A "C" Check (as Norman stated) is accomplished every 12 to 18 months and is a structural inspection that may or may not include the "A" and "B" checks as part of the work package. This inspection is usually a general visual inspection (GVI) with the aircraft opened up to expose critical structrual elements. SSID (Supplemential Structural Inspection Detal) items may be included along with CCP (Corrosion Control Program) items. This inspection takes 40 to 60 men 10 to 15 days to accomplish depending on the work scope.
http://www.airdisaster.com/forums/archive/index.php/t-54137....

SI -- structural inspection

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Note added at 46 mins (2005-02-26 21:49:18 GMT)
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я имел в виду категории 4С

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Note added at 47 mins (2005-02-26 21:50:18 GMT)
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т.е. 4С -- это тип проверки

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Note added at 59 mins (2005-02-26 22:03:11 GMT)
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According to Maintenance Plan, the scheduled maintenance of B737-300/400 includes daily check and low- or high- level check with the former including pre-flight check, transit check and after-flight check, and the latter single and multiple A check and C check. A check involves multiple A check such as 1A, 2A, 4A, 8A,. The A-check interval is 250 flight hours While multiple C check, such as 1C, 2C, 4C, 6C+SI (structural inspection), and 8C. The C-check interval is 4000 flight hours.
http://www.hnair.com/en/company/fleet/fleet3.asp