AT my US 121 airline, the "A" check is a service check done every 24 hours. The "C" check is done on a 20 month cycle, and has nothing to do with the accumulated flying time. It's a major overhaul, costing about $1,000,000 and takes about 4-5 weeks. I'm not sure if we even do "D" checks any more. I'll look into it.
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