Final Conclusions

The aluminum sheet with the “ALCLAD” marking, allegedly from Amelia Earhart’s Electra aircraft, is actually from a 1940s aircraft — precisely as the aluminum markings predicted. The artifact’s unique rivet pattern matches exactly the top of the wing of a surviving Douglas C-47B Skytrain built in the 1940s. (This is especially significant since a C-47 crashed on Sydney Island in the Phoenix Group.)

Case closed.


Even More ALC Markings

ALC24ST — ca. 1935.  Alcoa Aluminum advertisement, Aviation, May 1935, pg. 9. (Image also reproduced in Aero Digest article below.)  Link:

ALC24ST — ca. 1936.  Douglas fuselage frame in “Development of Aluminum for Aircraft,” C.F. Nagel, Jr., Aluminum Company of America, Aero Digest, Feb. 1936, pg. 27.  Link:

ALC 24ST — ca. 1936.  “Spot Welding vs. Riveting,” Aviation, Oct. 1936, pg. 29.  Link:

ALC24ST — ca. 1937.  “Thrills of the Navy Test Pilots,” Popular Mechanics, Aug. 1937, pg. 227.  Link:

ALC24ST — ca. 1937.  Alcoa Aluminum advertisement, Aviation, Nov. 1937, pg. 9.  Link:

ALC24ST — ca. 1938.  Rear cover, Curtiss Fly Leaf, December 1938.  Link:

ALC24ST — ca. 1939.  “Slots Are Here to Stay,” Kurt Rand, Popular Aviation, Oct. 1939.  Link:

ALC24ST — ca. 1940.  “Are You Qualified?”, Hurd Barrett, Flying Magazine, Oct. 1940. pg. 32.  Link:

ALC24SRT — ca. 1940.  “Assembling of a DC-3 / C-47 aircraft, ca.1940,” California Historical Society Collection.  Link:

ALC24ST — ca. 1940.  “A man working on fuselage in the factory,”  George Strock/The LIFE Picture Collection/Getty Images.  Link: (Another source:

ALC24SO — ca. 1940. “29th March 1940: At the Lockheed Aircraft Corporation at Burbank, California, large numbers of high-speed twin-engined planes, part of a previous British order, are being produced,” Photo by Fox Photos/Getty Images. Link:

ALC24SO — ca. 1940. HPM Fastraverse Presses advertisement, Aviation, June 1940, pg. 67. Link:

ALC24ST — ca. 1941.  “Curtis Wright,” Dmitri Kessel, April 1941.  Link:

ALC24ST — ca. 1941.  Multiple photographs, “Glenn Martin Plane Factory,” George Strock, January 1941. Links: 1, 2, 3, 4, 5, 6, 7, 8 

ALC24ST — ca. 1941.  “B-17E Flying Fortress Camera Pit Below Radio Compartment.”  Link: and

ALC24ST — ca. 1942.  Although other aluminum markings were introduced in the early 1940s, the leading edge of this wing still shows faded “ALC24ST” markings.  Link:

ALC24ST — unknown [DC-3].  Handbook of Instructions for the Transport Model DC3-277B for American Airlines, Inc., Douglas Aircraft Company, Inc., pgs. 11, 34, 64, 89, 106A, 145.  Link:

ALC24ST — unknown [P-36].  P-40 Warhawk in Detail, Part 1: Y1P-36 through P-40C, Bert Kinzey, pg. 26. 


1942 Alclad Marking Documentation: “The Aircraft Apprentice”

Further documentation of ALCLAD aluminum markings was published in the 1942 publication, The Aircraft Apprentice, Leslie MacGregor, Pitman Publishing Corp., 1942 (

From page 100 illustrated below: “It may be noticed that some codes are preceded by the letters ALC which signify that the alloy is covered on both sides with a coating of pure aluminum, and is known as Alclad. This coating prevents corrosion and amounts to approximately 10 per cent of the thickness of the sheet.”

image page100[Click to Enlarge]

1943 Alclad Marking Documentation: “Aircraft Production Standards”

Additional documentation of ALCLAD aluminum markings from the 1943 publication Aircraft Production Standards, Stuart Leavell and Stanley Bungay, McGraw-Hill, 1943. (

The entire text is not currently available online, but Google Books allows viewing of snippets of the work including the following: “A typical identifying code such as 24ST ALC may be used for an example. This group of numbers and letters will be found stamped repeatedly across the surface of sheet metal. Here the numerals 24 indicate a specific alloy. By referring to the table on page 253, it is possible to find the materials and their percentages used in the composition of this alloy.”


1942 Alclad Marking Documentation: “How to Do Aircraft Sheetmetal Work”

Another example of documentation of ALCLAD aluminum markings are provided in the 1942 publication How to Do Aircraft Sheetmetal Work, Carl Norcross and James D. Quinn, McGraw-Hill Book Co., Inc., 1942. (

The entire text is not currently available online, but Google Books allows viewing of snippets of the work including the following from page 50: “Alclad is used to a great extent on all-metal airplanes. Usually the sheets forming the fuselage and the wings are of Alclad. Sheets are easily identified because they are stamped “ALC” preceding the identification of the metal, for example, ALC24S-T.”


1941 Alclad Marking Documentation: “Aircraft Sheet Metal Work”

Written documentation of ALCLAD aluminum markings in 1941 is provided in Aircraft Sheet Metal Work: Bench and Repair Work, H. Edward Boggess, New York, Pitman Publ. Corp., 1941. (

From page 13 included below: “The letters AL or ALC preceding the alloy designation indicate that the material is Alclad, which means that it has been surfaced on both sides with pure aluminum.” And further on the following page, “Thus a piece of metal labeled AL-17ST-ANA would be heat-treated, Alclad dural which conforms to the Army and Navy specifications and comes from the mill at Alcoa, Tenn.”

image1[Click to Enlarge]

image2[Click to Enlarge]

Numerous photographs show the “ANA” mill/plant marking, but this photograph is the only example discovered to date showing the “ANK” mill/plant marking. (Source: National Archives,

ANK-marking[Click to Enlarge]

1930s and Early 1940s ALCOA Aluminum Markings

Documented examples of 1930s and early 1940s aluminum markings. Click each image to enlarge.

All known photographs of aluminum markings from the 1930s show Alcoa Alclad aluminum was marked “ALC24ST” and “ALC24SRT”.

Photographs show “ALC24ST” and “ALC24SRT” markings on aluminum in aircraft from:

  • Boeing
  • Douglas
  • Lockheed
  • Severski
  • Glenn L. Martin Co.

Boeing XB-17 (Model 299):

Boeing XB-17[Click Image to Enlarge]

Douglas DC-3:

douglas_dc3_upload[Click Image to Enlarge]

Seversky P-35:

seversky[Click Image to Enlarge]

Unknown Glenn L. Martin Co. Aircraft:

glenn-martin-unknown-aircraft[Click Image to Enlarge]

Unknown Douglas Aircraft:

unknown-douglas-co-aircraft-upload[Click Image to Enlarge]

Caltech Thesis:

alc24st-thesis[Click Image to Enlarge]

Earhart’s Electra:

alc24st-earhart-mechanics2[Click Image to Enlarge]
alc24st-engine-mount copy[Click Image to Enlarge]

Lockheed Electra:

  • Marking: ALC24ST
  • Font: serif
  • Aircraft: Electra
  • Company / Factory: Lockheed
  • Date: N/A
  • Source: “Lockheed Aircraft Corp. Report No. 421”. (Internal pages include dates from 1934 to 1936.)
  • Notes: Via private communication. No online source at this time.

electramanual2[Click Image to Enlarge]
electramanual1[Click Image to Enlarge]

Aircraft Template Development (1942):

lockheed-electra-image-one-inset[Click Image to Enlarge]
wing-image-book-inset[Click Image to Enlarge]

NACA TN No. 752, Investigation of Sheet-Stiffener Panels (1940):

test-specimen-dunn-alclad[Click Image to Enlarge]

George Arnold Photo:

alc24srt-china-inset[Click Image to Enlarge]

NACA TN 883 (1943):

  • Marking: ALC24ST
  • Font: serif
  • Aircraft: N/A
  • Company / Factory: Alcoa
  • Publication Date: 1943
  • Notes:

press-test-1of2[Click Image to Enlarge]
press-test-2of2[Click Image to Enlarge]


TBD Devastator:

tbd[Click Image to Enlarge]

Boeing 314 Clipper:

boeing-clipper[Click Image to Enlarge]

Airplane Metal Work, Vol. 2 (1941):

24so-image[Click Image to Enlarge]
alc24st-image[Click Image to Enlarge]

F-10-G Fleet:

fleet-1934[Click Image to Enlarge]

Mech. Prop. of Flush-Riveted Joints (1940):

  • Marking: N/A
  • Font: serif
  • Aircraft: N/A
  • Company / Factory: N/A
  • Publication Date: 1940
  • Source: Bruggeman, Wm. C. & Roop, Frederick C., “Mechanical Properties of Flush-Riveted Joints”. UNT Digital Library.
  • Notes:

naca pg39 high_res[Click Image to Enlarge]

To Do:
– Add Lockheed Electra
– Add DC-2 example
– Add examples from the Alcoa Aluminum book
– To add:

Other Sources to Investigate:
– Marking regulations. War Dept. TM:;view=2up;seq=16
– Airplane Metal Work:;view=2up;seq=36
– Aircraft Sheet Metal:;view=2up;seq=40
– PC and ALC abbrev.:$b317251;view=2up;seq=118
– Color markings (British?):;view=2up;seq=14
– General discussions of abbreviations: ($b65154;view=2up;seq=66,;view=2up;seq=124,;view=2up;seq=208,;view=2up;seq=88,;view=1up;seq=424 )
– Too small to read:$b317215;view=2up;seq=100

Aluminum Markings Online

Online photographs showing examples of ALC24ST and ALCLAD markings, primarily dating from the WWII-era:
[black ink? maybe not alclad] (–,_Long_Beach,_Calif.jpg
[not alclad, some other marking]

NEW!;view=1up;seq=87;view=1up;seq=89;view=1up;seq=48;view=2up;seq=76;view=2up;seq=50 (URI: ark:/34231/c6p26w46) (URI: ark:/34231/c6n014h4)’s%32Kansas%32City%32Assembly%32Facility,%32circa%321943-2F3XC5W5B2I.html,%32Douglas%32A-20-2F3XC5MNSFG.html,%32Rosie%32the%32Riveter-2F3XC5JZ48N.html’s%32Station-2F3XC5PX3M7.html



[24ST-PC?] //

[ANA] //

Stencilling and sheets, not necessarily ALCLAD:

Combination of plate markings and thicker piece markings:;q1=alclad%20surface%20scratch;page=root;view=image;size=125;seq=501;num=483

(Credit: Thanks to many other researchers for helping discover these images and links.)