Found Objects - Aircraft Skin

Volume 12 Number 2/3
October, 1996

Found Objects

Navigator’s Bookcase	Sheet of aluminum with red paint	Cut Strip of Aluminum	Aluminum Plate
Dado	Aircraft Skin	Aluminum Comb	Riveted Assembly
Channel Section	Cables	Plexiglas	Aircraft Safety Wire
Thermometer, broken	Threaded metal cap	Shoe parts	Artifact 2-2-G-8, Shoe heel

[L10] Aircraft Skin (TIGHAR Artifact 2-2-V-1)
Date Found:	October 1991 during TIGHAR’s NIKU II expedition.
Materials analysis:	NTSB
Report date:	March 5, 1990
Description:	This is a sheet of 0.032 Alclad measuring roughly 23 inches by 19 inches. There are four rows of evenly spaced 3/32 inch diameter rivet holes with a 1 inch pitch (space between centers) and one row of 5/32 inch diameter rivet holes with a nominal pitch of 1.25 inches along its long dimension. There is no line of rivet holes across the width of the sheet. The rows of rivet holes are not parallel but show a slight convergence. Nominal spacing between rows is 4.25 inches at one end of the sheet and 1/8 to 1/4 inch closer at the other end. One rivet is still present. The length of the undeformed rivet shank (distance between the manufactured head and the formed head) indicates that the skin was previously attached to an underlying member approximately 0.06 inch in thickness. The faint outline of 1/2 inch tall letters “AD” are present on what was the exterior surface of the skin.
Condition:	The sheet is bounded by fractures on all four edges (no manufacturer’s cut is present) and has a pronounced curvature across the short dimension. On one side the fracture runs generally along the line of 5/32 rivet holes. The fracture intersects all of the 5/32 inch diameter holes except for three unfractured holes which are unevenly spaced. The skin around the 3/32 inch diameter rivet holes is dimpled inward toward the concave side of the sheet suggesting that the surface was area loaded from the concave side while the rivets and underlying structure were intact. Scratch marks on the interior (concave) surface adjacent to some of the rivet holes suggests the further removal of underlying structure by prying with a hand tool. Post separation abrasion and erosion damage has obliterated the fracture faces and, in the opinion of the NTSB, positive determination of the modes of fracture can not be established. However, the NTSB did note that the fracture geometry along the line of 5/32 rivet holes is consistent with tearing separations in both directions away from the area of the intact holes. Deformation adjacent to the fracture along one edge of the sheet indicated that it had been folded 90 degrees toward the convex side at least twice prior to separation. The only place on the sheet exhibiting significant corrosion is an area roughly 1.5 inches in diameter located immediately adjacent to the “tab” with the three undamaged 5/32 rivet holes. The corrosion originated on the interior surface of the sheet and has perforated the exterior surface in three pinpoint-sized locations.
Identification:	This is a forcibly removed section aluminum which was once part of an airplane skin. The aircraft was of all-metal construction and was manufactured in the United States, probably before 1939. The surviving rivet is an AN 455 AD 3/3. The head style, known as the “brazier” head, was replaced in most applications by the “universal” head (AN470) sometime after 1940. The letters “AD” noted on the exterior (convex) surface are the last two letters in the word ”ALCLAD” which was stamped onto the aluminum in red ink at the time of manufacture by Alcoa Aluminum as part of the product labeling. Complete examples of this same size and style of lettering (ALCLAD 24S T3) have been noted on aluminum used in repairs or modifications to two surviving Lockheed 10s: c/n 1015, recently rebuilt as a replica of Earhart’s aircraft and currently registered NX72GT, and c/n 1052 in the New England Air Museum collection. Similar labeling has also been found on a small patch on the nose of a Douglas C-47 in the Dover AFB Museum collection. The font, or type style, of the lettering does not appear to match any of the styles used by Alcoa for aluminum manufactured during or after World War II. The fact that the lettering is not aligned with the grain of the metal indicates that the labeling was hand-stamped, a practice replaced by rolled-on labeling when aluminum production boomed after 1939.
Commentary:	This is, by far, the largest and most complex aircraft artifact found on Nikumaroro to date. It is also the most controversial. The skin’s material components, dimensions and rivet pattern are similar, but not identical, to some portions of the Lockheed Model 10. TIGHAR initially noted the artifact’s similarity to the underside of the Electra just forward of the cabin door and attributed the differences to changes made when the Earhart aircraft was repaired following its March 20, 1937 accident in Hawaii. Critics correctly pointed out that such structural changes could not have been made without a wholesale re-engineering of the aircraft for which there is no evidence. Subsequent research has identified another location on the Earhart aircraft which seems more promising.

Finding A Fit
The artifact has some distinctive aspects which provide important clues to its origin. The presence of 3/32 inch diameter rivets (the smallest commonly available size) in a 0.032 inch (relatively thin) skin suggests an aircraft of moderate, rather than large, size. Also, the absence of a crossing row of rivets in the skin’s 23 inch length is quite unusual. Other clues to the original structure come from the way in which the aluminum failed. The edge which has been fractured by being bent 90° exhibits a straight break indicative of a crossing internal structural member at that point. Likewise, in the lengthwise dimension, the change from 3/32 to 5/32 rivets suggests a boundary with a larger internal structural member. The peaked wave shapes in the ”tab” protruding from the edge where the sheet tore along the line of 5/32 rivet holes indicates the presence of another row of similarly sized and spaced rivets approximately 2 inches away. From these observations we can construct a more complete picture of what the structure must have looked like. To date, despite extensive research, no exact match to Artifact 2-2-V-1 in materials and structure has been found on any aircraft. There is, however, evidence to suggest that the sheet found on Nikumaroro may be from a repair patch installed on the underside of the Earhart aircraft on the left-hand (pilot’s) side of the airplane just forward of the main beam (wing spar). That such a patch was installed is documented in Lockheed Repair Orders for c/n 1055 (Earhart’s Electra) signed off as completed on April 19, 1937. The belly of the Model 10, in this area, features two 0.040 inch Alclad skins, “25R” on the right and “25L” on the left. When Earhart groundlooped the airplane in Hawaii, the right side of the belly was virtually wiped out while much less damage was done to the left side skins. The repair orders call for the replacement of most of the right side skins, including all of 25R. For 25L, the decision was made to replace only the damaged portion instead of replacing the entire skin. Fuselage Assembly repair item #6, therefore, says to, “Replace left hand bottom skin from a point 9.5 inches aft of slanting bulkhead to main beam – rivet new skin in place with double row of rivets similar to joint in slanting bulkhead.” There is no documentation or known photograph which shows just how this repair was effected, but if the patch was constructed of 0.032 (rather than 0.040) Alclad and reinforced with two additional longitudinal stiffeners added between the standard stringers, the resulting structure would be an exact match to the sheet found on Nikumaroro. Pursuing this hypothesis further, we know that the sheet failed due to area loading from the interior side – in other words, it was blown outward with sufficient force to rip the heads off many of the 3/32 rivets and tear the skin through the larger 5/32 rivet holes like paper torn through perforations. Walter Korsgaard, the FAA’s explosives expert for its investigation of the PanAm 103 bombing, examined the artifact and felt that it showed deformation typical of a low-grade fuel/air explosion. If this sheet was once part of the patch on the belly of NR16020, directly above it was a 118 gallon fuel tank. Immediately adjacent, on the centerline of the aircraft, and nearest to the spot of anomalous corrosion on the artifact, was the main battery. A fuel leak which trapped vapors between the floor and the skin, ignited by a spark from the battery, or even an explosion of the battery itself, might result in the weakest part of the structure – the patch – fracturing in just the manner seen in the artifact. Such a scenario is entirely consistent with other evidence which indicates that the Earhart aircraft was landed successfully on Nikumaroro and sent radio distress calls for two days until the signals abruptly stopped late on the night of July 4, 1937. Still attached by its rearward edge and some remaining stringers, the hanging flap might present an inviting piece of metal for a later Gilbertese discoverer to break and pry off and carry back to the village. Conclusion: Based upon the evidence presently available, Artifact 2-2-V-1 appears to have once been part of the Earhart aircraft.
Caution: Don’t Try This At Home
The drawings in this analysis will probably not match your neighborhood Lockheed 10. Of the 148 Electras built, at least a dozen survive today, all of which have had long and varied careers, often including accidents and modifications. To establish what the belly of Earhart’s machine probably looked like, TIGHAR started with microfilmed engineering drawings for the standard airplane on file at the Smithsonian’s Garber Facility archives. We next tracked down every accessible surviving example of the Model 10. TIGHAR volunteers traveled to these aircraft and taped large sheets of paper to their bellies (the airplanes’, not the volunteers’) and the made rubbings which traced every rivet, skin border, drain hole, access door – you name it, between the slanting bulkhead (Station 93) and the Main Beam (Station 129 5/8). TIGHAR researcher Frank Lombardo (#1806) then converted the tracings to mylar transparencies, enabling a full-scale, one to one comparison of the several aircraft. Not surprisingly, we found considerable variation. For example; c/n 1015, a 10A which was later converted to a 10E and has since been rebuilt as a replica of Earhart’s c/n 1055, has a stiffener added at Station 114 creating a crossing line of rivets not present on the original aircraft. Another surviving 10A , c/n 1052, features not only the added stiffener but also has larger rivets than the standard Electra. By contrast, the oldest surviving example, c/n 1011, has no stiffener at Station 114 and features the small AN455 AD3 rivets seen on the artifact. This aircraft also appears to have, at some point, been repaired with thinner than standard skin beefed up with supplemental stringers.

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