USAAF C- for transport designations

According information in Fahey's US Army Aircraft 1908-1946 the UC designation was used for transport aircraft that carried up to eight persons (incl. the pilot) or cargo of not more than 1400 lb.
It seems that sometime in 1946 or 1947 this designation disappered in favour of the C designation as Fahey's USAF Aircraft 1947-1956 does no longer mention the UC designation.
I have also other evidence to suggest the UC designation lapsed.
Does anybody know on what date the use of the US designation ceased and the aircraft were redesignated as C.

Jos Heyman said:

According information in Fahey's US Army Aircraft 1908-1946 the UC designation was used for transport aircraft that carried up to eight persons (incl. the pilot) or cargo of not more than 1400 lb.
It seems that sometime in 1946 or 1947 this designation disappered in favour of the C designation as Fahey's USAF Aircraft 1947-1956 does no longer mention the UC designation.
I have also other evidence to suggest the UC designation lapsed.
Does anybody know on what date the use of the US designation ceased and the aircraft were redesignated as C.

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Please note that "UC-" was not a designation as much as the apposition of the "U" prefix to the "C for transport" designations.
Although I can't be positive, I suppose that the "U for Utility" prefix (just like the "L for arctic" one) disappeared at the same time as the new tri-service system came into existence, i.e. 1962.

I cannot agree with your interpretation as Fahey's clearly states UC as a type designation. He does mention: "Auxiliary prefix symbols that indicate the usage of an airplane when it is modified so that its original usage no longer applies. Thus a B-17G-90-VE modified for cargo use would become a CB-17G-90-VE".
The Auxiliary prefix symbols he lists are C, E (for exempt), F (for photo), K (for ferret), S (for air sea rescue), T, V, and W. He also mentions R (restricted), X, Y, Z, the latter as Classification Symbols.
So no U prefix of any sort at the June 1948 status description provided Fahey's.
In fact the U for Utility was not introduced until 14 March 1955 and then only as a primary type designation, not as a prefix of any sort. This is from Fahey's USAF Aircraft 1947-1956.

Model Designations of Army Aircraft, 12th ed., 1 June 1946 (downloadable at http://www.designation-systems.net/usmilav/original-docs/index.html)lists only 3 prefixes - U, X and Y (the previous edition, Jan. 45, also had R ("Restricted") and Z ("Obsolete"). Therefore "UC" was not a separate type designation as the other two-letter prefixes, e.g. AT, BT, PT. A supplement to the 1946 edition, dated Dec. 1947 (also downloadable from the linked page) doesn't mention any changes to the designation system, so I assume that the U-prefix was formally still in effect at that time.

My best guess(!) is that the U was dropped when the USAAF completely revised the designation system in 1948. Circumstantial evidence for this is the fact that former UC types were listed as simple C types after 1948 (often with prefix Z-for-obsolete, though). Unfortunately, other than pre-1947 USAAF and post-1962 DOD documents, I was never able to get hold of USAF aircraft nomenclature regulations from the 1948-1962 era.

Forgot to mention: Apart from the two "Faheys" (1908-46, 47-56), I also have a three-page "supplement" to the 08-46 edition, which has some bits which are in neither of the two "regular" editions. I'll have a look tonight, if it says anything about designation prefixes.

Andreas Parsch said:

Forgot to mention: Apart from the two "Faheys" (1908-46, 47-56), I also have a three-page "supplement" to the 08-46 edition, which has some bits which are in neither of the two "regular" editions. I'll have a look tonight, if it says anything about designation prefixes.

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The supplement gives a detailed explanation of the USAF designation system, which became effective on 11 June 1948. The letter "U" is one of the few that isn't mentioned anywhere - not as "Type Designation", not as "Prefix Symbol" and not as "Classification Symbol". Also, the few remaining ex-"UC" types in the inventory listing are all listed without "U" prefix.

I think it's safe to assume that UC was dropped in the course of the 1948 designation system change.