411A:
I would doubt any regulatory agency would approve a 4,200 hour/no calender limit between C-Checks for the L-1011, without approval by Lockheed. Having worked in Lockheed product support for 20 years, where the approval would have to come from, I can assure you there is no way something like that would be approved. If fact like GotTheTShirt said only an experanced operator would get the extension from 12 to 15 months.