert those who are flying airplanes equipped with metal propellers that had been bent and straightened out or which had been cut down to be constantly aware of what little it takes to be the cause of an eventual propeller failure. The Metallurgist's Factual Report, Metallurgical Laboratory Report No. 69-19 is as follows: A.
FIG. 1. Appearance of the surface of the fracture in the propeller blade. Arrow "a" indicates the approximate origin of the fatigue fracture. X.43
Place: Ft. McCoy, Florida Date: October 29, 1967 Aircraft: Mooney M-20E, N-1302W NTSB No.: MIA 68-F-269 B.
Piece of propeller blade from a Hartzell Propeller, Model HC-C2YK-1A; Blade Serial No. A58932F; Blade Material 2025-T6 aluminum alloy. C.
Laboratory examination showed that the nucleus of the fatigue fracture in the propeller blade probably was at a corrosion pit in the camber surface. Numerous other corrosion pits associated with intergranular corrosion damage were found on the camber surface of the blade. Hardness tests on the blade material gave values typical of 2025-T6 aluminum alloy, which was the material specified for the blade. D.
DETAILS OF THE EXAMINATION
The piece of propeller blade examined was about six inches long and included the inboard surface of a chordwise fatigue fracture 9-":s in. from the tip. A visual examination of the fracture showed that a fatigue crack had originated on the camber surface of the blade approximately at the point indicated by arrow "a'1 in Fig. 1. The crack had propagated through about 75 percent of the cross-sectional area of the blade before the blade failed completely. After the failure occurred the fracture surface had been damaged by numerous scratches in the area of the fatigue origin. These scratches had obliterated surface markings that might have served to identify the exact point of crack nucleation. However, crack progression marks outside the scratched area indicated that the nucleus of the crack probably was at a 24
FIG. 2. Camber surface of the propeller blade in the area adjacent to the point on the fracture surface indicated by arrow "a", Fig. 1. Arrow "b" indicates a pit in the camber surface. Arrows "c" indicate some of the shallow grooves resulting from scratches on the fracture surface. X 20
corrosion pit (arrow "b" of Fig. 2) in the camber surface. Numerous other corrosion pits were found
on the camber surface of the blade, as shown in Fig. 3. An enlarged view of one of the pits is shown in Fig. 4. Metallographic examination of a section through the pit indicated by arrow "b'' in Fig. 2 showed that the pit was very shallow, as shown in Fig. 5. However, similar sections through other pits revealed considerable intcrgranular corrosion extending from the pits into the adjacent metal, as shown in Figs. 6 and 7. Also, some areas of intergranular corrosion without appreciable pitting (Fig. 8) were found. Although no evidence of intergranular corrosion in the plane of the fracture was observed in the section shown in Fig. 5, such evidence could have been destroyed by the scratches on the fracture surface. Hardness tests on specimens cut from the propeller blade gave an average value of 81 Rockwell K, which corresponds approximately to 110 Brinell. This is within the range of hardness normally found in properly heat-treated 2025-T6 aluminum-alloy material.
FIG. 5. Metal log raphic section through the pit indicated by arrow "b", Fig. 2. The top edge of the section intersects the camber surface of the blade and the right edge intersects the fracture surface. The deformed structure
Camber side of the blade near the fracture, show-
along the right edge is in the area where scratches were found on the fracture. Keller's etch. X 180
ing numerous corrosion pits (black spots) in the surface. X %
FIG. 6. Section through one of the corrosion pits on the camber surface of the blade. Unetched. X 150
FIG. 4. Appearance of one of the corrosion pits (arrow) in the camber surface at higher magnification than in Fig. 3. X 20
FIG. 7. Microstructure adjacent to one of the corrosion pits, showing the associated intergranular penetration.
Keller's etch. X 300
FIG. 8 Section through the camber surface of the blade in an area where intergranular attack had occurred without appreciable pitting. Keller's etch. X 540 SPORT AVIATION
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